A DIY Receiver for GPS and GLONASS Satellites
Marja: vidmar , 55 3MV tex YU 3 UMV. IT 3 MV)
A DIY Receiver for GPS and
GLONASS Satellites
0:'0 (;PS
Next to the ama teur radio satel lites, the
most irucresting sate lfncs arc weather
satellites. Rad io am ateurs have al ready
succeeded in build ing equipment to
recei ve Images from all blown ....eethcr
satellites and for all transmi ssion standards in use, and it was also radio
amateurs who .....ere the first to receiv e
T V satel lite signals. using partic ularly
small aeria ls. MJmC lime before these
became mas s-produced articl es.
It is already more than ;\0 years since
the first navi gation sate llites were
lau nched into the cos mos. But it is only
in the last few years that satellite
nav igation and posi tio n ing have
become really popular. with the i ll '
troduction o f mo re reliable. more: accurate and more user -friend ly systems.
s uc h a s th e Ame r ica n G lo ba l
Pos itioning System « iPS) and the
Rue..ian GLO bal N A v i~at ion Satellite
System (GLO:"lASS) .
Eac h "y,"cm will evcntuall y replace a
who le ran!!e of ground -..upported navigaric nal aids. As a usef ul by-prod uct.
they give anyone with the rig ht equi pment a very preci se lime base (l OOns )
and a very accurate frequency ( 10 12 ),
Ongtna uy both sy stem s. UPS and
GI.O;'>lASS, were designed for military
purposes. Bur since then there have
been considerably mo re ci vilian tha n
mi litary users .
G PS navigati on receiv ers (soon to be
jo ined by combined GPS/G LONASS
receivers ) can be manufact ured 10 be as
handy, user -friendly and favourably
priced as modem portab le radio sets.
These pieces of equipment can measure
thei r three-d imensiona l position with an
accuracy of app. 50 met res at any point
on the Earth's surface . So the se sets a TC
of interest. not only to leisure-time
pilots, lorry dri vers or mountaineers.
but to radio amateurs too !
Apart from the c hallenge o f building
your own satellite receiver. radio amateurs can also put (I PS and GLON I\SS
signals to ot her uses. Probably the
simplest applicat ion for a <ir s or
GLO NASS receiver is 10 usc it as a
hig h-precis ion frequency source. E xact
liming and synchro nisation can be used.
for example. for mod em transmission
techniques. or for precise experiments
conce rning the propagation path and
the propagation me cha nisms of radio
Finally. this syste m C,HI also he of usc
in the positioning. and alignment of
high -gain micro wave aerials.
Th is article will be split into se veral
part s: I sha ll begin by describing the
.sate llites them selves and their radi o
signals. Next will come II description of
the way II (I PS or OLONASS receiver
works . This will he followed by assem bly instruction s for a DIY GPS or
G LONASS rec eiver. togeth er with an
expla nation of the operationa l software.
These receivers ca n be built in two
versions - as independent portable recei vers with a small keyboard and a
liquid crys tal display screen. or as
add-ons with plug -in modules for the
DSP computer ( I) (2).
2.1. Ra d io navi gati on backgr ou nd
Like all areas of electro nics and radi o
eng ine e rin g, rad io nav igation I S
devel oping very fast.
The basis on which a ll rad io naviga tion
systems operate is that exte nsive
resea rch has been done into the prop aga tion mecha nisms o f radio wave s,
and that the propa gation speed of radio
waves is normal ly c lose to the speed o f
light in free space. Systems working.
with radio waves normally have a
sufficiently extens ive range to ma ke it
meaningful to lise them for detennining
locations. speeds and positio ns.
In the end, all m easurement s involving
radio waves, whether we 're talk ing
abo ut position finding, and therefore
direct ional search, measuring running
time, phase m easur em ent or the measurement of the Doppler frequency shift,
can he carried out - on the user 's side.
al least - using simp le and reasonablypriced technical aids .
Ea rlier radio navigat ion systems made
use of the directional effect of the
recei ver aerial, the transmitter aerial , or
both. In both kinds of system, the main
cause of measurem ent errors was the
lack of precision in the alignment
characteristic of the aerial. Since the
measured variable consists of an angle ,
the positional erro r increases linearly
with the distan ce o f the USCT from the
position of the navigationa l referenc e.
These systems arc therefore very mu ch
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limited with regard to their capability
of usc - in their range or precis ion. so
to speak. They are. of course . outstanding ly suitable for one app lication getting the user to a specific point. for
example guiding an aircraft onto the
runw ay by means of an instrument
landing system (ILS).
Time and freq uency are definitely the
physical variables which can he meas ured with the greates t accuracy. If the
propagation speed and propagation conditions of specific radio waves are
known, the distance can be calcu lated
in the simplest way by measuring the
running time. The abso lute precision of
such dista nce measure ments just docs
not depe nd on the order of magnitude
of the distance to be measured. irres pective of the uncertainties of the propagation speed of the radio waves used
on this path.
Por this reason, all high-precision radio
navigation systems which arc suitab le
for long distanc es are based on meas urements of running time or path
difference and/or on the derivativ es of
these variables in relation to time. also
known as the Doppler freq uency shift.
(~! -
- - - - - - - - - -----'-"'--'=
== = '-""=
The simplest way \0 determine the
distan ce LO a known location is to
install a converter there. transmi t a
signal and measure the runn ing lime
until the response signe t is received .
Although systems o f this kind are
indeed in usc (c .g. D!\.1E for civi l
aircraft), they do have their limitations.
smcc eac h user ha s 10 have a
transmitter as well as a recei ver.
In the civilia n sector. thi s syste m has
the addit ional disadvantage that the
equipment has to be licensed , and the
military try and avoid transmission as
much as possible. so as not to give
away their position. wh ich must be kept
secret. But the higgt'st handicap is that
only a limite d numbe r of people can
usc this system. for the simple rea son
that they ca n only use it one at a ti me.
As regards the user side. we could do
wit hout the transmis sion equipment if
we could usc some other means 10
achi eve and main ta in the synchronisation of lite two sides.
For example. both sides. the user and
the nav igation transmitter, could be
equ ipped with high-precision time standards. e.g. an atomic clock . The user
would merely have to synchronise his
or her clock at a known location. and
then lake thi s clock to the unknown
location as an aid 10 measurin g the
running time .
Hut since ato mic c locks are a little
cl umsy and expens ive. we need 10 find
a considerab ly simpler sotuuon, which
needs nothing bUI a recei ver.
Such a system must co nsist of a whole
series of synchronised tran smitters. as
shown in Fig.l .
However. since the preci se time is not
known on the receiv er side, we can not
mea sure either the delays or the d istances dl , d2. dj, etc. to Ihe transmit ters
TX t . TX2. TX3 . etc . directly. We ca n
measure only the varying arri val limes
of the different transmission signa ls.
The se lime differences directly correspond to the differen ces in distance.
TIle multitude of points for a give n
distance difference for two preset points
produce a hyperb ola (loo ked at in two
dimensions) or a hyperbolo i d (loo ked at
in three dimensions). Here the two
transmitters arc in the foci of the
hyperbo la (or the hyperboloid).
l-or two-dimensiona l navigati on (10cation-findi ng), signals mu st he received from at lea st three ..ynchroniscd
transmitters. ....or exa mple. the hype rbola dl • d2 = const. 12 can be plotte d
directly on a map from the d ifference in
the running times meas ured for TX I
and TX2. The hyperbo la d2 - d3
c on s t . 23 can
b e plott e d
correspondingly on the map from the
difference in the running times mea sured for TX2 and TXJ. The inte rsection
point of the two hyperbo las is the
unknown locat ion of the user!
For three-dimen sional nav igat ion .
signals must he recei ved from at least
four synchronised transmitters . Th e
three differen t runnin g time differences
then give three hyperbolo ids. The
surface.. of two hypcrboloids intersect
in a curved line. which intersect s with
the surface of the third hyperb oloid al a
single point. Th is correspond s 10 the
three -dimen sional position of the user.
="'-"""'--"''''------- --
Should even more transmitters be
availab le, we can sed : out the three or
four transmitters which give hyperbolas
(hyperboloids) which intersect a lmost at
right angles. The remaining transmitters
can then be brought in to chec k for
possible errors or ambiguous solutions,
since curved lines and surfaces can
produce more than one point of irucrseette n.
Hyperbolic naviga tion systems were
originally structured as ground-supported systems in the medium and
long-wave ranges. e.g . Loran. Decca or
Omega. Rut since these systems ope rate
from the ground upward s. no threedimensional position-finding can be
carried out - only a reliable determi nation of geographical latitude and
longitude. For it to be possible to
mea sure th e heigh t as well , a
transmitter must he as far above or
below the receiver as possible. or at
least outside the horizontal plane of the
Radio navigation systems insta lled on
the ground use relatively low frequencies of the radio spectrum to obtain
as wide a range as possible. and
simultaneously to avoid undefin ed
propagation through space waves. For
example, Omega uses a frequency range between 10 and 14 kHz, and thus,
using only eight transmitters, covers the
entire surface o f the Earth!
Longwave navigation systems were
devel oped at a time when digita l
computers were not easily available.
And navigation on two planes, using
transmitters at fixed location s, requires
only a minimum of calculation from the
user, Moreover. the multip licity of
- - - \ ..
hyperbolas for each transm itter pair.
incl uding the necessary corrections for
propagation anomalies. ca n be plotted
d irectly for use on corresponding maps.
One of the first applications (or artificia l satellites was radio nav igation.
Naturally, for their pan , artificial satellites needed rad io naviga tion 100. in
order to estimate the power of the
carrier rockets and determine the satellite ' s final orbit. Moreover. space is an
id e a l lo catio n fo r na vi ga ti o nal
transmitters . firstly. an enor mous range
is available for VHF and for the higher
frequencies. and seco nd ly the propagation o f the radio waves is calc ulable
and the influence of the continuously
chang ing ionosphere is insignificant.
fi nally. the locations of the nav igati on
transmitters in space ca n he selected in
such a way that thr ee-dimensional
position-find ing is possible all over the
Since orig inally satellites could he used
only in ncar-Earth orbits. the first
navigation satellites . such as the Amen can Transit satel lites or their Sov iet
counterpart, Cicada. were launched irao
low Polar orbits (abo ut 1.000 km. up).
Since a satellite in a near-Eart h orbit
travels a long its path very quickly. even
a sing le satellite can be made use of for
position-finding. The accuracy of a
quartz watch is sufficient to measure
the few minutes required by a satellite
for an overflight. The change in the
sate llite 's position roughly corresponds
to a quantity o f transmitters at various
points along its ni ght path.
In practise, we measure the Doppler
shifl of the satellite signal for a certain
time and then use the satellite path data
-,, ,
RX luser)
_ d1'U
Vu = dT
...,vJ • ~
/ dj u
Ne vlgauon Eq uation for ca lcula ting Doppl er Shift Differe ntial:
(r, - ru) ' ( v, -vu )
82 Relativ e
-- -r:
6 1,)
= - c·f,
- - ...........-..
speed of T XI
spe ed of TX J
""a \'il:ation Eq uat ion for calculating Running Time Differe nce;
I r, - r"l - I r) - ru l = c-
6. 1~
Dista nce to T XI
Instance In TX J
F ig,2: Eq uat ions for the Calculation of Running Times and Doppler Shin
to calculate our own unknown position.
Although only a single satellite is
needed to determine a position. these
systems usually consist of from six
satellites (Transit) up to twelve, in
order to cover the surface - after all, a
satellite in a low orbit is visible from
the Earth' s surface only for a certain
pe riod of lime. And since th e
ionosphere has a certain influence on
radio waves in the VHf and U H~
ranges, both systems - the American
and the Soviet satellites - operate on
two frequencies. at 150 MHz and at
400 MIll.. The actual frequencies arc in
the exact ratio 3/8 and the transmiucrs
arc kepi phase-synchronised, so that the
influence of the ionosphere can be
balanced out.
The most serious disadvantage of navigation satellites in low orbit is the fact
that we have to wait for a satellite: to
fly over, and we then need several
minutes for the measurement. Finally.
ou r own speed and course must be
known precisely. so that they can be
taken into account in determ ining our
position. Several sate llites are needed
for a position 10 be determined very
rapidly. U at least four satellites are
visible at various po ints in the sky . our
own location call he dete rm ined immediately a.. regards longitude. latitude
and height. without having to wait Ior
the satellites 10 move through the ~ L:y.
To keep the number of satellites requi red as low as possible. they must be
put into higher orbits. The American
GPS satellite navigation system and the
Soviet GLONASS system arc intended
eventually to cover the entire surface or
the Earth. each having 24 sate llites.
including reserve satellites in space.
AI least four of the satel lites from
either system should be visible anywhere on Earth. distributed over the sl y
in such a way as to make threedimellSionalnavigation poss ible.
Nor should we forget the enormous
amount or calculations required to carry
out three-dimensional position-finding
using satellites. The fact that the satcllite's position is constantly changing
means a computer must be used.
Perhaps this explains why satellite
navigation is only now becoming popular. Suitable satellites have indeed
been available for thirty years - but
reasonably priced computers have not.
2.2. Eq uations Ior sate llite ne vlgatlon
In order to understand satellite nav igation systems (SNSs). we: must first
look at the mathematical background to
satellite navigation. First we must
define a co-o rdination system . Most
satellite navigation systems operate by
means of a right-handed Ca rtesia n co ordinate system. like the one shown in
fi g.2. The co-ordinate system is rigidly
connected to the Earth and is thus a
rotating co-ordinate system. and thus
deviates hom the inertia co -ordinate
system for Kepler elements used for
most satellites.
The zero poi nt of the co-ordination
system nonnall y lies in the centre or
the Earth. The 7. axis corresponds to the
rotational axis of the Eart h. and points
to the North. '1l1C X and Y axes arc in
the plane or the Equator. with the 7.
axis pointing in the direction of the
Greenwich meridian, whil st the Y axis
is orientated in such a way that a
right-handed orthogonal co-ordinate
system is produced
Naturally it is also possihle to convert
the data into a more popular coordinate system. c.g . into degrees o f
latitude and longitude. plus hcif.ht
above sea level (height above surface
or an ellipsoid). These conversion procedures are always based on the final
result. since most or the calculations
required for position-delennining in
navigation receivers can be carried out
considerably more easily in a Cartesian
co-ordinate system.
Finally. we should not leave out or our
reckoning the fact that there are various
co-ordinate systems in use with the
same basie definition . Meanwhile. SNSs
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Fig.3: Hefght a nd Angle of I nclin ation or Or bits of GPS and G LONASS
Sat ellite s
have improved the absolute positioning
accuracy until it is now within one
metre. as a result of which the small
discrepa ncies in the different local
geographical co-o rdinate systems become noticeab le. Thus GPS uses the
WGS· 84 co -o rdi nate system and
GI.ON ASS u se s SGS - S5 . The
differences between the two systems
add up to a difference in position of
about to m. in the East-West direction
and (he same in the North-South direclion for the author' s location in Central
calculated by determining the scalar
- - - --
If we use a vector repres entation.
navigational equations can be expresse d
much more simply. Using a threedime nsional Cartesian co -ordin ate system, it can easil y be understood that an
individual vector describes three independent variables.
Th e nav ig a ti ona l equ at io n for
differences in running time consists
merely o f tbc area vectors which give
the positions o f the transmit ters [satcllites) and the receivers (users). The
d ifferences between users and satellites
are calculated as absolute va lues from
the area vector differences. On the
other side o f the equatio n stands the
difference in running time mea sured,
multiplied by the speed of propagation
of radio waves (c).
If the user' s location is unknown, which
means the area vector is too , three
variab les o f the scale are missing, and
th ree inde pe nd ent ru nn in g t ime
difference eq uations have to be solved
to ca lcu late th em. AI lea st four
transmitters (and thus four visible sate llites) arc needed \ 0 solve these three
equations. The absolute value of a
vector is a non-linear function, which is
determined by ca lculating ..quares and
roots. These equations are thus solved,
eit her by numerical iteration or ana lytically (3).
I\. navigation equation for calculating
the Doppler shift d ifferential contains
both area vectors and speed vectors.
The speed differential for the Doppler
frequency shift must be calc ulated first,
so that the projection of the speed
differentia l vector onto the direction of
prcpagauon o f the radio waves can then
be calculated. Vector projections are
product of two vect ors .
On the other side of the equation. we
have the Doppler frequenc y shift as a
dependent variable. the absolute va lue
of which is obtained by dividing by the
carrier frequency ro' T he relative frequency diffe rential can then he coovencd into speed va lues by multiplying
the variable by the speed of prop-a gation (c ).
The Doppler frequency shift navigation
equations contain both the positiona l
vector and the speed vector of the user.
Th is can mean up 10 six unkn own
sca lars. Rut since we normall y do not
have six independent equations. the
foll owing rou te can he taken:
1. If the localion is a lready known
from the equations for calculating
the running time difference, the
use r' s speed ve c to r c a n he
determined using three independent
equat ions to calculate the Doppler
frequency shift.
2. If the user' s speed vector is known,
or if the user' s speed is ' .CTO (in
stationary operation), then the location can be determined using three
independent equations for calculating the Doppler frequency shift.
3. Various com binations of the equalions for running time difference and
Doppler frequency shifts are possible.
Apart from the visibi lity problem, the
navigation equations impose additiona l
restrict ions and requirements on the
orbital paths of the naviga tion satellites.
T he accuracy with which the loca tion
(t ,- -- - - -- - - ---"-"-""'=="-'==
or speed is finally determined depends
on the constructio n of the equatio n
system .
If the equation system is badly selected,
every measuremen t error appears enlarged even in the final result. In terms
of geo metry. a badly selected equation
system is tantamount to having intcrscctions of lines and surfaces at very
shallo w ang les.
The impairment of accuracy due to an
unsuita ble equat ion system is referred
to 3." UDOP (geomet rical dilution of
precision). Naturally . satellite orbits
mu st be se lec ted in suc h a way that th e
GooP is as smal l as possible for the
biggest possible number of users. Hut
since we arc dea ling with non-linear
equations. the GlX) P a lieni with the
location of the user. Users must therefo re select four satellites which arc
favourable for them . It is certainly
abso lutely possible that several satellites will be " visible", possibly even a!
a rather high elevation. which also
increases the GOO P.
The most remarkable case of a large
GDOP when running time difference
equa tions arc used occurs when two
navigation satellites arc close to eac h
other in the sky. A more common case
is when all four satellites arc in almos t
the same plane. For the same rea son, a
geos tationary orbit is also unfavourable
for navigation satellites. A further
disadvantage would seem 10 be the low
relative speed of the satellite. since the
equations for ca lculating the Doppler
frequency shifts are not designed for
cases where the positional vector is
multiplied hy very small numbers.
2,3. The GPS & Gl,ONASS s.alellile
UPS and GI.ONASS are the first satellite systems which require the simult aneous ope ration of severa l satellites.
Other systems are already operati ng
with one satellite, and each one improves the system further.
In the GPS and GLONASS systems. the
satellites have to he sync hro nised and
can in all cases operate only in sds of
81 least four salellites visible to the
user. The requirements for a low GIXW
should not be forgotten here .
UPS and GLONASS satellites have
been put into similar orbits. Fig.3
compares the orbits of (iPS and
(; I,ONASS satellites with other kno wn
satellite orbits, such as the geostationary or near-Eart h contra-rotating Sunsynchronised orbits.
(iPS and GLONASS satellites have
circular orbits, with an inclination of 55
- 65 degrees and an orbital period in the
order of 12 hours. which corresponds to
8 height of app. 20,000 km. (about I Ih
Earth diameters).
(To he conlinul"d)
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Mc ajaz Vidmar , S5 3MV(ex YU 3 UMV , IT 3 Af Vj
A DIY Receiver for GPS and
GLONASS Satellites
2.3 .
crs & G LONASS Salcllilc
System s
G PS and GLONASS arc the first
satellite systems that require the
simultane ous operation of a number of
sate llit es. In othe r satel lite systems,
includ ing curl ier navigat ion systems,
the operation of every single satellite
was almo st autonomous and any
addit ional satellites only improved the
capacity of the system.
Tn <ii 'S or n J,oN'i\SS the satellites
need 10 be sync hronised and can only
pe rform as a constellation of at least
four visible satellites for every possible
user location without forgetting the
Gl'Op requirement! Both GPS and
GLONASS satellites arc launched into
similar orbits . A comparison among
GPS, GLONASS and more popular
satellite orbits like the geostationary
orb it or the retrograde sun-synchronous
Low-Earth Orbit (LEO) is made on the
scale drawing on Fig.3. Both GPS and
GLONASS satellites are launched into
circular orbits with the inclination
ranging between 55 and 65 degrees and
the orbita I period in the order of 12
hours, which corresponds to an altitude
of around 20000kin (one and a half
Earth diameters).
The GPS system was initially planned
to \LSe three different orbital planes with
an inc lination of 63 degrees and the
ascending nodes equally spaced at 120
degrees around the equator. Each
orbital plane would accommoda te R
equally spaced satellites with an orbital
period of 11 hours and 58 minutes,
synchronised with the Earth' s rotation
rate [4].
During a 10-year test period from 1978
to 1988 only 10 such " Hlock rsatellites were successfully launched in
orbital planes " A" and "C" as shown in
FigA. The GPS specification was
changed afterwards [5] and the new
"Block II" satellites are being launched
in 55-degrees i nclination orbits in six
different orbital planes A, B, C, D E
and F, with the ascending nodes
equall y spaced at 60 degrees
around the equator. The new GPS
con stella tion should also include
24 sate llites, having f OUT sa tellites
in each orbital plane. including
acti ve in -orbit spares. The orbital
peri od of the GPS satellites
sho uld be increased to 12 hours to
avoid repeat-tra ck orbits and
reson a nces with the Ea rth' s
gravity field. fina lly, the new
"Bl ock II" satellites 3 1.~O include a
na sty feature called "Se lective
Availability" (SA): the on-board
h ar d w ar e
may .
Bcgmning in L988 and up to
March 1993. 9 GPS " Blod : II"
10 new
Sate llile
La unc h Orbit
(iPS BI-02
OI'S Bl-04
GPS 8 1-06
(iPS 131-08
(i PS Bl -09
7R 20 I\.
7R 47 A
7R 93 A
A -?
78 112 A
c -?
GPS 131-10
80 32 A
PRN# Deeom m
Launch fai lure
IB n A C-3
84 59 A C- l
84 97 A A- I
85 93 A C-4
luI 85
lui 81
May 92
Oc t 89
Nov 83
Mar 9 1
May 93
gro un d
comma nd. intentionally degrade
the accur acy of the navigation
signal s for civilian users while
m ilitary users still have access to
the full system accuracy.
" Bloc k I1A"
satellites have heCII launc hed
using "Delta' rocket s. The SAmode is currently turned on and
degrad es the accura cy to be twee n
50 and l(XlM.
The G LONASS system is planned
to usc three different orbital
planes with an incl inatio n of 64 .8
degrees and the ascending nodes
equall y spaced at 120 degrees
around the equator. Each orbital
plane would accommodate 8 (or
12) equally spaced sate llites with
an orb ita l period of II hours, 15
minut es and 44 seconds. so that
each sa te llite repeats its ground
track after exactly 17 revolutions
or 8 day s 16).
(iPS Oll-Ol 8913 A
(i PS BlI-02 89 44 A
(i PS HIl-03 89 (~ A
(i PS BII-04 89 85 /\
(;pS BII...Q5 &997 A
U PS RII-06 90 8 A
(i PS RIl-07 90 25 A
( ;1'8 BII-08 90 68 A
UPS BIT-09 9088 A
0 -3
[\ -2
0 -2
(i PS BIIA- I()l)O IOl A
(i PS O[lA-119147 I\.
GPS BIIA-1292 9 1\
UPS BITA-1392 19 I\.
GPS HlIA·149 2 39 A
UPS BIIA-I592 58 A
(i PS HIIA-I69 2 79 /\
GPS HIIA·1792 89 A
(i PS BIIA-1893 7 A
UPS BIIA-1993 17 A
(I PS 8111\.-2093 32 A
GPS 8 11A-2193 42 A
UPS BIIA-2293 54 A
f -2
f -I
0 -1
A- I
wa s
Fig. 4: Published GPS Sat ellite Ope r ation
Sa tellite
Launch O rbit CHN# Decommissioned
G lonass 34
Glonass 36
Gl ona ss 39
G lonass 40
88 4 3C
88 85C
89 Ii\.
Glonass 41 89 IB
Gl onass 44
G tonass 45
Glonass 46
Gtonass 47
G lonas s 4 8
Glonas s 49
Gtonass 50
Glonass St
Glonass 52
G lonass 53
G laoass 54
Glonass 55
Olonass 56
Glonass 57
Gfonass 58
Gtonass 59
Gloua ss 60
Glo nass 61
0 ."''''
~. '. , . n,_
90 45A
90 45B
90 45C
90 110C
9 125A
3-1 8
93 lOB
3-2 1
3· 20
92 SB
92 47H
92 47C
93 lOA
Jan 92
Rep laced Mar 93
Rep laced Feb 92
Formerl y #21
Mar 93
Sep 92
Formerly # 19
Jan 92
Feb 92
Jan 93
Formerl y # 17
Formerly 3-18
Former ly 3-2 1
r , . ~" . , ,,
._ - -
L1_Q nAg. "'. """"'
An,. , n.
' _ I _ T ,~ g.~ , "" ,," n ,
, oot"
Formerly #23
I ~ - ".g .''' ." '' .~'
F ig. 5:
Recently obser ved
GLO NASS Satellite
O peration
l ' :P
I1OQUt "'I QIl " T . -.
'll.1Jt1Hl: Gf";
C'....c Od. ·
" on...''''
( lOCK
l O1Jt1H, ijf";
·'....' GLCW:
( l OCK
~~o,on kod; ~ ,.n
S 53 MV
SOb~. N "'~ 0""'"
OAT'" ... lo<mo' ;..""
Fi2.6: Block Diag ram of GPS and G LONASS Satellites
:,f.-p- Hn
l 1. ( I'"
MOOOl ""()Il
r ''''loon, ·
======='--- - - - - - - - - - - \'~..
Since the beginning of the GLONASS
program a large number of satellite s
have been launched into GLON ASS
orbital planes 1 and 3, the orbital plane
2 has not been used yet Some satellites
never transmitted any radio signa ls,
since the GLONASS system also
inclu de s passive "Eta Ion" satellites used
as optica l reflectors for accurate orbit
For the navigat ion function alone. the
satellite s could be muc h simpler.
carry ing a simple linear tran sponder
like on civilia n co mmunicat io ns
satellites. The requ ired navigation
signals c ould be generated and
synchronised by a network of grou nd
stations. However, roth GPS and
GLONASS aTe primari ly intended as
military systems.
GLONASS satellites arc launched three
at a time with a sing le " Proton" rocket.
Due to this constraint all three satellites
can only he launched in the sallie
orbital plane. Recently observed
GLON ASS sate llite operation is shown
0 11 H g.S. The observed lifetime of
GLONASS satellites seems to be
shorter than that of Americ an GPS
Uplinks arc undesired since they ca n be
easily ja mmed and a network of ground
stations ca n be eas il y de stroyed.
Therefore, hoth UPS lind ULONASS
satellites arc des igned for completely
autonomou s operation and generation of
the required signals. Synchron isation is
maintained by on- hoard atomic d ocks
that arc on ly periodica lly updated by
the ground stations .
A crit ica l piece of on-hoard equipment
are the atomic clocks required for
system synchronisation. Although each
satellite carries redunda nt rubid ium and
caes ium clocks. these caused the fai lure
of many GPS and GLONASS satellites.
In add ition to this, GLO~A SS satellites
have had problems with the on-hoard
co mputer , Unfortuna tely, the (iPS or
GLON ASS orbit altitude is actually in
the worst ionising-rad iation zone , Ihe
same ra diation thai already de stroyed
the AMS A'j"-OSCAR-I0 compute r
memory .
Both (iPS and GLONA&'<'; satellites
carry a caesium atomic clock as thei r
primary time/frequency standard, with
the accur acy ranging between 10-12
and 10-13,
GPS & G LO ~ASS Sat ellite
O n-hoa rd Eq uipm ent
Since th e two systems are similar, GPS
and G LONASS sate llites carry a lmost
the same on-board equipment as shown
in Fig .6.
Much smaller and lightwe ight rubid ium
atomic clocks are used as a backup in
the case the main time/frequency
standard fail s, a lthough rubidium
atomic cl ocks ar e an orde r o f
magnitude less accurate . Due to the
stable space enviro nme nt these atomic
clocks usually perform better than their
ground-based counterparts and any
tong-term drifts or offsets can he eas ily
compensated by upload ing the required
correction coef ficients ill the on-board
comp uter,
The output of the atomic time/
frequenc y standard drives a frequency
synthcsiser so that a ll the carri er
frequencie s and modulati on rates are
(f'! -----------=~~~-""-"'''''-'"
derived coherently from the same
referen ce frequency. The on-board
com put er generates the so-called
nav igati on da ta . These in c lude
inform ation about the exact location of
the satellite, also called precis ion
ephemeris, inform ation about the offset
and dr ift o f the on-hoard atomic clock
and information about other satellites in
the system, also called almanac. The
first two arc used directly by the user's
computer to assemble the navigation
The almanac data can be used to
predict visible satellites and avoid
attempting to use dead, malfunctioning
or non-existent satellites, thus speed ing up the acquisition of four valid satel lite
signals with a reas onable Glx)P.
Be s i d e s t he transm itt ers for
broadcasting naviga tion signals, GPS
and GLONASS satellites also have
tclccom mand and te lemetry radio links .
In particula r, the tclecommand link is
used by the com mand stations to
Satellite Channel
LI • Carri er
GPS (all Satelli tes)
GLONASS Channel 0
GLONASS Channel 2
ULONASS Channel 3
(iLONASS Channel 4
GLONASS Channel 5
GI .oNASS Cha nnel 6
GLONASS C hannel 7
GI, ONASS Channel 8
GLONASS Channel 9
G I .oNASS Channel 10
GLONASS Channel 11
GLO NASS Channel 12
ULONASS Channel 13
GLONASS Channcl14
GLO NASS Cha nnel 16
GLO NASS Channel 17
GLONASS Channel 18
GLONASS Channel 19
GLONASS Channel 20
GLONASS Channel 21
GLONASS Channel 22
GLO NASS Chann el 23
GLONASS Channel 24
1602 .5625
1604 .250
1604 .8125
1609 .&75
161 1.fXlO
16 12.125
1613.8 125
1614. 375
1615 .500
1246.43 75
1246 ,&75
1247 ,750
J 249.9375
J 250375
1250.& 125
M H,
M fu
1.2 • Ca rrier
Fi g.7: Carrier F re q uencies for G PS a nd G LO!'lASS Satellites
======="----- - - - - - -- -- - \'~.
£ ~
1. . .
,"" ....,
C/ A·Code
", ' .
'. ~,
5e>_O,"'" G;>
" " "
0 ' _50010 00 '
--<:1:= (]::
, ,
Fig.S: The GPS CIA Code Genera tor
regularly upload fresh navigation data
into th e on-hoard computer. Usually
this is d one once per day. although the
on-hoard computer memory can store
enough data for several weeks in
adv ance .
In add ition to dedicated telemetry links.
part o f the tele metry data is also
inserted in the navigation data stream .
GPS & G LO J\ASS Satellite
(iPS and GLONASS satellites use the
mi crowave Lband (0 broad cast thre e
separate radio-navigation signals on
two separate RF cha nnels usually called
LI (around 1.6 GlIz) and L2 (around
1.2 G Hz)- These frequenc ies were
chosen as a compromi se between the
required satellite transmitter power and
ionospheric errors. The influence of the
ionosphere decreases with the square of
the carrier frequency and is very small
above 1 (1Hz.
However, in a precision na vigation
system it still induces a position error
of about 50m at the L1 freq uenc y
dur ing da ylight and medium solar
acti vity. On the other hand. GPS and
GLOKASS were designed to wor k with
o mn i d ire c t io na l . hemi s ph eri c a lcoverage receiving antennas. The
capture area of an antenna with a
defined radiation pattern decreases with
the square of the operatin g frequency.
so th e power of the o n-boa rd
transmitter has to be increased by the
same amo unt.
Both GPS and GLONASS broadcast
two di ffer ent signa ls: a Coarsel
Acquisition (CIA ) signal and Precisi on
(P) signal. The CIA-signal is only
transmitted on the higher frequency
(LI) while the Pcslgnal is transmitted on
two widely-separated RF channels (L1
and L2 ).
Since the frequency dependence of
ionosph eric errors is known. the
absolute error on each carrier freq uenc y
can be computed from the measured
differe nce bet wee n th e two Ptransmissions on LI and L2 carriers.
The LI CIA - and Pccarricrs are in
quad rature 10 enable a single po.... cr
amp lifier to be used for both signals. as
shown on Fig.6. The LI and 1.2
transmi tter outputs arc combin ed in a
passive network and feed an array o f
helix antenn as. These produce a shaped
beam covering the whole vi sibl e
hemisphere from the
(J PS/G I.o~A S S
orbit with the sam e sign ul strength.
All t hr ee G PS or G LO NASS
tran s m i s s io n s are con tin uo us.
stra ig h tfo rward BI' S K mo d ulate d
carriers. Pulse mod ulalion is not used.
Th e riming information is tra nsm itted in
the modulation : the user' s receiver
mea sures the time o f arrival o f a
defi ned hit pattern. which is a known
code. If desired, the modulat ion code
phase can he rel ated to the carrier
phase in the recei ver to produc e even
more acc urate measurem ents , since both
the carrier frequency and the code rate
are derived cohe rent ly from the same
refere nce frequency on-board the
satel lite.
T he GPS CIA-code is 1023 bits long
and is tran smitted at 1.023 Mbps. The
CIA -code repe tition period is therefore
1 rns. The GLO NASS C/A~eod c is 5 11
bits long and is transmitted at 5 11 kbps.
so it has the same repetit ion period as
the GPS CIA-code. The P-codc is
transmitted at 10 times the spee d of the
CIA-code: 10.23 Mbps for G PS and
5 .11 Mbps for GLON ASS . Th e
trans mitter po wer level for the P-code
on Ll is 3dB belo w the LI C/A-codc
and the P-code on 1.2 is 6dB below the
LI C/A-c odc. The P-code repetiti on
pe riod is very lo ng. making an
autonom ous sea rch for synchronisation
not practical. All Pccode rece ivers first
acqui re lock on the CIA-transmission.
which also carri es information that
allows a qu id; P-code loc k. Both CIAand P-cod es are generated by d igital
shi ft -r eg ist ers with th e fee d back
selecte d to obtain pseudo-ran dom
codes. The navigat ion data is modulo-Z
added to the pseudo -random codes.
Since the navigation-data rate is vcry
low, only 50 bps. il docs not affe ct
significantly the random ncw prope rties
of the codes used.
The navigation data at 50 bps Is
synchronised to the C/A-eode period to
resolve the timing ambiguity caused by
the relatively short I ms CIA-code
repetition period . GPS " Block II"
satellites may encrypt the published
P-codc into the secre t Y-codc. T his
process is called "An ti-Spoofing" (AS) .
Its purpose is 10 prevent an enemy from
jamming the GPS with false ( JPS-like
Details of the .GLO:--JASS Picode arc
not published. In fact. the GLONA SS
P-code is even not mentioned in [6l.
although these transmissions can be
eas ily obse rved on a spectrum analyser.
The GPS and GI.ONA SS RF channe l
carrier frequencies are shown on Fig.7.
\ .~
Cod e l'\umher R C21..ter Taps Clock C ~dcs
GPS PR:,\ 3
UPS PR:--l" 4
GPS PR!\ 8
U I'S PR." 9
GPS PR ~ 13
G PS PR X 14
UPS PRt\ 18
(iPS PRr-; 19
GPS PR t\ 24
GPS PR ~ 25
(iPS PRN 26
G PS PRr-; 28
Fig.9 :
2& 6
3& 7
6 ctks
' &8
7 elL:s
8 d ks
17 dh
5 &9
1 &8
3 & 10
5 &6
6& 7
9& 10
2& 5
.1 &6
4 &7
5& 8
6& 9
1& 3
4& 6
5& 7
7& 9
8 & 10
2 &7
18 elks
139 dks
140 ells
141 ells
25 1 elks
252 elks
2.~ ctks
255 el ls
256 clh
257 elks
469 el ks
4 70 c1l;..
471 elh
472 elks
473 elks
474 d b
509 db
5 12 c1h
5lJ e1l;s
514 elks
515 elks
5 16 c1k'i
859 elks
8fJO elks
86 1 elks
862 db
(irS Ci A Codes a nd the tr
eoer etation wllh th e Regtsters
All GPS satelmes tra nsmit o n the
same LI a nd L2 carrie r
freq uencies: 1575.42 Mfll and
1227.6 Mill. which are he ld in
the exa ct ratio 77/60 and are
intege r mu lli p lcs o f th e
fu n da mental G P S doc k
frequency o f t 0.23 Mil l .
Every GPS satellite transmits its
own set of CIA · and P-ro<ks that
have good c ro ss-corre ta rio n
properties with tbc codes Uk-d by
other GPS satellite" . Since a GPS
a ntenna
r e c e rv m g
omnidirect ional and receives
many satellites al the same li me.
the rece iver is using Code Di vis io n M ultipl e A cce ss
(COMA) techniques 10 separate
..igna ls co ming from d ifferent
GPS sate llite,", arc therefore
ident ified hy th e Pseudo Random -N o ise code number
(PRN#). ",be GLONASS 'ItcllilN
use 2S d ifferent Rf< cha nnel s.
Channel 0 is reserved (ur testing
spare satellites while channels 1
to 24 are dedicated 10 operational
Gf ,ONASS satellites.
All GLON'ASS satellites transm it
the same C/A -rodc and arc
usually identified by the C hannel
Numbe r (C HN# )'Ibe 1.1 and 1.2
carrier frequencies arc in the
ex act ratio 9{7 and the channel
spaci ng is 562.5 kHz at Ll and
437.5 kHz at 1.2. Although there
exi st civ ilian Pccodc receivers.
the majority o f civilian GPS or
GLONASS recei vers are CIA .
o n ly re c eive rs , Si nce th e
511 kHz
~ @ ll e 'l
9 BIt Sc hie be' e<j1 5t e'
"I "I I "I " "I " "I "'
CIA-Co d e
FIJt.l0: The GLONASS Ci A Code Ge nerator
advanta ges of using the Ic codc arc
limited, especially with SA. AS or both
act ive, only the C/A-code transmission
will be discussed in detail here.
registers arc started in the "all-ones'
state and since both sequences have the
same length , the shifl registers maintain
the synchronisation throughout the
operation of the circuit.
Gold codes arc obtained by a moduto-z
sum (another exclusive-or operation) of
the outputs of the two shift registers (11
and G2 . Different codes can be
obtained hy changing the relative phase
of the two shift registers. Instead of
desynchrcn isiug the shift registers it is
easier to dela y the output of one of
them (G2). This variable de lay is
achieved with yet another modulo-Z
sum (exclusive-or) of two G2 register
G PS C/A-T ransmls.<;;ion Fo r ma t
G PS satellite s use co de -d iv is io n
multiplexing on both CIA· and p,
transmissions. Since CIA-codes are
relatively short sequences (only 1023
bits), the codes have to be carefully
selected for good cross-correlation
properties. GPS C/A' l.-OOes are Gold
codes (named after their inventor
Robert Gold) that can be generated as a
modulo-Z sum of two maxi mum-length
shift-register sequences. The GPS C/
A-code generator is shown on Fig.S.
II includes two lO-bit shift registers Gl
and G2, both clocked at 1.023 MHz.
eac h with a separate feedback network
made of exclusive-or gates. Both
feedback net works are selected so thai
both generated sequences have the
maxima l length of 1023 bits. Both shift
Exclusive-or feedback shift-register
sequences have the property that a
modulo-2 addition of a sequence with
its delayed replica produces the same
sequence. but delayed by a different
number of clock cycles. Choosing two
G2 register taps ,
differe nt delays can
be generated yielding 45 differen t Gold
codes with good auto-correlation and
"""-''''''''''= ''''-''''''''-'=--
- -- - -- --
cross-correlation properti es. Out of
th ese 45 possible codes, 32 are
a llocated 10 GPS satellit es as shown on
required imm edia tely and are
suhco mmut ated in 25 consec utiv e
frames. so that the whole almanac is
transmitted in 12.5 minutes.
Th e c ross-correlation propert ies of GPS
CIA -c odes guarantee a cros s-talk
smalle r than ·2 1.6dB between the
desired and undesired satellite signals.
'The 50bps navigation data stream is
sync hro nise d with th e CIA -code
ge nera tor so that hit transitions co incide
with th e if all-ones" state of both shift
registers GI and G2. Al 50hps one data
bit corresponds to 20 C/A -cooe periods.
The allocation of the single data words
is co mpletely described in (5j. Most
numerical paramete rs are 8-, 16-, 24- or
32-bit integers , either unsigned or
signed in the two's complement format.
Angular values that can range from 0 to
360 degrees are usually expressed in
semi-circles to make better use of the
available bits. GPS is also using its own
time scale. The units are seconds lind
weeks. one week bas 604800 seco nds
and the week co1I1I1 is incremented
between Saturday and Sunday. Gi'S
time starts on the midnight of Jan uary
5/6. 1980.
Th e nav igation data is formatted into
word s, subframcs and frames . Words
are 30 hits long includi ng 24 data hits
and 6 parity bits computed over the 24
dat a bits and the last two bits of the
previous word. Parity bits are used to
chec k the received data for errors and
to reso lve the polarity am biguity of the
npSK demodulator. 10 words (300 bits)
form a subframe which always includes
a subframc sync patten! " 1000 1011"
and a time code ca lled "Time-OfWeek " (TOW). One subframe is
transmitted every 6 seco nds. Five
subfrarnes form one frame ( 1500 bits)
that co ntains all of the informa tion
req uired to usc the navi gation signals.
One frame is transmitted every 30
The first suhframe in the frame contains
the on -board clock data: offset, drift
etc. The second and third subframcs
cont ain the precision ephemeris data in
the fonn of kcplerien clements with
severa l correct io n coefficients (0
accurately describe the sate llite's orbit.
Fina lly, thc fourth and fifth subframcs
co nta in almanac data that is not
GPS tim e is a continuous time and
therefore it diffcrs by an integer number
of lea p seconds from UTe. 'the
difference between In C anti (IPS time
is includ ed in the almanac mes~age .
GLONASS C/A-T ra nsmis.<;io n
GLONASS satellites usc the more
con venti ona l
fr equenc y -d iv i s ion
multiplexing at least for the CII\ -code
transmissions. All GI.ONASS satellites
use the same CIA-code, generated by a
9-bit shift register G as shown on
Fig. 10. The GLONI\ SS C/A ~ode is a
maximum -length sequence and thus has
an ideal auto-correlation function.
Prequcncy-civiston multiplexi ng allows
a better channe l separation than code division multiplexing. The separation
between two adjacent GLONASS
channel s should be better (han -48dB. A
large channel separation is useful when
the signal h om one satellite is much
wea ker because o f reflected waves
and/or holes in the recei ving antenna
radiat ion pattern .
O n the other hand , the GLONASS
satellites requ ire a wider RF spectrum
and a GLONASS C/A-rc('civ{'r is
necessarily mere complex than a GPS
'111e G LONASS navigation data stream
is synchronised with the C/A-cooc
generator so that level transitions
coincide with the "a ll-ones" state of the
shift register. 'Inc navigation data
strea m is formatt ed into lines o f the
dura tion o f 2 seconds. Each line
c o nta ins 85 i nforma t io n b it s .
transmi tted at 50hps for 1.7 seco nd s
and a "ti me mark" sync pauer n
" 11111000 1101 11010 10000 10010 110",
which is a pseudo-random sct.j ucnce of
30 hits transmitted at 100hp~ for the
rem ainin g 0.3 second s.
Th e 85 inform ation data bits always
start with a leading "0" , followed by
7 6 h il s c o nt a in in g n av ig a ti o n
information and 8 pa n ty -che cking hits,
co mputed acco rding to the (85. 77)
Hammi ng code. After comp uting the
parity hits. all of the 85 hits arc
d ifferent ially encoded to resolve the
phase ambiguity in [he receiver .
Finally , the 85 differenti ally-encoded
bits arc Manchester encoded, so thai a
" 10" pattern corresponds to a logical
"one" and a " 01" paUcrn corresponds to
a log ical "zero ". Th e additional
transition in the midd le o f the data hits
introduced by the Manchester encoding
speeds-up the synchronisatio n of the
rece iver. 15 navigation data lines fonn
one frame of the durat ion of 30
seconds. The alloca tion of the sing le
data bits in the frame is completely
descri bed in (61. Th e first four lines o f
a frame contain the time code, on-board
clock offset and dri ft and precis ion
epheme ris data of the satellite orbit ill
the form of a state vector (position
vector and velocity vector). To simpli fy
the computations in the user' s rece iver.
the corrections for the Sun- and Moongravity forces are also supplied. The
alm anac data is tra nsmitted in the
remaining I I lines of the frame.
Almana c satellite ephemeris is in the
form of kcplcriau elements and is
transmi tted in two consecutive lines III
a fram e . Th e whole a lmanac is
transmitted in five co nsecutive frames
also called a superframe of the duration
of 2.5 minutes . The various numerica l
parameters arc transmitted as different
size, either unsigned or signed integers.
Signed integers arc transmitted in the
form of a sign bit followed by all
unsigned integer re presenti ng the
absol ute value 01' the num ber (this is
different from the two 's compleme nt
notation!). Angular values are usua lly
expressed in semi-circles.
T h e GL O NA S S ti me is ke p t
synchroni sed 10 U'l'C. GLONASS uses
more conventional time units like days.
hours, min ute s and seconds. The day
count begins with a leap year (cnrrently
1992) and counts up to 1461 days
before returning back to zero.
(fa be co nti nued)
(References overleaf)
Inst itute of Space Dev ice
Eng inee ring. GLAVKOSMOS.
171 Raben C.
M atjaz Vidma r: "Digital Signal
Processi ng Techniques for Radio
A mateurs. Pan-2: Desi gn of a nsp
C om puter for Radio Ama teu r
Applicat ions"; VIJf
Communica tions IJ91.pp 2-24
12} Matjaz Vidmar: "Digital Signal
Processi ng Techniques for Radi o
Amateurs. Part -S: Construct ion
and usc of the DSP Computer",
VIIF..communicaliom 2/89, pp
74- 94
Dixon : "Spread
Spectrum Systems". (422 pages ).
1984, Second Edit ion , John Wiley
& So ns, Ne w Yore, USA.
18J Matjaz Vidm ar: "Di~ita l Signal
Proc-essing Techn iques for Radio
Amateu rs•• Theo retical Part".
v lrlt-Communications 2/8 8, pp
19) P. Mattos: " G lobal Positioning by
Satellite", (16 rages), lnmos,
Technical note 65. Ju ly 1989 .
[31 Jonathan S. Abd. Ja mes W.
C ha ffee: "Ex istence and
U niqueness o f G PS So lutions" ,
pages 952-95616-91. VOL.27.
1101 J. n. Th oma s: "Functional
Description o f Signa l Processing
in the Rogue (iPS Receiver" . (49
pa ges), Ju ne I, 1988. Je t
Pro pulsion Laboratory. Cali forn ia
Institute of Technology, Pasade na.
Californ ia.. USA .
14) "Interface Co ntrol Doc ume nt
MHO~ "()(xx}2 -400. rev- E", (84
pages), August 7th. 1975.
R oc kwell Interna tional
Corporalion. Spa ce Divi..ion.
12214 Lake wood Boulevard.
Do wney , Cali fornia 9024 1, USA.
1111 Char les C. Kilgus: "Sha ped Co nica l Radiatio n Pattern o f (he
Bac kfire Quadri filaT He lix" ,
(pages 392-397). IEEE
Transaction s on Amennas and
Pro pag ation. May 1975 .
[51 "Inter face Control Doc umen t
G PS-200", (102 pa ges ), Novem ber
20th, 198 1. Roc kwe ll
In te rnational. Space Operations
and Satellite Sys te ms Division.
12214 Lake wood Boulevard,
Dow ney, Ca lifornia 90 24 1, USA.
16 J "Global Sate llit e Navigation
System GLONASS Interface
C o ntrol Docume nt" (46 pages).
19 88, Resea rch-and-Productio n
Associatio n of Applied Mechanics,
11 2J Matjaz Vidma r: " A Very
Low-Noise Aeria l Amp lifier for
the I.-Band"; VHf
Com munications 2/92, pp 90-96
11 31 Matjaz Vidmar: "Radio-amateur
ap plications o f ( a 'S/GLONASS
satellites: Using G PS/G I.oNASS
satellite s as an acc urate
freq uency /l ime standa rd", strani
186· I9O/Scripl1lm dc r vortraege.
37. Weinheimer U KW Tagu ng,
19.· 20. Se ptember 1992
Marjaz Vidmar S53MV (ex Yll 3 lIMV, YT 3 M)
A DIY Receiver for GPS and
GLONASS Satellites
GPS/G LO NASS Receiver
Pr inciples of Op eration
Since the signals transmitted by GPS
and GLONASS satellites arc similar,
th e re ceiver design [or any of these
systems follo ws the same guidelines.
The pri nciple block diagram of a (IPS
or GLO;.TASS receiver is shown in
Fig.l l . Only a single chan nel recei ver
is sho wn for simpli city. The problem of
sim ultaneously receiving more than one
signa l (like the CIA-signal and both P
signals from four or more satellites)
will be discussed later.
Since the user's pos ition, velocit y and
attitude are unknown in a navigation
problem, satellite navigation receivers
gene rally IIS C eithe r one or more omnidire ctional antennas. All satel lite navigation sig nals arc circularly polarised
(usually RHe l') to allow the user's
receiver to further atte nuate any reflected wave s, since circu larly polarised
waves change their sense of polarisation on each reflection. Refle cted waves
are a major nuisance in precision
navigation system s: they rep resent an
un predicta ble pr opagat ion anomaly
which is a major source of measuremcnr errors.
The radio signa ls co llected by an omnidirectional receivi ng antenna arc
weak. A low-noise ampl ifier will pre vent any further degradatio n of the
signal-to-noise ratio, but it can 110t
reduce the thermal noise collected by
the antenna nor unwanted naviga tion
C/o, -----------'.!:!!:.-""""'""""''''"''''~'''''
Fig:.l1 : P r inciple Block Diagram of
GPS/<i l ,O NASS Receiver
satellite transmissions on the same
frequency. OI'S and GLO NASS satellite
signals arc widcband, ranging from I
MHz (GI.ONASS CIA-code) to 20
MHz (UPS p-coa c). lind the satellite
transmitter power is lim ited 10 around
25dBW EIRP (Ll e lk -c ode for both
GPS and ULONASS) Of even less than
this (Pctrausnu ssions). making the signal usually weaker than the thermal
noise co llected by the antenna .
A GPS or G LO~ ASS receiver will first
dow nconvert the signals 10 a suita ble IF
an d amp lify the m be fore further
processing. At this stage a wide IF
filter, corresponding: to the com plete
original signal bandwidth, can he used
to improve the dynamic range of the
receiver. T he downc on vcrtcr may be
ma de tunea ble if widely separated
channels arc to he received, like th e
(1J.ONI\ SS C/I\ -transmissiom .
Although buried in thermal noise and
interference. these signals can still be
used. since the given bandwidth and
megabits-per-second rates apply to a
know n code and not 10 the inform ation
ban dwidth. which is smaller than I ltlz
for hoth ti mi ng and Doppler shift
mea surements and the navigation data
transmitted at 50hps. In other words,
GPS and GLONASS signals are directsequence spread-spectrum signa Is, using:
Cod e-Di vis ion Mult iple Acc es s
(CDMA) techniques [7).
The wideband IF signal is then multi -
plied by (mixed with) a Ioceny-gencrercd sate llite signal replica, mod ulated
hy thc same code . If the locally
gene rated code is synchronised to the
sat el lite transmission. the band width of
the desired mixing product will collapse
do wn to almost zero, since two identica l Oli gO-degrees BPSK modu lation
processes exactly cancel each other, on
the ot her hand" the bandwidth of all
unwanted signals, like noise or interfercncc. will be further expanded by this
ope ration to a double band width.
Since the bandwidth of th e desired
signal collapses, this operation is usuall y caIle d signal spectrum desprcading .
The des ired signal can now be filteredout with a narrow IF filter having a
ban dwidth ranging from 100 Hz 10 10
kl Iz in a GPS or GLONASS receiver.
After the narrow IF filter, the signalto-noise ratio finally achiev es usable
valu es and typically reaches 20d H.
The filtered IF signal is then used for
sev era l purposes. First, it is used (0
acq uire and maintain synchroni sation of
the loc ally generated code. Dithering
the locally-generated code hack and
fort h b y a fraction of the bit period
ge nerat es an amp litude modula tion on
the filt ered signal. The phase o f this
modulation cont a ins the information
required to keep the synchronisation of
the loca l code generator.
The filtered IF signal is also fed to a
BPSK demodulator (usually a squaring
I'LL o r a Costas PLL) to extract the
50bps navigation message data. The
BPSK demodulator also provides a
re generated carrier that is used for
Doppler-shift measurements. On the
other hand, the code -tim ing in formation
is obtained from the local code generator. A ll three signals, cod e timing ,
Doppler shift and 50hps nav igation data
arc fed to the receiver CPU to compute
th e user position, velocit y, accurat e
time etc.
Fo r Eart h-located" slowly-mov ing users, the Doppler shift on the satellit e
signals is mainly due to the satellit e
motion and amount s up to +/- 5 kHz on
the LI frequency. In most cases some
fin e tuning will be required to com pensate the Doppler shift in front of the
(to: - - - -- - - -- ---''-''--'=====
navi gation data and synchronise the ir
local Pccode gen erator to the CIA-code
transmission first. Since the Pccode rate
is only 10 times the CIA -code rate,
there arc very few possible Pccodc
phases left 10 be tested 10 lock on the
GPS and GLONASS have been desig ned 10 supply liming codes. the user
position be ing computed from the
mea sured propa gat ion tim e differences.
Add itio nally. the user velo city can be
computed from the alrea dy known posilion and the measured Doppler-shift
differences on the signal carriers.
Although the Doppler shift can also be
measured on the cod e rates, this rucas urcmcnr is usuall y vcry noi sy. On th e
oth er hand , no abs olute delay diffe rence
ca n be me asured tin the ca rrier. since
the carrier phase becomes ambiguous
after 360 degrees.
Finally, relating the carrier pha se to the
code phas e may prod uce excelle nt results, hut requires un accura te compcnsauo n of ionospheric efforts, which
have opposite signs : the ionosphere
dela ys Ihe modulat ion and al the same
time advances the carrier phase!
Besides the described principle of operation of a GPS or CiLO NASS receiver, there arc some other possibilities. Por example, the CIA-code sync
could he obtained muc h fasta using an
analo gue (SA W) or digit al (rFf) corrclutor. To evaluate Ionospheric errors,
codeless reception techniques can he
used to receive both Pctrausmissions on
Ll and L2 frequencies witho ut eve n
knowing the cod es used .
3 .2.
DigilllJ Signal Processi ng (DSP)
In G P S/G L O NASS receivers
After the princ iples of opera tion and
the required funct ions of an electronic
ci rcuit are know n. one has to deci de
about the tec hn olo gy to practically
implem ent the ci rcuit. III most ca ses
UPS or (JJ .O NASS recei vers are mobile
units inst alled on vehicles or eve n
port a ble handhel d units. The recei ver
weight. size and power-con sumption ar c
a ll importa nt. Wh ile every <iPS or
GL ONASS receiver m ust have an antc nna. a RF front -cud and a d i~ ital
co mpute r to solve the navi)!alion cq uattcns. the IP signa l prot'cssillg may
include just a single chann el in a simple
CIA-only receiver or more than 10
channels in a full-spec Ll & 1.2 It-code
recei ver.
when the same circuit function needs
to I:'C duplicated several times. like the
II' proces sing channel s in a radionaviga tion re ceiver, it is usually COIlvcuicnt to use Digita l Signal Proc essing
(I) SP) techniq ues. An impo rtant advanrage of nsp over analogue ci rcuits is
that d uplicated cha nnels are comp letely
identica l a nd require no tuning or
calibrat ion to accura tely meas ure the
difference in the time of arriv al or
Dop pler shift of radio-navi gation signals. A single nsp circuit ca n also he
easily multiplexed am ong severa l signals. since the inter nal variables of a
f)SP circuit like a PL! . veo frequency
or phase can be stored in a compute r
meOlory and recalled and Updated when
needed again.
Th e bandwidth of the navigation sarellite signals is several MHl and this is a
rather large figure for nsp. Implement-
l<~r>:l """" .
"'n1t M e lR~P)
fnl t !j<altd
141,;t Im~
(""" ", TiJo,,,,"
(haNoi 5 ~ j .( j
Fig .12 : Principle Bloc k Dia gram
or a
l -hil DSP G PS/GLONASS Receiver
ing the whole IF signal processing of a
UPS/G LONASS receiver completely in
software (like described in the introduc tion t o nsp technique s in [liD is
difficu lt although it has been done 19]
fo r the GPS CIA-code using powerful
m icro c omputers. Most GPS/GLONASS
receivers usc a com bination of dedicated DSP hardware and software for lf
signal processing. Dedic ated DSP hardware is only used where the bandwidth
is large and the functions arc relatively
simple , like the local satellite signal
replica generation and the signal despreadin g, while all other functions,
includin g all feedback loops, arc implemented in software.
When designing a DSP circuit and in
particul ar when designing dedica ted
DSP hardware it is essential to know ,
bes ides the signal bandwidth or sampling frequency, also the resolution or
numbe r of bits per sample requi red to
represent the signals involved [10]. A
GPS or GLONAS S signal is a constant
amp litude signal and limiting is there fore not harmful. Howe ver, after the
wide If filter in the rece iver there is a
mix of many satellite signals of diffe rent strength and lots of thermal noise as
well. If such a mix of signals is limited,
the re sulting intcrm odularion distortion
degrades the signal-to-noise ratio by
around 2dB.
Since navigation satellite signals arc
pseudo-random sequences, all undesired
signals and all inrcrmodulatlon product s
only af fect the desired signal in the
same way as thermal noise. The refo re,
in a GPSjGLONASS rece iver, very few
hits arc required to represent the wideband IF signal. Most GPS/GLONASS
receiv ers simply limit the widcb and IF
signal, thus accepting the 2dB scnsirlvity degradation and represe nting each
sample with just two qua ntisation levels
or one single bit. Increasing the numb er
of bits per sample only increases the
(~, -
DSP hardware complexity while bringing marg ina l sensitivity improvements,
so that no known receiver des ign uses
m or e than 3 bits per sample (8 -level
On the other hand, an I -bit/sample DSP
GPS/ULONAS S receiver may have a
really simp le IF signa l processing as
shown 011 the prin ciple block diagra m
on F ig.12. The IF signa l is limited, so
no AGe is required. Signal samp ling
an d A(D convers ion is perform ed by a
sing le D type flip-flop. Signal despreading or mult iplication with the locally
generated signal replica is accomplished with an exclusive-or gate. Since
the narrow IF can be selecte d clo se to
zero, the narro w IF bandpass filter may
be re placed by a Jowpass filter or an
integrator. In the case of I -bit samp les,
the latter is simply a counter with the
d ock set to the sampl e rate and gat ed
by the input signal .
However. after the narr ow If filtering
the resulting signal can no longer be
repres ented with a sing le bit per sample , i f the sample rate of the narrowhand signal is significantly reduced. In
a CIA-code receiver, the integrator is
read and then reset each millis econd, to
match the perio d of either GPS or
GLONASS CIA-codes, since the autoand cros s-correl ation properti es o f these
codes are only main tained over an
integer number of code periods. An
integration period of 1 m s corresponds
to a narrow IF bandwidth o f +1· 500 lIz
aro und the cen tre frequency. T he latter
is a very good choice for a GPS or
GLO NAS S receiver.
Any furthe r signal processing after the
integration can be conven iently per156
= '-'==
formed in softwa re, since an interrupt
rate of only I kTIL can be accepted by
any mi croproce ssor. The accumula ted
data in the integrator has a resolution of
12 to 14 bits. so any further software
processing can he done without any
loss of quantisation accuracy nor
proce ssing speed of a general-purpose
16-bit microprocessor.
Dedi ca ted hardwa re is also required for
the generation of the local signal replica. Ca rriers or rates are conveni ently
generated in Nume rically Controlled
oscillators (:--leOs). An 1\CO inc ludes a
digital adder and an accumulator. In
eve ry d ock cycle , a constant represen ting the desired output or rate is add ed
to the accumulato r. If an ana logue
ou tput were desired, the accumulator
conten t could be fed to a RO M contai ning a sine ta ble and then to a D/1\
converter, formi ng a direct digita l frequency synthcsiscr.
In a I-hit
navigation-receive r the
si ne table and Df" converter arc not
required . Si nce the nsr hardware operates with l -bit dat a. it is sufficien t to
take the MS!l o f the NCO accumulator
as the frequency out put Two NCOs arc
required: on for the carrier frequency
and another for the code rate. The
code-rate NCO supp lies the clock to a
code generator like the ones shown all
Hg.S or 10. The output of the code
ge nerato r i ~ excl usive-or gated with the
output of the carrier NCO to produce a
BPSK-modulated satellite signal replica.
Of co urse both NCOs have to he
accura tely steere d to the required frequency and phase to maintain lock on
the incoming signal. T he feedback
~~=~~~---------- (I:,
funct io n can be performed hy the
m icroprocessor, si nce the feedback
speed is vel)' low: a 100 Hz update rate
is usuall y fast enough. Fina lly. the NCO
frequency can be ea sily steered modifying the addition constant and the NCO
phase can be easily steered modifying
the acc umulator content. In a timemultiplex ed W cha nnel. both can be
eas ily stored by the microproce ssor and
recall e d when th e cha nnel ha rd ware is
switched hack to the same satellite
Prom the tec hnology point of view, a
DSP I F channel can he built on an
"Euroc ard" size printed c ircuit hoard
using just bare 74TICxxx logic. I\. single
IP channel may also be programmed in
a programmahie-logic int egrated circuit.
f ina lly , the comple te If signa l process ing with 6 or S indep e ndent ch an nels
may be integrated ill a single custom
inte grat ed circuit. Commercia l satell ite
navigation receivers usc custom integrated circuits essentially to prevent
una ut ho rised duplication. On the other
hand, hare 74HCxxx logic is preferred
for an amateur, home-made rece iver.
IIopefu lly program ma ble-logic Ies will
some da y become standa rdis ed and the
necessa ry programming tool s cheap
cno ugh to allow amate ur applications.
Multi-channel reception of
naviga tion signals
A sate llite navigation receiver should
be able to rece ive the signals from four
or more satellites at the same tim e, to
be able to measure time and Doppler
differences. When the GPS specifications were published back in 1975 [4] ,
the digi tal computer was t he largest and
most comp lex part of a satellite navigation receiver. Both GPS and GLONASS
receivers were initially inte nded to have
severa l analogue IP processing channels, one per each signal type per
satellite . Since these receivers were
intended for military vehicles like
fighter aircraft, tank s or battle ships, the
price and complexity of several ana logue IF processing channels wa s almost unimportant.
Early civili an ( ; PS receivers also used
analogue If proce ssing, although initially limit ed to the C/I\.-code and one
or tw o time -multiplexed IF channel s.
Time-multiplexing is d ifficult w ith analogue IF channels, since the latter have
10 reacquire lock each time the sate llites arc changed. Lock Acquisition
may take 15 to 20 seconds, so tha t the
me asureme nt loop through four or more
satellites takes severa l minu tes . These
receivers were only suitab le for sta tionary or slowly-moving users.
The introduction of Dxl' technique s and
inexpensive computers allo wed much
faster multiplexing. Since the variables
of a DSP circuit call be stored and
recalled, a nsp IF channel docs not
need 10 reacqu ire lock each time it is
switched to anothe r satellite signal. A
DSP IF channe l is typically switched
among satellite sigua ls around a hundred times per second makin g the
whole loop among all required signals a
few ten times per second. I Icwcvcr,
because of the avail able signal-t o-noise
rati o, the navigation solut ion in a
CIA- code receiver only needs to be
computed about once per seco nd.
AH current commercial GPS and GLONASS receivers lise DSP IF process ing .
C/'· - - - - - - - -- - ----'-"'-==
= "'-"= =
Sma ll handheld C/A-code receive rs
have one, two or three time-multiplexed
IF c hannels. Mobile C/A-codc receivers
hav e 5. 6 or eve n 8 independent
cha nnels so tha t no multiple xing is
required . TIme multiple xing males the
carrie r lock and Doppler measurements
di fficult and unrel iable. so it is undesired in mobile receivers.
Un fo rtu natel y. multi-cha nnel 01.0NASS rece ive..-rs require a wider raw
sig na l IF and a milch higher sa mpling
rate due 10 the wide FD:\1A channel
spaci ng. On the other hand. UPS rccciv C!S require the saute raw IF ba ndw idth
reg ardle ss of the number o f cha nnels
thanks to C IJMA. The higher sampling
rates required for n I.O~ A S S ace a little
im pra ct ical with curre ntly avail able integ rated circuit s. Maybe this is another
reason why (J I'S rece ivers are more
po pular and ( jI.O NASS is almost unkno wn. Since faster Ie s will ce rtainty
be available in t he future. one can
,. 1,
t llA,
-.-,, 1
ffl ~
expect that com bined GPS/GLONASS
receivers will become stand ard.
In this article I am going to descr ibe a
singte-channel C/A-only recei ver usi ng
fast time mu ltiplexing . Th is receiver
ca n be built in two versio ns: GPS or
GLO:-JASS. Although hoth versions use
the same modules as much as possible,
this is not a co mbined GPSJ{jLONAS S
receive r yet. The main limitation o f a
single IF channel. tirnc -mulriplc xcd reccivcr is that the m aximum number of
sim ultan eo usly tracked satellites is limited to four or five. so that a com bined
GPS/GLO:"JASS receiver docs not m ake
much sense.
Praclica l (j PS r ecei ver design
Th e bloc k d iagram of the described
(i PS receiver is shown on Fig..!3. In the
micro wave freq uency range, at Lcband ,
the an tenna needs a direc t v isibility o f
the satellites. Therefore it has to be
102" " 1
B!nr ;I.
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~H I
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V"" ,. IIn<Mr ·
~ . tt .
S·SFX 89
6139 kf!l'
f0- _.. _.f)-=~
l .lnllll
!lFX! 9
Fi~. 13 :
( i PS Receiver Block Diagr am
==== = =='---- - - - -- - -- - ';;.-
installed outdoor, 011 the vehi cle roo f or
on top o f a portable receive r. Due to its
ex cellent performance, a half-tum quad rifila r helix is used as a circu larly
polarised, hemi sphe rica l-coverage antenna,
the temperature range from 0 10 30
degrees C. as encountered during normal receiver ope ration, the TCXO was
replaced by a much less expensive
The L NA is install ed directly under the
antenna. Using two ine xpensive GaAs ~ETs it achiev es 30dB of gain ma king
any following (reasonable) cab le loss
a lmost unimpo rtant.
Sampling the 10 Mill. wide IP signa l
with 6139 kH7. produces a th ird down conversion \ 0 a 230 3 kl lz nomina l
centre frequency. TIle Inter is the final
carrier frequency that needs to he
rege nerated in the dedica ted DSr hard wa re . The dedicat ed nsp hard ware i.s
dcsigucd as a mic roproc essor peri pheral
with read and writ e registers and is
interrupti ng the MC6RO[O C PO once
every millisecond to ma tch the UJ'S
CIA-code period .
'111C Gl'S receiv er incl udes a fixed tu ned downcon vertcr to a suitable IF,
a n IF amp lifier and limiter, a ded icated
nSf' ha rdware. a MC68010 based microcomputcr with a sma ll keyboa rd and
a LCD display and a single master
crys tal oscillator for all frequency con ve rsions and s':lln p l i ll ~ rate s. T he dow n co nve rsion from the C PS 1,1 frequenc y
( 1575 .42 MHz) is made in two steps for
co nve n ient image filteri ng. '111C first
wide IF is in the 102 MHz range and
the second wide IF is ill the 10 MHz
ran~e. The wide IF bandwidth is set to
around 2 M fJ/.. The actual va lue of the
wi de IF bandwidth is not critical, since
filtering is only required to prevent
spec tru m aliasing ill the signal samp ling
A freq uency of 6139 k il l. was selected
as ma ster cry stal oscillato r frequency o f
the de scribed riPS rece iver. since the
best TeXOs arc usually avai lab le for
the frequency range betwee n 5 MHz
and 10 MIT/..
T he output of Ihe 6139 kl lz master
oscillator is used both as the samp ling
freq ue ncy for the I}-, AID conve rsion
an d as an input to a cha in of multiplier
stages to supply a ll of the frequencies
required in the do wnconvcrrcr. Limiting
con ven tional crysta l osc illator in all of
the prototype s built.
In th e portable, stand-alone (fI'S rece iver. the operating soft ware is stored
in a com pressed form in a 32l:hYlc
EPROM . After powe r-oil reset. the
software is decompres sed ill 12RkhYlcs
o r battery -bac ked CM OS RAM , whic h
is also use d to store the sys tem almanac
and other data to speed-up the acquisiti on o f four va lid sate llites. For the
same reason the CPU also has tlCCCSS to
a sma ll battery-backed real-time clock
A smal l 8-key key hoard is used to
select the va rious menus o r the opcratlng so ftware and manuall y set some
receiver param eters if so desired. T he
portable version of the (iPS receiver is
using a I .eD mod ule with integrated
driving elect ronics and two rows or 40
alp hanum eric (ASCII ) ch aracters eac h.
to display thc receiv er status . the al manac data or the rcsuhs of the navigat ion
(~, 3.5 .
- - - - - - ------'""--''''''"===
Practical GLONASS rece iver
The block diagram of the described
G LONASS receiver is shown in Fig.l4.
The G LONASS receiver uses the same
type o f antenna and LNA and the same
dedicated DSP hardware and microcompute r as its G PS co unterpart. TIle
main di fferen ce bet ween the two receivers is in the dow ncouverter. The
GLONASS receiver includes a tuneable
downconvcrter, otherwise the wide
H ) MI\ cha n nel spacing would require
Impractically high sampling rates in the
dedicated DSP hardware.
The downconversicn from the nLO-
NASS Ll frequency range (1({)1 to
1615.5 MHz) is made in two steps for
convenient image filterin g. To reduce
group-de lay va riations, the first conversion is made tunea ble and the second is
l~ .. t-.'
,,. ,,,
1601,"'15 "K>
"'" ~ I
F i~. 14:
1 ,1F -~ .
11'1lt II",""
Both wide . Ps are fixed tuned at II R.7
Mllz and 10.7 Ml T/, respectively. To
avo id any gm up-dclay v aria tio n ~ in the
wide , FIi. the frequency symh csiscr
steps must accurately match the channel
spacing so that all signals are co nverted
to the same , F values. Finally. the , F
limite r sho uld no t introd uce a variable
.,. .,
111.1 "'"
fixed. . n this case the only contribution
to group-delay variation s across the
GLONASS Ll frequency range arc the
tuned circuits at 1.6 G Uz. Group-del ay
variations introduce errors in the mcasured time difference... so they immediately affect the accuracy of a navigation receiver. This problem does nor
exist in a GPS receiver. since all GPS
satelljtos transmit on the same carrier
frequency and any signal flltcn ng produces the same group delay on all
satellite signals that exactly cancels-out
when com puting the differen ces.
~, -
"U.91 ~
G LONASS Receiver Block Diagr a m
cse~_ .
del ay as the input signal strength is
changed when switching amo ng chanriels. The second wide IF signal at 10.7
MI ll. is sampled with 4 500 kHz produc ing a third downcon verslon to a nominal centre frequency o f 1687.5 kllz,
T here are several d ifficult-to-meet requireme nts for the freq uency synthesiscr supplying the signet for tile first
convers ion. This synt hcslscr has to
provide a clean signal in the frequency
range from 1483 to 1497 MHz in steps
o f 562.5 k Hz. Its phase noise should he
low enough to allow carrier lock and
50b ps nav igatio n data demodu latio n: its
spectral Iincw idth should he abo ut to
times JI3lTOwer than required ill a voice
SSB receiver . Fina lly, ill a time -multiplexed, sin gle -chann el rece iver the syntheslscr should he able 10 switch and
settle to another frequency ill less than
I ms the CiT.ONi\SS CIA-code per iod,
to avoid increasing the switching: dead
li me.
Th e freq uency synrhesiser is a PI J .
with a frequency convener in the
feedbac k loop, 10 decrement the d ivider
modulo, increase the loop gain, speedup the settling and improve tbe output
phase noise performanc e. Th e feedback
.signal is downconvertcd to the frequency range 25 10 38 M IIz, so that a
ve ry lo w programma ble loop-divider
modu lo betwee n 45 and 69 is required.
The co mparison freq uency is sci to
562.5 kHz.
A well -designed PLI. will settle in 100
to 200 cloc k peri ods of the comparison
frequency and the described PLL
achieves this performance with a set tling time between 200 and 300 mic roseconds .
Th e described GLONASS rece ive r is
using a master crystal oscilla tor at
18.000 MH/.. This frequency is multiplied by 6 to obtain the 108 MHz signa l
required for the second conversion and
by 81 10 obtain the 1458 MHz signa l
required for the PLI . feedback-loop
convers ion, The master osci llator frequency is divided by 4 to o btain the
4500 KHz sampling frequency and by
32 to ohtai n the 562.5 kHz PLI.
reference frequency. Lik e in the (i PS
receiver, in place o f an ex pensive
TCXO conventional crystal oscillators
were used in all of the prototypes built,
lim ithig somewhat the operating temperature range.
In the describe d GI.O NASS recei ver,
the microcomputer has o ne function
more . Besides controlling the dedicated
Dsp hardware, keyhoard and LCD
display, all identical to the UPS courtterparts. the mic rocomputer has to set
the frequency synrhcsiscr when switching among chan nels. The ope rating
software is very sim ilar to that ill tbc
(iPS receiver and has the same hard wa re re quire ment s: 3 2 khy les o f
EPRO M, 128kbytes of battery- backed
CMOS RAM and a batte ry-backed
real-tim e clock.
(' I' S/(, I.ONASS dedi cat ed
ha rdware design
Althoug h the theo ry o f ope ratio n o f an
I -bit USP (I PS or GLONASS receiver
ha s already been discusse d, the practical imp lementation still offers ma ny
different choices and some additional
problems to be solved. For example,
from the theoretical po int-of-view it is
uni mportant whether the code lock or
(~ -
- - --
- - ---'-'-"-'== = "-'==
the ca rrier lock is achieved first. In
practice . the code lock should be
ach ieved first and shou ld be completely
In practice, two separate signal-dcspread ing mixers arc required when
downconvcrting 10 a narro w IF of
al most zero. The mixers arc dri ven with
the same satellite signal replica, moduIatcd with the same code, but with the
carriers in quadrature . In th is way no
information is lost aft er signal despreading, downconversion and integration. The code lock can he made
com pletely independent from the carrier
lock, since the narrow If signal amplitude-can he com puted out of the J and
Q integration sums withou t knowing the
carrier phase. The same 1 and Q
integration sums arc used in a different
way to achieve carri er lock and extrac t
th e Subps navi gation data . Due to the
low sample rate (1 1.:1Lt,) the biter are
conveniently performed in software.
independ ent from the ca rrier lock, both
to spee d -up the initia l signal acqulslrlon
and to avoid loosi ng lock at short signal
dropouts (obstructions, fading) or rccciver frequency refer ence instabilit ies.
Th e block diagram of the practicall y
implemented G PS/GLONASS dedicated
DSP hardware is shown on fi ~ . 1 5 .
Allhotlg.h the imple mented hardw are is
intended for a single channel, time
mult iple xed operation. it dirk rs siguificann y from the theoretical block dia gra m shown on Fig.12. The mai n
d iffe rence is that there arc four signa ldcsprcadin g mixe rs (multipliers. ex-or
ga tes) and four integ rators (count er s)
for one single channel.
101+\1 G'S
f-S; rlII e-,
1M lnt'9"O!f(ll"er1
Dcll er1lTrrig.tl KoOo
1e.t AbtQI!-Oc!.I . "
f\ -- -IH< I-Early
::::{)---t ~. o.·[tlrt y
I ,-,
=D---t'- Q"L~'.
I-EIl' ly
6139 kH',..,ii>S
Tg kl
" ''J.»icH~!UWo.ss
i>1l9 1~Sll Dki1t
H If"Iy
f l<
• l.SOollIiUlI
l - l gl,
Zli/I t...
Zd~l ...
~" ~ '''. T." r"J-L
Hog,"".. .
[ Mhln
rriigtrl Kodt
F i~ ,t5 :
St ! ~tr104j
llU IO
~ IIItl1t
G PS/G LO NASS dedicaled OSP Hardwa re Block Diagr am
('U 8l,ls
Although code lock may be maintained
by dithering the locally generated signal replica, two separate narrow IFs for
an "early" and a " late" local signal
replica provide a 3dB imp rovement in
the signa l-to -noise ratio on time-delay
measurements. The former solution,
code dithering, is usually used in
receivers with an analo gue narrow IF,
since it is difficult to build two identica l circuits in analog ue technology. The
latter solution is used in receivers with
a nsp narrow IF, since nsp circuits
perform the sum c nume rical ope rations
and arc therefore mathema tica lly identica l.
To achieve the 3dB signal-to-noise
improvement, two separate sets of I and
Q signal-processing cha ins for the
nearly" and "l ate" signal replicas need
to he used . This brings the total number
of signal-desprcading mixers and inte grators to four. Of course the local
signal replica genera tion includes the
generation of four d ifferent signals:
Q-LAlTI. All these signals can he
obtained from a single carrier and code
generator, since they arc merely de layed versions of the same signal: either
the carrier or the code or both are
delayed . In nsp, dcluys can he easily
obtained with shift registers.
On the other hand , the loc al satell ite
signal replica gen eration can be simplified with a look-up table. Since the
integra tion period is I ms and the input
sample rate is 6139 kHz (GPS) or 4500
kHz (GLONASS), there are on ly 6139
or 4500 different hits to he stored in the
look-up table for each dcspreading
mixer and integrator. The look-up table
sampli ng rate or any odd multiple of
this value: 1536 kHz for GPS or
1125 kHz for GLONASS.
In pract ice 613 9 kHz was selected as
the sampling rate for the GPS rece iver
to av oid interferenc e with the GPS
CIA -code rate (1023 kIlz), since the
described look-up table generator maintains a fixed phase relationship betwee n
the code transitions and sampling rate.
Considering the various conversion frequencies obtained from the same
source, a n IF of 23m kl Iz resulted after
signal sampling.
In the GLON I\SS recei ver. any intcrferencc betwee n the sampli ng rate and
cod e rate are unimportan t since all
satellit es use the same C/A -<:oue . The
sampling rate of 4500 kl lz was chosen
for convenience. Considering the operation of the frequency syn thcsiscr. the
final wide IF value could he chosen in
562.5 kHz steps. The value o f 1687.5
k Hz was selected 10 avo id some spurious frequencies generated in the synthcsiscr.
Finally. the described dedicated DSP
hardw are alw ays requires the support of
a microcomputer. TIle latter should
compute and load the look-up tables
first. After each interrupt request (every
mi llisecond) the microco mputer reads
all four integrate d sum s. From the I and
Q components it computes the early
and late magnitude s used to search and
maintain code lock . The code phase
required to maintain lock is at the same
time the result of a time-delay me asurement, referenced to the rece iver clock.
T he difference o f two such measurements is a parameter of a navigation
eq uation.
O n the other hand. the T average li nd Q
average are supplied to a Costas-loop
demodu lator to recover the carrier and
de modulate the .) O hp ~ nav igation data
hils. Then the subframe or line sync is
detected 10 fonn nt the data stream and
c heck the parity bits before the navigat ion data is used in the computations.
il /1 literature refcrences in this article
are 10 he found on IXl?,f'- 77 of issue
(To he cont inued)
Very low noise aort al amplifier for th e
I. -band as per the YT3MV articl e on pag
90 o f VHF Comm unications 2/9 2.
Ki t complete with housing An No. 6358
DM 69. Orders 10 KM Publications at the
address shown on the inside cover, or to
UK w -Bcricme d irect.
Maljaz Vidmar . S53MV
A DIY Receiver for GPS and
GLONASS Satellites
Quadrifilar Backfire Helix
Al1h ou ~h
lon l.: r a nge,
prt'CI ~ 101l
n a vl-
ua t lon systems like (iPS or G LONASS were d l's i~ nl'd to he lnucpcnd c n t us much HS possib le or the perIor mn nce or eith er tran smlulnn o r
reeelvt ng lInll'nnlls. the antennas IISfiI
sHU have so me influen ce on t he
system per for ma nce.
T he tra nsmitting anten nas installed on
the spacecra ft have a shaped beam to
supply any Eart h-located users with the
same signal strength and usc the onboard
transmi tter power
more effi-
Maintniniug till' same ~ iJl: ll a l strength is
es peci ally impo rtant ill CnMt\, since
the GPS C/A--codes are too .\ II(m to
offer a very g-ood crosstalk perform alice. The ideal recei ving- antenna
should have a he mispherica l radia tion
pau c m. offe ring the same sig nal
strength from a satel lite at zenith and
(m ill an other sate llite just above horizon. Furthe-r, the receiv ing antenna
should match the transmitter polarisalion (RHCP) ill a ll va lid directi ons .
Hnally, the receiving antenna shou ld
attenua te an)' signals coming from undesire d d irecti ons, like dguals coming
from negative elevarious, since these
arc certainly reflect ed waves and the
latter arc a major source
measurement errors due 10 their unkn own
propa gation path .
ciently .
Although a turns tile anten na
(two crosse d dipole s fed in
qua drature) with or without a
reflector is frequently used
for satellite reception, this
ante nna is-not very suitable
for satell ite navigat ion for
several reasons . The polariseli on of a turnstile ante nna is
circular only in the zenith
direction and is complete ly
linear in the horizon plane.
Therefore, a turnstile antenna
offers no discrim ination be'w een the desired RHCP dircct wave and tile unwanted
t He]> reflected wave, since
cir cularly polarised wave s
c ha nged thei r sen se of po lari -
sati on on cuch reflection. Re-
flected waves cause severe
mea surement e r rors and a
relativel y slow and deep signal fading, so that the reccrvcr even looses lock on
the signal.
A better alternative is a
microstrip patc h ante nna . A
single mic rostrip patch rcso-
Solder julots
O.1f.1"_ SO.n.
S5 3MV
Fig.I: The Quadrifilar Backfire Antenna with
Right-Hand Circular Polar isation
="""''''''''''"''''-'''''=''''----------- (~.
nator pro vides a useful rad iation pattern
with a reasonably circ ular pola risa tion
over a wide nl11ge of eleva tions. Un fortunatel y the radia tion pattern o f :I
microstrip antenna fall.. down to zero in
the ho rizon plane. Micros trip antennas
ar c usu ally used when a simple, lowprofi le antenna is requ ire d, usually 10
he in sta lled 011 u vehicle roof. Since
low-elev ation satcllitc,s can not be rece ived , a mlcrostrip antenna us ually
lim its the avail able ntX}P.
Th e be st antenna for satellite na vigati on
and o ther applic at ions requir ing hcmis pherica l eovc l';l ge see ms to he the
quudrifilar back fire hel ix (als o ca lled a
"vol ute" anten na) . Sud , an antenna
prov ides a shaped conical beam. '!11l'
beam shaping and cone aperture can he
co ntrolle d by ad justing the hel ix radius.
turns p itch dist ance an d number of
turns as desc ribe d in II 1], By the W:lY,
the same type o f an tenna is frequen tly
used o n low-Earth orbit satellites, like
the ~ Oi\A weat her satel lite s.
As till' GPS and G I.O:.rASS satellill's
a lready prov ide a const ant signa l
strengt h for Earth -located users re gardless o f the satellite elevatio n, no par·
ticular beam shapi ng is required for the
re c e iv ing antenn a . T he opt imum
number of turns of a qua drifila r ha ckfire hel ix used as a (iPS or GLOr\AS S
receiving antenna seems to he between
1.5 and 3. M ak ing a qna drifiJar backfire
hel ix lon ger hy incre asing th e nu mber
of turns docs no t have much effect on
the gain or the bea m-cone apertu re, hut
it improves the bea m shaping a nd
further attenuates the undesired lobe in
the opposite di rection (downwards).
Altho ug h the best GPS receiver s use
such a quadri filar heli x with 1.5 or 2
turn s. suc h :111 antenna is di fficult to
manufacture and teet. In pa rticula r, the
four he lica l wire s have 10 he fed in
quad rature and ther e is very lill ie Space
on top of suc h an antenna to inst all 111(:.
feeding net work. Further, a z- turn hackfire helix is rat her large (Zucm hi gh) for
a po rta ble rec eiver. If its im proved
pattern per formance is to he fully
ex ploited. the direction o f its axi..
should 1101 d eviat e too muc h fro m
vertical and this is 110 t a ve ry practi cal
requirement for a porta ble receiver .
Most UPS/( H.O NASS receive rs the refore lise a sim pler ante nna . us uall y a
short one -hal f 111m backfi re hel ix like
shown 0 11 Fig . 16. Making (he q uadrifilar heli x sho rter resonance e ffects can
be used to feed the four helical wires
with the prope r sig na l phase.... Tn pal'·
neuter. one pa ir of wires is ma de
short er to mck c its impe dance ca peci ti vc at the operating frequency and the
ot her pai r of wires is made longer 10
make irs impedance inductive at the
ope rating frequenc y.
To ob tain Rlle p a con ve ntional endfire heli x ha ~ 10 be wound like a
rig ht-hand screw, The back fire hel ix is
jus t opposite: 10 obtain R1ICP th e
backfire hel ix has to be wound as a
left -ha nd scre w, bes ides th e proper
pha sing of the four hel ical wires, of
(~ - - - - - - - --
- - ---'-"'--====
course! Further, the back fire hel ix re-
hand, the performa nce of the antenna is
not de graded much if no balun is used
as shown on Fig. 1.
quires no reflector. The four helical
wires arc fed at one end of the helix
and shorted to geth er at the other end o f
the hel ix. Since the main (desired)
rad iatio n lobe is directed towa rds the
fe edp o int and away from the shorted
end, such an antenna is ca lled a
bac kfire antenna .
T he fccdpoint impedance is in the 500
ran ge, symmetrica l. A good matc h to
500. is usually sacrificed for the radia tion pattern wh ich is much more im por tant. U sually one o f the [om helical
wires is replaced by a semi-rigid coaxial cab le of the same miter diameter
to fo nn an " infin ite ba lun". 011 the other
Sup p ression 01' in terfer ence in 70 ·cm
ATV mod e us tng high ly selective
n otch IiIter, by E.B erbe rich j 1/94
Some errors crep t into fig . t 2 on p. 52.
so here' s the circ uit again.
I x~;r :: :®"Ik1
Sm ool~ lll ~
' 'l
Im provements and additions to the
Sp ect r um Analyse r by Dr.L jlrmann,
So me points were not cl ear rega rding
thc structure of the spec trum ana lyser
and need correcting :
In the prac tical construct ion of a ha lftum quadrifilar helix it is espec ially
important to respect the ex act lengths
of the helical wires, since the antenna
uses resonance effects and is rat her
narrowband. The dimensions shown on
f ig. 16 arc for the GPS L1 frequency
(1575.42 MHz). A GLONASS LI antenna sho uld be approximately 37"
sma ller. Fina lly. an antenna for both
(IPS and GLO NASS Ll channels can
he built by des igning it for the ave rage
of the two frequency bands.
I. Printed circuit board 00 7 (LOfPU .):
Circuit dia gram and components diugram gave different values for resistance of 17, pin- 2: the version with a
56 k resist ance to earth is correct .
The capacitor at pin-a o f 11 (l\ E 551 4)
has a purely block ing function. It co uld
be given a value o f. for example. O. l ufo.
2. Print ed circuit hoa rd 009 (run-off
control) : The tendency of the em itter
follower to oscillate did not become
apparent until the layout had been
completed . It can be remed ied by
means o f a l nl- (not lnl-) ce ramic
capacitor on th e foil side.
C ircuit diag ram and components dia gram gave dif ferent values for resistance of 12, pin 2 to eart h. The correct
value he re is 150k; at 39 k, the tuning
d iode in the seco nd La would have a
bias voltage in the condu cting direction.
"-"-==='-'='-='''-=------------ (~,
Matja z Vidmar , S53MV
A DIY Receiver for GPS and
GLONASS Satellites
In this part of lh e se r ies complete
construction details will he shown for
the (i PS RF Module Hod lh e IF
Co nver ter. lh e G LO;'llASS RF Mod u le Hod IF CURve-TIer Mod the <;1..,0·
PLL Sym hcslscr Conver ter .
Th e RF and IF Stages of the
Con ver ters.
Th e Anlenna
111e Quadrifilar Backfire Antenna used
by the Author for this project was
described in part-Sb of this project in
VHF Communications 4/ 1994, pp.197 .
Low-Noise Am plifier
T his unit, which is common to both the
GPS and the GLONASS receive convertcrs was described fully in VHF
Communications 211992. pr . 90 - 96
and is availa ble as a separate kit from
KM Publ ications (see the advertisement
on page 17 of this issue).
GPS RF Module
The (IPS receiver only requires 3
single-frequency (1575.42 MHz) downconverter and its design is relatively
straightforward. The (i PS do wnconverier includes two modules: a RF
module built in microstrip technology
and an IF strip built on a simple,
single-sided printed circuit board.
The circuit diagram of the UPS RF
module is shown on Fig.l9. The GPS
RF mod ule includes three RF amplifier
(~ -----------"="'-""''''''''''''-'''''~'''-''-'''
,: L2
. 12','
2. HP2%Ol8A481 l
55 JMV
Fig. 19: (i PS RF Module
stages and the first downconvcrsinn
mi xer. The amplifier stages arc identi cal and usc silicon MRF 571 transistors .
M uch of the gain provid ed hy these
tra ns istors is lost in the microstrip
filter s, since the latter arc etched on a
lossy but inex pensive glass fibre-epoxy
lam inate.
Th e first dcwnconvcrsion to 102 M Hz
is performed by a harmo nic mix er using
tw o anti- par allel Sc hott ky d iod es
HP2()OO. 13 1\4X\ or simil ar. Such a
m ixer has a hi gher noise figure than
convcnriona l diode mix ers, es pecially
when using th e suggested low Froqucnc y diodes . On the other hand, th e
re quired local oscill ator signal is at
736 MHz. only h;Jl f of the frequency
required for the downconvcr sion
(14 73 MHz).
The Rf modulo circ uit includes a
network to supp ly with +12V the <';al\s
FET preamplifier through the RF cab le.
On th e othe r hand. the + l 2V supply
voltage for the RIo' module itself is
taken out of the If.' con vert er, after
II ill .Ii .'-"-'""
GPS RF Module,
upper side
(top view)
= "- - -
15 7 ~'1l
fiF I N
102 HH'1 I
1"1716 HHl
I F aUT! ! ! !L O IN
F i~.21:
G PS RF :\tlndule Component Overlay
4 . 20~
VIr. ...I·
f4<h. r
'il. G35"1 Hl
Hi" hor
BfX 89
I, '''' "
i "
4 ·20~
8 tX8 9
1H .ltHHI
" ,.., "" .,.
2. ll p
55 3MV
Fig.22: GPS IF Con verter 1\11111iplier and Mixer
\'/'. - - - - - -- - - - - - ----""'--'=
=== = ="
,. ,
. 11~
"" '"
'" '"
m HH,
\'tt,Ifl •
t~( 1wr
~ lKht r
Fig .23: <IPS IF Converter Loca l Osdllato r
bein g filtered by a choke and a IOOuF
capacitor. T he RF module is built in
microstrip techno logy on a dou hie-sided
ho ard made of O.79mm thick glass
fibre -epoxy. Th e lIpper side is shown on
rig.20 while the lower side is not
etched . '111c loca tion of the compo nents
is shown on Fig.21. Befo re insta lling
the components. 1.3, I,5, L7. 1.9 and
LJ 2 should he grounded by soldering
small Unsha ped pieces o f wire ill the
m arked local ions.
r.r. 104.
Ili, L8 and Lt l are quarterwavelength chokes. These arc made
from about OCIll of O.15mm thick
cop per enamelled wire. tinned for about
5mm at eac h end. The rema ining wire
is wound on a lmm inner diameter and
the finished chokes are small sel fsupporting coils. on the other hand. L2
is a commercial IOOuH " moulded "
It is recomme nded to use thin Te flon
coa x like RG· 1811 for the internal RF
wiring of the ( iPS recei ver. Th e bra id
of the cable shou ld he soldered dir ectly
to the rnicrostri p grouudpta nc while the
ce ntral cond uctor reaches the upper
tracks through a hole in the printed ci rcuit board. To avoid shorts. the
copper platin g around this hole on the
groundpla nc side should be caref ully
rem oved using a much larger (.1mm)
drill tip,
The (i PS Rl- mod ule need s some adju stment s of the striplines and these arc
best performed a ncr aII of the receiv er
hardware is assembled. 1.3, L5, L7 and
1.9 usua lly need to he trimmed shorter
by about I mm at the open end 10
achieve the maximum gain at 1575
MIl,.. On the other hand, LI D and Ll 2
may need some smaII pieces o f copper
fo il (abo ut 7mmx 7m m) at d ifferent
locations along these striplinee 10
achieve the best noise figure from the
diodes actually used in the mix er.
= -'!EFig.24:
GPS IF Converter,
bottom view
, Hz
" 11102M
136MHz l:
I '
: : I F IN
Fig .25: GPS IF Converter Component Overlay
(~ -------------'-"-"-'''''''''=~~~
GPS TF Converter
The GPS If strip includes a second
dow ncon vcrsion to 10 MHz. signal
amplification and limiti ng at 10 MHz
and the generat ion of all required local
oscill ator and clock signals from a
single m aster frequency reference.
The second downconversiou to 10 MHz
and the LO frequency generation is
incl uded in the GPS IF converter
module shown on fig .22 and Fig.23.
The GPS IF converter module includes
a 61 39 kHz crystal oscillator (Fig: .21).
This frequency is used both for signal
sampling and suitably multiplied for
both downconvcrsions . Since the rcquircd short term stab ility is vcry high,
in the J.E-9 HlllgC . to be ah le to
demodul ate the 50 HI'S I'SK navigation
dat a. the crystal oscillator has its own
supply regulator no:; :.I1lU is followed
by tw o buffer stages.
TIlt' crystal oscillator output frequenc y
is first multiplied by five to obtain 30.7
MH z and then by three to ob tain the 92
MHz required for the second downconversion. Three additiona l Frequencydouble r stage s <Ire required to obtain
the first downconvc rsion signal at 736
MHz from the availa ble 92 MHz signa l.
The design of ali mult iplier stages is
simil ar and is using two tuned cir cuits
in eac h stage except for the first stage,
where three tuned circ uits arc necessary
due to the hig her multiplication factor.
The 102 MHz IF signal is first amplified (BFXR9) and then filtered (L9 and
LlO ). The second mixer is a simple
dual -gate MOSFET mixer (BF98 l). The
selectivity prov ided hy the tuned circuits at 102 W Iz (L9 and LI O) and at
10 NITIZ (Ll ) is alrea dy comparable to
the GP S CIA-code signal bandwidth (2
MHz) ln fact, LI 1 already requi res
dam pin g resistors to achieve the required bandwidth .
Th e GPS IF converter is built on a
single -sided board as shown Oil Fig.24 .
The location of the com ponents is
shown on Fig.25. Due to the limit ed
spacc all o f the resistors are installed
vertically. The ca pacitors arc convcntional cera mic discs (except for toOuF)
wit h a pin spacing of 5mmCapacitive
trimmers arc pla stic foil types of 7.5mlll
diameter: green 4-20p1' and yellow
2-10pF. There is also a wire jumper
ma rked with "A".
Th e BFX 89 is used as an univ ersal RF
transistor in this module and has n1<lny
possible repla cements: TIFY90. TIFW.10
etc. The four leads of the BF981
MOSFET are bent so that the device is
insert ed in the printed-circuit hoard
with the marking towa rds the boa rd.
The 7805 regulator doc s not require a
heat sink prov ided that it is-a TO -220
The GPS IF converter incl udes several
ind uctors. Most of them are air-wound.
self-supporti ng coils wound with copper
enamelled wire of either O.5mm or
l mm diameter. The turns of the se coils
arc not spaced and the leads go straight
through the printed-circuit boa rd with om any additional bend ing or forming.
In these way the coils themselve s have
about 1/4 of a turn less than speci fied
in the following para graph.
L1 and L 2 have 3 turn s each of 1mill
wire wound on a 4mm inner diameter.
L 3 has 5 turns of O.5mm wire wound on
a 3mm inner diam eter. L4 and L5 have
two turns each of l rnm wire wound on
3 311111I inner d iameter . L6 has ::l turns
o f 0.511I 01 wire wound on 301m inner
diameter. 1.7 and L8 have one slugfc
turn (or "U" loop) of imam wire with a
3mm inner diameter. L9 and U O have
5 turns each o f O.5mm wire 0 11 a 4mm
inner diam eter. l-inalty . L1 3 and 1.14
have 6 turn s each of O.5mm wire
wound (111 .:1 4m m inner diamet er. 1.13
has all add itional coupling loop o f one
sing le tum around the mai n winding .
Lt I ,1.I 5. 1.16, 1.17 and LI S arc wound
on standard cores for IF trans for mc, TS
(To ko or Mits umi) with the external
d imensions of lO111m x lOmm. Lli
should have a bo ut 4.5uH and in pra \:rice this means 15 turns of O. 15mm
diameter copper enamelled wire 0 11 a
10.7 MIII. IF transform er core set
includ ing. a fixed central ferrite core, all
adjustable ferrite (,.·up, various plast ic
support parte and a me tal shiel di ng. can.
Ll 5, 1.1 6 and L1 7 should have abo ut
DAuB and in practice have 6 turns (If
0. 15111 111 diameter copper enamelled
wire 011 u 36 MH7. IF trans forme r core
set including a plastic suppo rt with a
central adjustable ferrite screw . a plastic e;) p and a meta l shielding call .
Th e exact val ue of LI S de pends on the
crysta l used and the frequency required .
In all o f the prototypes built inexpen sive computer crys tals desig ned for
6 144 k ll z were used. These require
quite ::I large iml uctivity to be pulle d 5
kl-lz down to about 6139 kH2". An
indu ctivu y around 40u H is requ ired for
this shift. The exact val ue depends
much on the crys ta l used and the
parasitic capacita nces of the circ uit.
Since the performance o f the (IPS
recei ver depends on the stabitiry of this
master crystal osci lla tor. also 1. 18 need s
to be very stable. Th erefore :I 36 MII:f.
IF transformer co re set is recom men ded
and the latter requires about 60 turns o f
O.08m m diamet er copper enam elle d
wire .
Finally, Ll 2
cho ke .
a 100 ull "moulded"
T he G PS IF conven er has severa l
connections. The two coa x ca bles canying IF and LO signals to the RI; module
and the +12V supply wire for the Rft
mod ule are all so ldered directl y to the
bottom side of the IF conv erter module.
T he 10 MHz IF output, the 6 139 kl-lz
clock output and the + l 2 V supply
voltage are availa ble on a 'z-pin counce tor obtaine d from a piece of a goodqua lity Ie socket wit h round contacts.
Th e (;l 'S IF module req uires seve ra l
adjustme nts, but the crysta l osci llator
should he adju sted first III roug hly 6 139
kf lz. T hen the multiplier chain sho uld
be adjus ted . Each mu ltiplier stage
should he adju sted to provide the
muximutn signal at the required Irequcncy to the next stegc . The levels of
the Rf signals can be easily mon itored
vo ltmeter, sin cc they arc
with a
rectified by the BE junction o f the next
stage . Without any Rr input, the rx ~
volta ge is set to about O.7V acros s the
BE juncti on. When the multiplier chain
is operating correctly, this vo ltage
should decrease down to about zero and
may even become negat ive
If the transistor base !l0e.s mo re negati ve than -O.5V, RP transistor s ma y be
damaged and th is shou ld he avoided by
decreasi ng the values of the coupling
ca paci tors.
(,/" -
- - --
Of COlJrSe, the voltmet er required for
these adjustments should only be COIlnectcd through a RF choke to avo id
disturbing the RF ci rcuit. A lOkohm
resistor may also be used as a RP
choke. In this way all of the multiplie r
stages can be adjusted except the last
one to 736 MHz, SblCC 110 BE juncti on
follows this stage. The level of the 736
Ml Iz signal is monitored ill a different
way . by connecting a DC ohmmeter to
the IF output O f the mixer. The higher
the LO signal level . the lower the
resis tance measured by the ohmmeter.
the signal circuits is best performed on
a real GPS signa l ob tained from a
directiona l antenna (a 15 tum helix or a
small dish) pointed to a GPS satelli te .
f inally, the crystal oscillator shoul d he
adjusted to the exact freq uency requ ired
by the software. f or the current version
V l 22 the exalt freq uency is 6139 .050
kl lz, but this may change in the future.
The exact frequency is specified in the
program listing.
The signal circuits (L9, LlO and LI l)
arc best adjus ted after the receiver is
completely assem bled, since the following I f ampl ifier has a Scmctcr output. A
grid-dip meter can he use d 3S a signal
so urce at 102 MIlz. The trimmers in
parallel to 1.9 and L 10 tunc almo st to
the ir maximum ca pac ity and I.10 may
sometime, require an additional cap aci tor in parallel. The Fina l adjustme nt o f
The GLONASS receiver requires a
tuneab le dcwncouvcrrcr across all of
the 25 Gr.ONASS ch annels spacing
from 1602 MHZ to 1615.5 MHz. ther efore its design is more complicated than
the GrS coun terpart. Thl~ GLONASS
downconvcrtcr is d ivided into four
mo dules for shielding purposes and
differe nces in the construction tec hnology: au RIo" module and a PLL synthe-
MRF 511
1602 .
. \61S.SMHl
, 11
I 120~H
Fig .26: GLONASS RF Module
1490M HI
HF it!
,. '"
" 0
. 1496.6'12 11H~
12{l ~ H
= ===== = '---- - - - -- -- - - \'f>..
slscr converter buill in microsmp technology :md an IF conve rter and synthcslser logic built on simple. single-sided
printed ci rcuit boards.
111e circu it diagram of the GLONASS
RF module is shown on Fi~.26. The
GLONASS RF module includ es two
select ive RF amplifier stages. the first
receiver mixer to the first (fixed) I ~ o f
118.7 MH1.. and a y m followed by II
buffer stage. The two RF amplifier
stages are identical and use MRF57 I
transistors. Since the GLONASS Rr:
modu le is built o n a lossy. bUI thicker
laminate thun (IPS, the losses in the Rf
filt ers arc lower and two amp lifier
stages prov ide eno ugh ga in.
T he veo inclu de s a n am plifie r
(fiFR9 1) and a highly-selective interdigital filter feed back network . Such II
veo can only cov er a very limited
frequency rol nge (abont 10% around the
central frequency ). hut its phase noise
ill v\."I)' lo w. The veo is tuned by a
nnlO; vari cap in the central finger o f
Ihe irucrdigual feedba ck network.
The Veo is followed by a buffer slage
with another BFR91A microstrip coupler takes part of the y e O ompnt
signal to dri ve the PLL circui ts. The
yeo and Rf signals arc then com bined
in an inlcrdigiu l filler network to feed
the mixe r d iode I JP2900 or BA48 1.
The GI.ONASS RF module circ uit
includes a network to supply with + 12V
the GaAs rET pream plifier through the
Rr cable .
T he G LONASS RF mod ule i.'i built in
microerip technology on II double-sided
board made o f 1.57mm thicl.: glass
fibre-epo xy. The uppe r side is shown on
Rg.27 while the lower side Is no t
etched . The locat ion of the com ponents
is sho wn on Fig.28. Before installing
the components, the resonators o f L3.
1.5. L6 and L7 should be grou nded by
solde rin g short pie ces of Imm diameter
copper wire at the marked locations.
Tbe transistors and diodes are installed
in 6mm dia meter ho les in the printed
circ uit boa rd.
L1. L4. 1.8 and L12 arc quarter wavelength chokes. These arc made
from about ecm of O.15mm thick
co pper enamelled wire, tin ned for
About 5mm at each cud. T he remaining:
wire is woun d Oil a I mill inlier diameter
and the finished chokes arc sma ll
self-supporting coils . On the other hand,
L2. L9 and 1.11 are commercia l 120u 1l
"mou lded" chokes.
Rr interconnection s inside the GL().
NASS recei ver are made wilh thin
Te no n coax like RG · 188, installed just
like in the UPS receiver fronl end. On
the other hand , GI .ONASS mlcmstrip
modules include Iconbrongh capacitors
to save spaC'C 011 the prira ed-circun
boards. The fecdtbrou gh ca pacitors are
soldered to the mtcrostnp groundplanc
from the bottom side . Some compo·
nears. like chokes and resistors in the
supply network , are also installed on
the bottom side of the microstrip
The GLONASS HF mod ule on ly needs
few adjus tme nts, mai nly to the yeo
feedba ck network. To co ver the desired
freq uency range. the centra l finger
usua lly needs to be tri mmed shorter by
several mra. The two side fingers-may
need adjustments if the veo stops
oscillating at band edges.
Module. upp er side
(lop view)
Th e rC Jnilll11ng lnrcrdig uul filters usual ly d,) not need any adjustments to
pro vide the best performance i ll the
desired frequency range . If the veo is
o perating correctly, the mixer diode
will pro vide a rccrifc-d voltage o f about
-OAV across the 1500hm resistor.
GLONASS IF Conve r te r
The GLON/\SS If strip includes a
second downconvcrsion 10 IO.? MII:t.
signal amplification and limiting al 10.7
Mlh a nd the generation of the required
local osci llator and d ock signal s from a
single master frequency reference.
: vee OUT
I I U ti 3 J ' 4 97 ~ l
' lOp
118.71'11'1 1
55 3HV
Fig. 28: GLONASS RF Module Component Over lay
1602/ 160!i.511M1
The second downconver sion to 10.7
MH z and th e LO frequency generation
is include d in th e GLONASS IF conve ner modu le shown on H g.29 . The
GLONAS S IF conve rte r module inetudes an 1S MHZ master crystal
oscilla tor. T his Frequency is used, di vi ded by four , for signal samp ling,
divided hy 32 ;JS the PLL refere nce
frequen cy and suitably multip lied lor
the second sign a l downconvcrsi un and
for the PLi . downconvcrsion. The Ci-LONI\ SS IF module only inclu des the
oscillator a nd some mul tiplier stages.
The dividers are 10C;Jl ed in the PLL
synthcsiscr log ic m odule and the last
frequenc y m ultiplie r is ill th e PLL
synthcsiser conv erter. Li ke ill the GP S
re ceiver, the required short rerm staoitiry is ve ry high. in the I-E-9 ran ge. to
he able to demodulate the 50 BPS PSK
navigation data Therefore the cry stal
osc illator has its own suppl y regulator
?S OS and is followed hy two buffer
stages j ust like in the C;PS If convener
modu le .
The crystal oscill ator output frequenc y
is first multiplied by th ree to obtain S4
MHz. This signa l is then do ubled to
lOS MITz for the second down convcrsion and multiplied hy three to obtain
162 MHz to drive lite I'LL synth csls cr
converte r. using two sep arate multiplier
stages fed by the same 54 MH z signal.
Th e 162 Mil l. signal is further amplified in a buffer stage (BI'R 96) to dr ive
the SRD m ultiplier in the Pl.L synthesiscr conv erter.
Since the des cr ibed GI DNASS receiv er
inclu des a more complicated RP frontend than G PS, mo re filtering is req uired
in all multiplier stages to avoid spurious
freque ncies. Therefore multiplier stages
may have three or even m ore limed
circ uits on the ir outputs. The 118.7
:M1 h; If< signal is filtered (L9, U 0 and
LJ 1) and amp lifie d (BFXS9). T he second m ixer is a sim ple dua l-gate MOS :rET mix er rnrcs I ). The selectivity
pro vid ed by the tuned c irc uits at II R.7
MITz (L9 , UO and LlI) and at 10.7
MITz (L12) is al ready compara ble to the
Gl .DNAS S CIA-code signal bandwidth
(1.2 MHz). In fact, Ll 2 already requires
da mpi ng resisto rs to ach ieve the re quired ban dw idth.
The GLONASS IF converter is built on
a single-sided board as shown on
Jo'i g.10. The location of the components
is sho wn on Fig.J I. Due to till' lim ited
space all of the resistors arc installed
vertically. The capacitors arc convcntiona! ceram ic disc s (except for 100ur)
with a pin spacing o f Sr nn-Capacitivc
trimme rs 4~ 20pF ar c a plastic foil type
of 7,Smm diameter. marked with a
green body , There is also a wire jum per
marked with "A",
The IH'X :'l9 is used as an universal RF
transistor as in the (i PS If converte r.
Also t he n f<9S 1 is installed just like in
the (i PS lf convener module and a
1'0 -220 case 7805 reg ulator is recommended so that no heat sink is req uired.
The (JL ONASS IF conven er incl udes
severa l inductors . Most of them arc
air-wound. self-sup p orting coils wound
with copper enamel led wire o f 0.5H1m
diam eter. The turns of these coils arc
not spaced and the leads go straight
through the printed-circuit ho ard with out any additional bend in g or forming.
In these way the coil s thems elves hav e
about 1/4 of a turn less than specified
in the following paragraph.
(t- - - - -- - - -------'-"'--''''''''='''''-''''''''-'=
. ,, ,
. .I
\- ,
. •• e
e 5
, .••
-< -T
,; •
\ .
Converter, bottom
10.7 MHl
18MH l
118. 7MHl
1112 MHl
Fig.31: GL O NASS IF Converter Component Overlay
L5. L6 and L7 have 4 turns each wound
on a Jmm inn er diam eter. L9. L10 and
Ll l have 4 turns wound on a 4mm
inner d iameter. 1.13 and L 14 have 5
turns wound on 11 4mm inner diamet er.
Ll4 has an additional coup ling loop of
one single tum arou nd the ma in winding. r.r. L2. D , lA and Ll2 arc wound
on standard cores for IF tranefon nc re
(Toko or Mu sumi) wit h t ill' ext ernal
d imensions o f Imm x IOmm.
1.12 should have about 4.5ul1 and in
practice this means ISlum s o fO .15mm
d iameter copper en am elled wire Oil a
10.7 Mllz IF tran sformer core sci
incl ud ing. a fixed centr al ferrit e core, <I ll
adjust able ferrite cup, vari ous plastic
suppo rt parts and a metal shielding can.
L2. 1.3 and L4 should have about
O. D ull and in practice have 3 turns o f
O.Jmm diameter co pper enamelled wire
011 a 36 MHz If transformer core set
Includ ing a plastic support with a
centra l adjustable ferrite screw, a plastic cap and a metal shieldin g can,
T he exact value of 1.1 depend s 0 11 the
crystal used and the frequency required.
In all of the prototypes bu ilt inexpensive computer crysta ls desig ned for
18000 kl lz, ser ies resonance, were
used . Th ese require a small induetivity
in ser ies to compe nsate for the feedback
capacit ors of the oscillator net work. In
practice about 211 1I were requir ed , correspond ing to 16 turn s of O, 15mm
diam eter copper enamell ed wire on a 36
MHz If trans former core set.
Finally, Ul is a VK200 "six -hole"
ferrite cho ke and LI 5 is a 100u1l
" moulded" cboke.
T he GLONA SS IF conve rter module
has several con nect ions. Th e two ca bles
carry ing the 118.7 Mi ll If from the RF
module and the 162 MH z LO to the
PL L synthesiser co nven er are all so ldered directly to the M ilam side of the
If converte r modu le. Th e to.7 MH:l If
outpu t, the 18 MIl l. clock outp ut and
the + Il V supply voltage are available
011 a 7-pin connect or obtained from a
piece of a good-qual ity Ie socket with
round contacts.
In the GI.ONASS If converter module
the multiplier stages should be aligned
first, just like in the similar G PS
m odule . However , only the outp ut o f
the first mnlrip hcr stage to :; 4 MIIz can
be monitored as a d ip o f the followingstage base volta ge. The output of the
l OX MHz mult iplie r may be observed as
u dip in the drain voltage o f the HF9RI
mixer. while the out put o f the 162 MII/.
multiplier ma y be measu red av the
rectified voltage by the SR O multiplier
in the PI.L synthcsiscr converte r.
The signal circui ts (LQ, L10, Ll I and
1.12) an ' bes t adj usted after the receiv er
is completely assemble d. since the
following IF amplifi er has a S-mdcr
output. 1\ grid-dip meter can used as a
signal SO\1Tee at 118.7 Mill'. Th e trimme rs in parallel to 1.9, 1.10 and Ll I
tunc a lmost to their maxi mum capacity.
Th e final adjustment of the signal
circuits is best performed O il a real
GLO;.lI\SS signal obtained from a direct ional arucrma (a 15 tum helix or a
small dish) pointed to a GLO;.lASS
fina lly, the crystal oscillator should be
adj usted 10 the exact frequency requi red
by the software. For the current versio n
V39 the exact frequen cy is 18000.000
kj-lz, but th is ma y change in the future.
" '"
14aB1 'l,
. 1496.61'l MH,
.. 1pS
rc I.. PUT
Fig.32: (; J,ONASS P LL S.\'nthesiscr Converte r
The exac t freq uency is speci fied in the
program li "li n~"
C:; LO NASS I'LL S-",nth.;siscr
Conver ter
A sing le -channe l GLONASS receiver
requires :I Iast-scufing frequ ency synthcsiscr, since the receiver is continuously switching among diffe re nt frequency chan nels . Be sides this require-
ment the symhcsiscr should have a low
phase noise. To limit gro up-de lay variatio ns the synrbe-iscr should supply a
varia hlc frequenc y already to the first
downtonv erter. AI[ these requirements
ask for a PJ.I. synthcsiscr with a
frequency do wncon vcrt cr in the feedback loo p, to decrease the divider
modulo and increase the loop gain .
Therefore. the GLONASS I'I ,L syntbc slscr incl udes a veo in the R io' mod ule ,
Fi ~.;l;l :
Con vcrtcr, upper
sid e (to p view)
55 3MV
(" . 21.
, _ ~ na,
nl~ ~J<1
Fig .~''':
" ,
6 f ~'IO
(iLON ASS PLL Syntbcstscr Converter Com ponent Overlay
a downconvertcr and conventiona l PI J .
sY lll hc~i~n
log ic like va riable modulo
di viders and a frequency/phase cOlllparate r.
'illC circ uit diagram o f H l~' CiT .oNi\ SS
PT.], synrhesis e r COIlVCI1cr i~ show n on
Pig.n . The circuit inc ludes an other
buffer s l;J~c for the y e O sig na l around
14 1Xl MIIz. a step-recovery diode
(SRI)) frequency multiplie r hy 9. 10 gel
145R ~lI z from the available 1 6~
Ml lz, a m ixer diode and an fro ampli4.lcr stage. The veo buffer sta ~c
(BrR90 ) is requ ired 10 avoid ge tti ng
:my unw anted spur ious signals hack in
th e Gl .ONASS RF mod ule.
The SRI> mult iplier uses a w ry lncfficient silicon I'N-jnoction d iod e IK4 14&.
Ot he r diode s like VIIF T V tune r band
sw uc hing diodes (Oi\ 182 or UA4 82)
provide an lip to 20dB stro nger signal
ar 1458 MH z in the sam e ci rcu it, hut a
higher signal level is not required here
and it is ev en harmful, since it m ay get
in the Rf mod ule and cause unwanted
mix ing produ cts. In practice it is thu s
conve nient to keep the 1458 MHz
signal level low and drive the m ixer
. ~­
,..Il""1 ~",
f lo • V'
(0 ' NPUT
'U ~~ '
di ode into the non- linear reg ion with
th e 1490 MH z veo signal.
To avoid any spurious generat ion a ll
sig na l levels are kept low. Even the
buffered y eO sig nal amou nts to onty a
few hundr ed mY on the mix er diod e
IIl '2900 (or Hi\ 4S I) while the 14.'lX
M l lz sij!llal level is m uch lower. To
ope rate e fficie ntly at low signa l levels
the m ixer diode receives a IX: bias
The PLL IF signal then nee ds much
am plification III reach the m . level
required hy the va ria ble -mod ulo l' OUlIrcr. Till' first I'LL l f ampli fier stage
(fl FRQO) is built in the I' lL converter
m odu le. The following PLL IF cmplific r stages arc loc ated in the PI.I.
synt hcsiscr lo!!ic module for shield ill!!
purpo ses, si nce harmonics o f the PU .
rr fall in the first IF ( 11 8.7 MI L,,)
frequenc y runge of the described 0 1.0:--lASS receive r.
The ClLON ASS PLL symn csiwr conve rier is built in mic rostri p technology
on a double -sided board made of
15 7mm thick glass fibre-epox y. '111C
uppe r side is shown on Fig..3 3 while the
lower side is not etched. The location
of the components is shown on Fig.34.
Before installi ng the components, the
resonators of 1.I . 13 lind L4 should I'IC
grounded by soldering short pieces of
1mm diameter copper wire at the
marked locations. The transistors and
diodes are installed in onun diameter
holes in the printed circuit hoard.
1.2. L5. L7 ami U: :lTC quarter-wavelength chokes. The se are made from
about 6cm of 0.15nuII thick copper
enamelled wire. tinned for about Smm
lit each end. The remaining wire is
wound on a lmm inner diameter lind
the Fin ished chokes are small selfsupporting coils. L6 is a self-supporting
coil with 3 turns of O.5mm diameter
copper enamelled wire wound on a
Smm inner diameter.
The microstrip filters in the GLONi\ SS
I'LL synthcslscr conven er usually do
not require any trimming. The l Okolun
trimmer for the SR I) bias current is
usually set 10 Skohm'lhc SRI) multiplicr will operate correctly if the rectilied IX.: voltage hy the IN4148 d iode
amounts to about 2V.
/lP" APAIMllf
1970 - 1994
Order your copy now
£2.50 by cash or cheque
£2.75 by credit card
K.\1 Publications, 5 Ware Orchard, liarby,
CV23 8UF, U.K.
Tel: 01788 890365
Fax : 01788 891883
Email : [email protected] serve.com
(~, ---------~~~~~~
Matjaz Vidmar, S53MV
A DIY Receiver for GPS and
GLONASS Satellites
This part or the ser ies conttn ucs wil lt
th e co nst r uct ion details for th e
GLONASS PL L Sy nthcstse r, Second
If' Amp lilier and lite W'S/( iLONASS
USI' Hardware.
GLONASS I'L L Synthesiser
co nve rt the freq uency range
160 2 Mill. to 1615.5 Ml lz down to
118.675 Ml Iz the veo must operate in
the freq uenc y fl Jn g C 14 83.3125 MlT... to
1496.8125 MIll',. Subtract ing 1458
MHz in the PI. I. syr ahesiscr converter,
thls frequency range is downconverted
10 2'U125 MHz to 38.8 125 MHz. The
Jailer frequency range corresponds to
integer multiples rang ing between 45
and 69 of the ( iLONASS channel
spaci ng of 562.5 lllz.
The design of the PI J . synthcsiscr logic
j.'i therefore straightforward and the
corresponding circuit diagram is shown
ill f ig.36. The I'LL synthesiser logic
includes a PLL IE a fixed divider hy
32 to obtain the 562.5 kllz reference
and a frequency/phase comparator.
The PLL IF signa l in the range from 25
MHz to 39 MHz is first amplified to a
TIl . level in a two stage amplifier. The
gain of this amplifier is set higher than
required to have a considerable safety
margin. The base bias resistor of the
second stage may need some trimmi ng
although the suggested value of 2.21dJ
will usually work.
The programmahie modulus counter is
built from two sync hrono us counters: a
74F l 61 and a 74IIC 16 1. A 74Fxxx
counter is required in the first stage to
operate rel iably at the hi ghes t
frequ ency, since the PLI. IF may be
severa l tens of MHz above 39 MHz in
the unlocked state!
The counter feedback network includes
an inverter and a 74HC157 multiplexer.
The mod ulus of the sec ond counter
(74HC16l) is programm ed direct ly.
The modulus of the first counter
(74F 16 1) is set to 10, except during the
last state of the second counter, when
the 74HC 157 switches the 74Pl 61
preset inputs 10 the 4094 outputs. In
this way setting the modulus of the
, ,••
, ,
, ffi '
.,. ~.,.
" '"
O. ,._o-J~I
•• ' I"
'·~ 1
OIl' .... '
.' ..".
55 3MV
. 1('
",,,",~, t
'".... "...
Fig.3!': GI.ONASS Synthcslser Logic
second counter changes the whole
divider modulus in steps of 10. while
setting the modulus of the first counter
during the last cycle only provides the
single-count steps.
The 4094 is an g-bit shift register with
output latches, It is used as a serialto-para llel interface driven by the
computer. Of the eight available output
lines, four are used to control the
modulus of the 74HC161 counter and
the other four to control the modulus of
the 74[-161 during the last cycle of the
7411C l 6 1. One should he especially
careful when programming the modulus
of the divider: the data is inverted and
the first divider modulus should never
be set too low to allow for the delays in
the slower 74Hcxxx logic!
T he 18
level in
MHz master reference
is also amplified to TfL
a single -stage amplifier
A 74HC393 counter divides
this frequency by four to obtain the
4500 klfz sampling frequency and by
32 to obtain the PI.L reference
frequency, The 4500 kl lz signal is
attenuated with a resistor network to
limit spurious radiations. It is then
amplified back. to TIL level in the
dedicated DSP hardware module.
The frequency/phase comparator is a
charge-pump circuit. including two Dtype flip-flops (741lC74), a feedback
network with a NAND gate and chargepump switches with Schouky diodes.
The backlash problem is solved by
making the charge-pump circuit faster
than the feedback. network. This does
not make the phase detector linear yet,
but provides a stable locking point for
the PLL with no dead zone. and
produces a very clean synthcsiser signal
spectrum. An additional NAND gate
provides a LOCK signal for test
(~ - -- - -- - ----'-"'--===='-'''-'=
Fl g.36:
PCB Lay out
the GLO~AS S
Synlhesiser Logi c
installed vertically to save space. All of
the capacitors arc ceramic with a Smm
pin spa cing , inclu di ng th e ) uF
The GLONi\SS PLL receiver logic is
built on a single-sided primed circuit
board as shown in Fig.36. The location
of the components is shown in Fig.37.
Th ere arc three wire jumpers on this
hoard and two of them arc installed
belo w the 74IIC I5 7 multip lexer.
A complex single-sided board also
places some constraints on the
installation of the resistors: those with a
IOmm hole spacing are installed
horizontally, whilst the others are
.. rr-,
L1 is a commercial 1(J(}flH moulded
cho ke. The module has th ree
connectors with 3, 4 and 7 contacts,
made from pieces of good quality Ie
sockets. The integrated circuits should
he soldered directly to the board except
for the 4094. It is recommended to
install this Ie on a socket, so that it can
- -
o0,m ~"
'; 0 0'",.
1"1216~ {I
1I)k -<>-
Pll IF
~ l9_
' H " ,~ ::
~'>«I 2~21
T 'Kt'n
i '0 00
l~ T,"
lk-o-lOO ~
ie c-
- 0 - :-it1_
100 1
F ig.37: Com ponent La yout for the GLONASS Synth esiscr Logic
~ , SMlil
.I 120p
I gn
~ k5
l1-' L,-p ~
o Bf ~
( A3089
I( A3189)
t;;~' I"UF
553 MV
: WO~
, ,
In n
~ '"
WO k
. 12V
Fig.38: GPS.!GI.ONASS Second IF Ampli fier
he removed from the circ uit and
re plac ed by wire jumpers, to he ab le to
test the sy nthcsiscr witho ut th e
computer runnin g.
T he GLONASS synthcsiser logic may
require a single adjustment: the bias
resistor for the second I'LL IF amp lifier
sta ge . This may be adjusted if the
o utput DC voltage dev iates much from
1.3V (w ith 110 input signa l) or if the
programmable counte r does not operate
relia bly .
Both G PS and GI,ONASS re ceivers
require a limiting IF amplifier at the
final 11' frequency around 10 MHz. The
circuit d iagram of this amplifie r is
shown in Fig.38 . The GPS/GLONASS
second If am plifier includes a first
stage w ith a bip olar transistor, n r X89,
and a sec ond stag e using the integrated
ci rcuit C A3089.
T he ga in of the first stage is lim ited by
the 220 resistor in the emi tter circ uit.
The firs t stage is followed by a tuned
circuit (Ll) to lim it broad band noise
and avo id amplifying var ious spur ious
signa ls from the ma ny osci lla tors inside
a GPS or ULONASS receiver. T he
damping resistor in parallel with LI
sets the bandwidth of th is tuned circui t
to he comparable wit h (iPS or
GI DNASS signa l bandwidth.
The second stage uses a popular FM IF
strip integrated circu it. The latter
provides widehand amplification and
limiting, while the disc rim inator section
of th is integrated circuit is not used
here. The limited JF output is available
on pin -S and the signal level amount to
a few hundred millivolts at 10 MHz .
111is is not enough to drive the
following TIl. logic direc tly an d the
remaining gain is built inside the
ded icated DSP hardware mo dule .
The CA3089 inte grated circuit includes
an S-meter outp ut. This is of little use
during actual rec eiver oper ation, since
the sa t elli te signa l l ev el s are
comparabl e to noise in the widcband
If. In the case o f a GPS receiver, the
S-me ter output can on ly show the sum
of all the signa ls present. On the other
PCB La yout for th e GPS/( iLONASS
Second IF Amplifier
hand. the S-meter output is very useful
during receiver testing and alignment o f
the RP and the first and second IF
tuned circuits.
central ferrite core. an adjustable ferrite
cup. various plastic support parts and a
meta l shield ing can. The primary
(resonant) winding of LI has 10 turns
o f O.I Smm diameter enamelle d copper
wire. corresponding to an induct ivity of
abou t 2JlH. The secondary (li nk)
winding has two turns of the same wire.
1.2 is a l OO ~LII moulded choke.
n oth GPS and GLONASS receivers
include an S-meter function inside the
narrowband IF process ing. Since the
latter is done in software, the real
rece iver Svmeter as displayed on the
I .CD is just another software function
a nd is NOT related to the hardware
Scmctc r output of the IF strip.
The second U; amplifier module has
two co nnectors; a 2-pin connector for
the input and a 5-pin for the output and
supply voltage. both obtained from
pieces o f good quality Ie socket with
round contacts.
T he GPS/GI.ONASS se co nd IF
amplifier is built 0 11 a single-sided
primed circui t board as shown in
Pig..]\), with the com ponent location
layout shown in fig. 40. Due to limited
space all the resistors arc mounted
vc rt tca tty . Th e ca paci to r s ar e
con ventiona l disc ceramics (exce pt for
the 22jlf) with a pin spacing o f Smm.
T he tuned circuit with Ll is best
adjusted after the GPS or (;LONASS
receiver is complet ely assembled,
find ing the maximum IX~ volta ge on
the test S-meter output in the sam e If
modu le . The final adjustment o f all
signa l circuits is best performed on a
rea l satellite signal obtained from a
LI is wound on a 10.7 Mill. If
transformer set. including a fixed
1k 100k
-0- .l
-n-- -n- -n-
1k5 ~
9)f( lr
6FX89 -0. - tl-6&k 1'2'OD
.w: Co mpo nent Layout for
Fi~ .
to r
.L .L
2iT or
-o- 1k
- 1122n
1100'~i I:=+~= bOU~~zUTZF
(he GPS/GLO NASS Second IF Amplilier
,• •e• ·,• ,
. . .
- · !r- •
- .. .
~ I'~
.. . , .. - .;:.r -
,, ,.
... !;'
' il " 91 "'~I
~ ~_
. '"
~_ ~
• a. : ,. •. ,. : e.
'" ,
.. •
. ,•
! .
• I
t- -
..., lr.l
ft-- t - ---1':
.... 1:-
. ~--
o n nnu o n
.j JJ
L..c ~
? .¢.
• '1
·· ·• - ; ..
__ _
01 . . . .
<>OOco <> o o
U>III ... ~ n." l
• ....
i~ E"
directional antenna (a I 5-tum helix or a
small dish) pointed at an operating
navigational satellite. A directional
antenna should provide a higher than
usual signal-to-noise (SNR) ratio of
more than IW O already available in
the wideband IF. A high SNR is
required to tune the circuits to the
signal peak and not to noise or some
spurious signals while observing the
voltage on the test Scmctc r output.
Ha r dwa re
The theo rv of operation of the
dcdica t e d U PS /G L O N A SS nsr:
hardware has already been discussed in
3 . 6 . Th e re fo re t h e p ract ic a l
implementation only will be described
in the following section.
The dedicated GPS/GLOXAS S DSP
hardware is built as a peripheral plug-in
module for the 1)S1' computer (1), (2).
The circuit diagram of the GPS/
GLON ASS hardware IS shown In
Pig's.a t and 42. The nsp hardware
module includes two amplifiers for the
limited If signal and master d ock
coming from the analogue part of the
GPS or GLO~AS S receiver, a look-up
tahle RAM. four signal dcsprcading
mixers, four correlator counters. all of
the timing logic to scan the look-up
table and generate interrupts to the
CPU and all of the interface circuits
necessary for the DSP computer bus.
The input signal amplifiers are built
with 74HC04 inverters to amplify the
input signals of a few hundred
millivolts up to TfL levels. In this way
the signal levels in the analogue part of
The look-up tables are stored in a 32k x
8 static RAM. The RAM area is
divided into 8 separate areas of 4
kbytcs each, selecta ble through a
microprocessor output port. Tn this way
the look-up table does not need to he
rewritten when switching 10 another
satellite. The receiver is usually timemultip lexed alllong four different
satellites and all four different look-up
tables are stored in the RAM. When
switching to another satellite. the
hardware is simply switched to another
look-up table and this only requires
executing a few instructions instead of
rewriting the whole 4 kbytc ta ble.
The 4 kbytes of each look-up table arc
written as bytes by the microproce ssor.
The microprocessor writes all of the
bytes to the same location, since the
address co unter is inc reme nted
au tomat ic all y a fte r ea ch write
operation. In read mode the look-up
table is scanned hy the same hardware
counter (74ITC4040) clocked at half the
sampling frequency (3069.5 kl'Iz for
GPS or 2250 kHz for GLONASS). The
byte data is latched (74HC273) and
then multiplexed to 4 bits (74HCI57)
- - -- - - - - - - --'-"'-'==
= -"==
Fi g.44: 1)(:1\ lA, yoUI for tbe
{~ rS/(j LO:\ASS
to get all of the four required local
signal replicas to he multiplied (EXO R
7411C8 6) wi th each input si gna l
sample, Each look -up tab le may he rhus
up to 8192 samples long. Th e unused
samples nee d not he writt en since th ey
arc not used by the hardw are.
Th e dedicated DSP hardware requires
six programm able counters: four
corre lation accumulators. a sampling
frequency d ivider and a variable dela y
count er. all contained in two PD7 1O.54
J)sr Hardware (lop view)
(82C5 4 ) integ rated ci rcuits . Each
P 0 7 10 54 contai ns t hree a lmos t
independent 16·bit co unters that ca ll he
programmed in di fferent ways. for
exa mple. the four co rrelat ion cou nters
arc cl ock ed with the same signa l
sampling frequency and th e signals are
fed to the GAn~ inputs, which are
programmed as dock ena bles,
T he signal sam pling frequency (6 139 or
4500 kHz) is divided down 10 1 kHz to
mat ch the CIA -code period (1 ms).
Flg.44: PCB Layout for lhe GPS/GLONASS IlSP Hardware (bou om view)
This signal is also used 10 request
Interrupts from the CPU. since the
co rrelatio n counters need to be read
eac h m illisecond. After receiving an
interrup t request. the CPU will latch the
co ntents of all counters in a single bus
opera tion and then read the latched
co nten t of every single counter in
separate bus operations. The sampiing
clock divider is also latched and read
and its content is used as an accurate
timing reference.
Every interrupt request sets a flip-flop
that needs to be re set by th e
microprocessor after the interrupt has
been serviced. Interrupt ann (cnable)
and reset (disable) is performed through
one (04) of the eig ht output port bits
provided by the 74HC259 addressable
latch. Of the remaining 7 bits, three
(Ql , Q2 and Q3) are used 10 select one
of the eight look-up tables in the RAM
and another bit (QO) is used to select
either write or read mode for the
look-up table logic. The lasl three bits
(~ ------------"-"-===-"==
~~~~~ H~H
-!I- lOOn
00 l!J [i]OO~
~G ~
H S(H ~
+lOOn I
F i~..a5:
-'T 100n
I 553N V
Component Lec a uon Ior the GPS/G LO NASS I)SP Hardware
GI.ONASS PI.L modu lus or as spares
in a GPS recei ver.
The bus interface \0 the DSP computer
includes a hi-directi onal data -bus buffer
(74 HC"245) and an add ress se lection
lo g ic (tw o 7 4 11<": 1 38 and o ne
74HC245). The bus interface docs not
MC..68010 CPU. The address decoding
fo r th e PO ? ) 054 programmab le
counter s must allow simultaneo us write
operations to both control registers of
both peripherals. to be a ble to latch the
contents of all of the counters at
exactly the same time. Finally. the
RESET signal is fed to the 74IK.'259
addressable latch essc nnau y to prevent
(Q5 , Q6 and Q7) arc used to co ntrol the
8 - 10k
any interrupts or other unintended
operations before the whole DSP
hardware is correctly initialised.
'I'he bus addresses are assigned as
shown in Tahlc-l . However, one should
notice that the remaining addn:sscs in
the range (ro m SEO(X)O to SfFFFf arc
not fully or correctly decoded, although
the module will ackn owledg e the
access to the MC6801O. Accessing
other addresses in thi s range (cither
reading from, or writing to) will
probably cause an erratic operation of
(he whole module. Tbe 74I1C2.'i9
addressable latch is programmed by
writing to the specified locations. Since
only the address is important and the
data is ignored. G..R.B instructions are
The bus addresses are assigned as follows:
Disable look-up table write mode
Enable look-up tab le write mode
Look -up
Look -up
Loo k-up
$E004 3
$E008 1
tab le address
table address
table address
table address
table address
table address
A2 reset
A2 set
A I reset
A l set
AO reset
A D set
Reset and d isable l ms interru pt
Enable 1ms interrupt
Reset and enable l ms interru pt. long transfer!
$E8IX lH
Common write to both 7 1054 command registers
$E804 1
#1 C r RO - data 0.4 accumulator
#1 CT RI - data 1.5 accumulator
#1 CTR2 · data 3.7 accumulator
#1 command register
7 1054
#2 CTRO #2 CT RO #2 CTRO #2 CfRO -
SE800 1
WRITE byte to look-up table
PL!. modulus STROBE reset
Pl.T. modulus STRO BE set
PLL modu lus DATA reset
PLL modulu s DATA set
PI.L modul us CI.OCK reset
PLI . mod ulus Cl .OCK set
used 10 write to single bytes and a
CLiU . instruction is used to reset and
arm the interru pt flip-flo p.
The dedicated DSP hardware board is
built o n a doub le-sided print ed circuit
bo ard as shown in Fig's,43 and 44, with
the co mponent overlay shown in
Fig,45. The single resistors. diodes and
the 10 0 J.111 c ho ke are inst all ed
horizonta lly. The eight lOkU resistors
variable CIA-code dela y
(iPS 16139 or GLONASS 14500 dock
data 2.6 accu mulator
co mmand register
arc in a single 9-pin SIL package. The
capacitors are ceramic except the
l OOfW tantalum. and all have a spacing
of 5mm.
Thc 74 HC 4040 sho uld NOT be
replaced by the standard 4040 device,
since the latter is too slow for correct
operation in this circuit. To allo w for
troubleshooti ng it is recommended that
at least the two 7 1054 cou nters and the
4 3256 RAM are installed in good
qualit y sockets. Th e spee d o f the AM is
unimportant, since even the slowest
150m static Ram devices arc fast
enough for this project.
T he ded icated DSP hardware modu le is
inserted in the DSP computer bus with
a 64- po le Eurocard A+C connect or.
T he remaining connectors incl ude a
5-pin soc ket for the IF input signal and
d ock , a a-pin soc ket for the Gl.ONASS
PLL modul us contro l and an 8-pi n
socket for the interrupt select ion. All of
these are made from parts of good
quality Ie sockets.
The dedica ted DSP hardware module
requires no ali gnme nt or sett ing up .
(to be continued)
Details of kits and PCR's fo r this
project appear on page 127 of this issue
Item No.
SW PA for 13cm
Com plete kit wit h LP's
£266. 00
281144 'Fransverter
£ 139.00
DJ 8 1 ~S -201
13cm FM AT V Exciter
06 3lUl
t: 70,m
4f) ]
Widcband Measuring Amplifier
£ 65.rXI
KM Publications. 5 Ware Orchard, Barby, Rugby, CV23 SUI"
Tel : (0)1788 8911365
Fax: (1I)1788 891883
Ema il: [email protected]
Mmju : Vidmar. S53M V
A DIY Receiver for GPS and
GLONASS Satellites
Th is pa ri or th e ~r i es continues wilh
lhe co ns tr uction derails tor the G PS!
ClLO NASS Pnrhtblc Receiver C P U.
th e 8. kcy Keyboard. th e LCD Display
Module and th e Power Supply a nd
Reset cireuilry. SUj:!.j:!.cstcd mount ing
ar ra ngements for the va r ious modules a rc also d iscu ssed .
·t tl
C; PSfGL01\'ASS Portabl e
Receiver C PU
The n!'s or OI.ONI\ SS receiv er described in this series can be built as an
interfa ce for the nsp computer lt l and
121, or as a stand-alone portable reccivcr. In the tarter case the receiver
need s its own microco mputer with a
key boa rd and an I.CD display, A fter
considering severa l possible ahcm arives. the siruplee t so lution resulted in
using a suitably modifi ed CPU board ns
described in [I] ami [21 as the microcomputer.
The circuit diagram of the modified
CPU board is shown in Pip!'> ,46 and 47 .
Since 11 C;PS or GLO;.;rASS receiver is a
portable unit. the power consumption i.s
an importa nt factor and conseq uently
74IICxx logic devices should he used
everywhere, This allows for the omis sion of three 3.3 kD pull-up resistor!'> ,
T he original VSP com puter C PU hoa rd
req uires the follo wing modifi cations:
The pad!'> belo w the EPRO M socket
should he connected so that pin-27
receiv es the " 14 sil!-nal requi red hy
the 27(: 256 EPRO M. Origina lly this
pin is connected 10 +5V when usin g
the 27 128 EPROM.
2 Th e
RA ~ should he Increase d from
64 kbytes up to 12R kbytcs . This is
achieved by piggy -bac k solder ing
I W O addit io nal 43256 RA M chips on
lop o f the e xivting IWo RA M chip s
on the CPU board . /\ 11 pins of the
additiona l RA M chips are connected
in parallel with the e xisting RAM
chi p pins, except for pin-20 (chip
select). The IW I) chip-select pins of
the add itional RAM chips arc then
wired 10 pin-ll (Q4) o f the middle
74JIC 138 address decoder.
- - - - - - - - - -----"-"'-===
= ==
.,.. 4~ '~
li Fr ~ ~a~; ~
F'i~.48 :
GPS/G LONASS C PU. upper side (lop "jew)
3 An HSCH I00 1 Schottky diode
should be conn ected between pin-l I
(<")4) of the tor 74Hf:13 S address
decoder and 'h e DTI\C K signa l
(pirJ-12 of the 74IIC05), to acknowl edge the additional RI\M chips 10
the CPU.
The keyboard connection remai ns unchanged. Th e total /partial reset switch
has a new function with the GPS or
GI,ONI\ SS software. In the case of a
stand-alone porta ble receiver this input
should a l~a y ~ be left open (+5V )!
The parallel output port (PD71055
c hannel-B) is now used to comm and
the HD44780 LCD controller. Since
there are only S output bits available,
the HD44780 is driven in the 4-bit
mode - write only. The real-time clock
chip PD4990 is required by the GPS or
GI,ONASS softwa re and must remain
in place. The IN'I7 jumpe r must remain
in place for keyboard Interrupt requests
while the INTI jumper is no longer
needed, although il may left in place.
T he printed circuit hoard is not mod ilied, as shown in Hg.~.48 and 49 , the
addit ions and modificat ions arc fully
visible on the component location plan
in Pig ..'iO. The connections of the S·key
keyboard and the LCD controller arc
also sho wn in Fig.50.
recommended thai the CPU hoard
tested first, if possible, in a DSP
computer. and then mod ified as described above only when it is fully
tested and working 100%. In parti cular,
the CPU board should be tested at
higher clock: freq uencies to find any
defect ive components. A 10 MIll. veri~
(; rS!G LO~ASS
C PU. lower side (bou om view)
sion of the MC68010 will u_~ u3 I1y work
lip 10 a clock frequency of 15 Mil, OIl
room tempera ture. so a 12 MHl clock
crysta l is a safe choice. The (i PS or
(i LONASS so ftware docs not requ ire
..uch a high clock frequency. till! the
accuracy of some measurements is
higher and (he updating of the display
is faster OIl higher c1IK:k rates .
-l.12 M.kr y Keyb oard
A portable GPSj( iLO!'lASS receiver
requ ires OJ small keyboa rd 10 issue
commands to the com puter. Since a full
ASCII keyboard is impract ical for a
portable piece of equipment , a sma ll
x-kcy keyboard was developed for
portable receivers.
The circuit diagram of the 8-key key-
boa rd is shown in Fip:.5I , The 8 keys
close towards grou nd. otherwise the
input lines arc held high by pull-up
resistors. A priority encoder (74HC I48 )
is used to encode the Kkeys into .3 bits.
A dou ble monc aaole (74HC45.38) is
used to generate the strobe pulse after a
key is depressed.
The s-kcy keyboard is assembled on a
small single-sided printed circuit board
as shown in H g.52. This printed circ uit
board was designed to fit on the front
panel of the rec eiver and carry e ight
sqllarc (12 x 12.7mm) push-buttons.
The location of the components is
sho wn in H p:.5J . Due to the space
constraints all of the compo nents
sho uld have a lo w profile and smal l
dime nsions. The capa citors have a
2.5mm pin sJ"'3cin l and the resistors are
(~ -------------'-"'--""=="'-'-"=~
.... "~ ...... "i'
~~ "JCI" '-'
Hr."4 ~iI~
( 0"1 11101.[ [ 1/
T, OO o
FiR50: GPS/GLONASS CPU. Compo nent
Ov~r 18Y
8 _ 10k
FI~.51 :
- -_ .
, ",
n •
11 ;1
""" Qih
74 HC4')18l
" Ii
10 I ~
l'" ""r
C1rcuil or the S.li:ey Keyboard
," ,
•, "
"I T , , '
. nt
S5 l MV
Fi~.52 :
g-key Keyboard
PCB (bot tom view)
installed horizontally . The 8 x 10 kO
resistor network in a SIT , package
should be first so ldered in place and
then ben t towards the c ircuit boardTh e
fi-pin co nnector is installed on the back
- so lder side of the hoard!
sizes, and mayor may not he equipped
with a display controller. LCD modules
with a built-in contro ller are easy to
usc, since the interfacing to any microprocesw r is very simple, it is identica l
to a parallel I/O port.
The GPS/GLO NASS receiver software
only acc epts ASC II characters from $30
to $37 as comm and s, corresponding to
numerals 0 to 7. These can be generatcd by a standard ASCII keyhoard
with a parallel output. or by suita bly
wir ing the 8-key key board described. In
particular. to obtain the codes betwee n
$30 and $37. the outputs DO. Dl and
D2 should be wired to the correspond.
ing inputs on the CPU board. In
addition. D3, D6 and D7 should he
connected to ground and D4 and DS to
+5V. All these connect ions, includin g
the supply rails and the strobe signal,
were already shown in Fig.5O.
Most small dot-matri x alphanumeric
LCD modules use the Hitachi HD44780
LCD controller. This integrated circuit
has an on-board, extend ed ASCn character generator, a display area RAM
includ ing up to two rows of 40 characters eac h and all of the liming circ uits
required to drive the LCD . Modu les
u ~jn~ the 11044780 co ntrolle r may have
a different number of characters per
row, or rows displayed. the total
number of characters is, however, limited hy the internal RAM to RO. Since
the HD44780 SMD flat package has
only RO pins. additional l .Cl r driver
chips (lID44 ICX) are used in most LCD
modu les.
4.13 LCD Displ"y Module
The only practical for a portable GPS/
GLONASS receiver is an LCD modul e
with built-in drivers. Such modu les arc
available in many d ifferent shapes and
.. .
~ .~. mH
L!.. 1
55 l MV
... e
AN LCD module with two lines of 40
ASCII charact ers each was selected for
this project. Such an LCD module
includes the liquid crystal display itself,
one HD44780 LCD cont roller and four
lI D44100 LCD drivers. Further. the
Is, / " I" /'0
/;7 / S", I;, I;,
"' iJ!:.~3:
X-key Keyboard
(j- - - - - - -...12V
10 k
liO On
r---->.- - 0UT1
EL suppl y
' - - 4_ _
Fig.::i4: LCD Raeklight Power Supply
display m odule may include some form
of illum inating the L CD, either EL foil
or LED. The tarter is recommended if
the GPS /GLONASS receiv er is to be
used at night as well .
Alth ough such di splay modules are
available from severa l differen t manufac turcrs, tile print ed circuit boards on
which th ey arc mounted all have the
same dimensions: IR2mm wide x 33.5
high x 1.1mm thick and all have the
same 14-pin electrical connector. The
pin numbers are usually marked on the
printed circ uit board and the pin allocations arc as follows :
Pin -4
Pin -5
Pin -7
Pin -9
Pin -IO
Pin -II
Pin -l2
Pin -14
+5V supply
LCD vo ltage. wiper of
tile contrast pot
Register Sele ct,
o = instruc tion, I = data
Read/Write, () = write,
1 = read
Enable, 0 = inactive,
1 = act ive
LSB data , x-hit bus
L SB data , 4-hit nus
MSB data
5S 3MV
I 9V I
_-_ uw
n ov
-ir,20p -l~ ,o p
Fig.55: LCn Backlight Power Supply
PCB (bottom view)
Fig56: LCD Backlight Power Supply
Component Overlay
; vv'
<1 21,1
100 H
1NIB 1
4 ~'f'
1N 5S1 S
680 1n
S5 3MV
_ _~~~ _
1--,-- - '" pEn
/1:: ~
Nj Cd
3.6 V
' SV
F ig.57: Power Supply a nd Reset Circuit
The EL or LED back-li ght may have
two additional pins on the connector or
solder pads on the printed circ uit board.
When L CDs are driv en in tim e multiple x the adjustment of the voltage
applied to the I .CD is crit ical to obtain
a good contr ast. A con trast co ntrol
potentiometer is us uall y provided to
adjust the nest available contrast for a
given vi ewing an gle . Thi s potcntiomctcr provides the Yo voltage to the
LCD module . Modem LCD modules
require operating voltages of less than
:'iV, so th e resistor termi nals ca n be
conveniently connected to ground and
to +5 Y, whilst the wiper is connected
to U1e V o input.
Due to the inte rna l circuits o f the LC D
controller an d driver chips, the internal
I.CD ground is vee (+5V). The volta ge
across the LC D equals the potenti al
di fference betwee n Ydd and vo. The
current thro ugh the Yo te rmin al Is very
small. so a 10 kG linear potentiomete r
is suffic ient
LC D contrast
If usin g an LCD module with an EL
foil hack -light, a suitable power supply
need s to he built. The J::L foil usu all y
requires a suppl y voltag e o f approxim ate ly 1I 0 Y at around 500 lI/" T h(~ EL
foil behaves electrically as a lossy
capacitor. T he vo ltage across its tcrminals affects the amount of light pro duc ed, whil st the frequency affe cts the
CO IOllf o f th e light.
Fig.58: Power Supply and Rese t PCB
(bottom view)
(~ -----------'-"'--"""'===~
Power Supp ly and
RN:l Ctreuu
Component O "cr lay
,,-: -
( PU· lloord
D~ P . 1' l atL ll ~
tcbee l
( u oh>n)
LCD Macli,..,
h .. - tcs tcto-
~ "'" A~""I'
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Slr tnl-
, l~oto
StrO lflwor s
fu r U O·
• '6~( l'u{ htunll
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Slrom v ~ sorgun9
ZF +Tell
Portable (IPS Receive r
An..,( ht von untl' n
HF. Tfl l
:\1odlllt" Loc ation
A suitab le power supply is shown in
Fig.54 and includes a power oscillator
(555), a step-up transformer and a few
EMI filtering components. For 500 Hz
operation a conventional mains transformer with a laminated core can be
used, either 220V19V or 220V/6V, of
course with the primary and secondary
windings interchan ged . A 1.2W or
Or aufs i(h t
,, ,,,
Synt hesizer :
r cu -e ccro
(ob ~n l
'---+ - - - - -~
,-- - 1-- --,,
OSP - P(llhne
(un ten I
Strom vers rgung I
und RES T
l unten )
LCD Mod ul e
fr ontplc tte
St rom -
v ers~
BN - Toslll tur
f ur LCO Be(euchlu ng
" "
Abs( hlrmung
Synt hesizer
ZF - 'lail
ZF - Teil
Portable GLONASS
Receiver Module
the load. Since the osc illator feedback
is taken from the out put (pi n-1 of the
5:~ 5 ) , some 555 IC:s may not operate in
a stable way in this circuit. In the latter
ea se the solut ion is to increase the
oscillator frequency hy de creas ing the
Pow er S up ply e nd Reset
C irc uit
rece iver is in-
tend ed to be operated from a 12V
battery, with the negative grounded.
This supply Voltage is common 10 all
portable and mobile eq uipment.
100nF capacitor.
Of course, an LED back -light is much
easier to me and usually only requires a
5C DC supply.
The ana logue circuits of the rece iver
arc already designed 10 operate from a
KEY # 4
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KEY # 5
( ;~f )
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KEY # 6
(CODE $ 35) (CODE S 35,
<, / '
C~ ': S ~) v:n
P'E;, I ' VE; '
sr - V A"
S E LE C ~
~' C [)E
(f .Hz STep S!
~ 'l :) G E 'l
SEt ev r o
SEL f C ~
(>';•• 62,
Receive r Status
+12VC supply rail , since thi s voltage
represent s a conven ient choi ce .
O f course, digita l circuits req uire a +5V
supply voltage, but bes ides the +5 V
ther e are other requirements. A GPS or
GJ D N ASS receiver should include a
real- time clock that operates even when
th e receiver is powered down . Similarly, th e almanac data including infor mation about the available satellite
orbits should he stored in the com pute r
memory whe n th e re ce iver is powered
Finally , since th e +SV po wer drain
am ounts to about one hal f o f the total
power drain of a C;!.lS or GLONAS S
receiver , the +5V supply re gul ator
should al so hav e a good effici ency,
especially in a portable rec eiver .
The requirements for the mic roco mputer power suppl y are th erefore the
sam e as for th e DSP computer published in l1J an d l2 1. The original DSP
computer power supp ly is however
about 10 times too large for this
appl icati on. so a scaled-down version is
shown in Fig.57. The latt er includes a
swit ching regulator from 12V to 5V, a
memory (clod) backup batt ery and a
very rel iable RES ET circuit.
The microcom puter pQlVer snpply is
built on a single-sided prin ted circ uit
hoard as show n in f ig5?; . The correspo nd ing component location overlay is
shewn in fi g.59 . A il of th e resistors,
diodes an d chokes arc insta lled hori zonta lly . All of the capac itors have a
Sm m pin lead spa cing.
Several mounting holes an d relat ed
pads are provided for different style
NiCad batt er ies.
(, PS/GLO NASS receiver
Module L oc a l inn
A (iPS or (jI.ONJ\ SS rec eiv er includes
borh low-level Rf signal ar nplif k-ation
an d processin g an d ve ry nois y di ~dtal
ci rcuits. so the module location has to
be sele cted car efully and some shielding is required in an y r use.
In the case of a (iPS or CiLON AS S
receiver operating as a peri pheral for
th e DSP co m puter, the Rf part o f the
receiver shou ld be built in its own
enclosure, whi le th e dedicated DS!'
hardware module is pl ugged into the
c om puter bus. O f co ur se, it is assumed
that the computer already has its own
shield ed enclo sure.
The GPS receiver Rf unit needs no
add itional internal shields among th e
thr ee modules: RF, IF co nverter and 1lam plifier. The GLONASS receive r RF
(~ ------------'-'-"-'~"""'''''-'-'''''''''-'''''''
unit is more ror npliru tcd and rcquircv
shielding . i ll particular. the ( ;1.0 ·
Ni\ SS 1'1.1. synthcciscr log it, need s 10
be wel l shie lded from the rema ining
mod ules: RF. IF co nverter. IF am plifi er
a nd 1'1.1 . conven er.
.~ OIll C
In t il t' ca cc o f a stand-a tone porta ble
c;flS or (iI.O NASS rccc i..-cr it is of
c rutrs c' dcvirable (0 ha ve th e com plete
rece iver packa ged in one single cn closure . It i.~ sugge sted that th is encl osure
id fabrica ted from unpainted alumi nium
shee t to have a goo d electrica l cont act
amo ng the va rious parts. Such a con .
lain cr is made o f a rect angular frame
and two ( 'O VI,.'rs installed with wlflocld ng S(' f l' WS .
The (ram " has an addit ion a l inte rna l
pla te that div ide s 11K' inte rna l volume
into two sections, shielded beth from
th e outside an d each other. One of nrc
sect ions is used for the noi sy digit;l!
circui ts and the other lor the low -level
Rr stages .
Non-I nterfering m odules, such a s the
keyboard and the power supplies, may
he install ed in e ithe r sect io n.
TIK' s ll1!gr st ~'d modu le loc at ion for il
po rtable (i PS rece iver is shown in
fi g.60, The sugge sted dimens ions arc
2rXlm m wide x ] oomm deep x 80m m
T he interna l plate is insta lled at a
height o f 30m m, so that the digital
sect ion has a volume of 200m m x
160mm x 50mm (top) and the analogue
secti on a volume o f 200mm x 160mm x
30mm (bottom), The internal plate ts
scre wed onto the frame on all four
sides with m any screws to ensure a
good elect rical shiel ding.
The two modules with (14- pin liurrx-atd
con nec tors arc insla lk d on :1 .\ 11011 hus
mother board with juct two fem ale con ncct ors with the correspon ding pins tied
together. The hns ca n he mad e hy
" ull ing a piece o f the DSI' com puter
bus hoard. or hy "imply i n sl ;l lh n l~ t wo
connector s on a piece o f a univ ersa l
hoa rd with hole s in a uniform 0 . 1 pitch.
The connections between the anal ogue
am i di)!it:ll uni ts do nOI requ ire
Ieodtbrough ca pac itors if they arc
ro uted ca refully . away from the scnslt i v (' or V ( ' ry noisy com ponents. SOIllCtimes it is ;11.'0 o f b enefit to addiuonall y
ground t he coaxial cable sbic klv whe n
crossing the internal scree n.
Th e suggested module 1IIl'31ion (or a
po rta ble (TJ -ONi\ SS receiv er is .• hewn
in Fi!!.6 1. T he sugpcstcd dimensions
are 24(hnm wide x ]60mlll deep x
xnmm high.
Th e internal p late is insta lled at a
height of 30m rn, so that the digi ta l
section has a vo lume o f 240rn m x
160rnm x 50rnm (top ) a nd th e a nalo~lI e
section 240m m x )(j()mm x 10lllm
( bouom ). As with till' GPS receiv er the
internal scree ning plate is screwed onto
th e frame on a ll four sides .
In add it ion, ther e is a small shie ld
between the RF module and the re maining mod ule, in the analog ue S~T ­
tion . On the other hand, in the (TI.ONASS receiver Ihe co mputer po wer
supply is installe d in t he digita l secti on,
toget he r with th e PLL synthcsisc r logic.
A list of PCBs and kia for this project
can hi' found on page-l v t of this issue
(References for this projec t can he
found overl eaf)
191UI. Research-and- Product ion
of Applied mechanics,
Ins titute of Space Device
Engineering. G I.nkmnux , Russ ia
As~ia tion
Matjaz Vidma r: "Oip:ilal Si!'nal
Proc ess ing Techn iques for Rad io
Amateurs , Pan -:!: Design of a nsp
Co m pute r for Radio Amateur
VHf Commu nicatio ns 1/9 1.
pp 2-24
12 1 Matjaz Vidmar: ~t)jJ! i la l Signat
Pr rcc....in!! Techniques fOT Radio
Amateurs. Part -S: Co nstruc tio n and
use of the
Co mpute r";
VflF Com munications 2/Hf),
fir 74 .94
131 Jonathan S. Ahd. James W.
Chat fcc : "Ex iste nce and
Uni queness o f CPS So lut ions";
ra pes 952-9SfJlf)-9 I , Vol. 27 IEEE
T ram. o n Aerospace and
Electronic Systems
(4) "Interface Control Docu ment
MII0 8.f lOfIU2-4{)fI. rov -E". (84
pages). Aup:U<;f 7th. 1975.
Rod.well Internationa l Co rp oralion. Space Division. 12214
La kewood Bo ulev ard. Downey .
California 90 241. liSA
151 "lmerrecc L'omro l Document
Gl'S-2 IJO", ( 102 pa~e~ ) , I\uj!u~t
71h, 1975,
Roc kwel l Internat ional Co rporalion, Space O pcra tiom and
Sate llite Syste ms Div ision, 12214
La ke wood Boulev ar d, Do wney,
Californ ia 9024 1. lJSA
~( ; I o ha l Satel lite Nav igat ion
System ( if .O NI\ SS Interface
Contro l D ocument" (46 pages).
(7] Robert C. Oixon: "Sprea d
Spec1J1Jm SYQcms", (4 22 pages),
1984, Second Edition, John Wiley
& Sons, New Yori:, USA
181 Malj:17. Vidma r: "(li ll.iu l Signal
Processmg Tech niques for Radio
Amateurs, 'Ih coreucal Part ";
VI Tf Com munications 2/88
pp 76-97
P. Matt os: "(i loha l Pos itio ning by
Sate llite", (16 pa ges ), lnmos.
Tec hnica l note 6.5, July 1989
IIOJ J. D. Th omas: "funct iona l
Descriptio n o f Sif!: na l Processing in
the Rogue UPS Receiver". (49
pages), June I, 1988, Jet
Propulsion La bora tory , Ca liforn ia
lnstir urc o f Tec hnology. Pasadena,
Ca lifornia, USA
1111 Chou b C. Kilgus: "Sha f'l.-'\I
Conica l Radiat ion Pattern of the
Backfire Quadrifitar Helix ", (pa ge."
391·397), IEEE Tra nsacti ons on
Antenna.. and Propagatio n, May 75
(121 Matj;u: Vid mar. "II. Very Low
Noise Amplifier for the I.-nand ";
VIIF Co m munica tions 2/92
pp 90-%
(13] M3tja/. Vidmar: "Rad io Ama teu r
A pplicatio ns o f G PS KiI .ONI\SS
Sa telli tes: Using G I'S,K;LONASS
Sa te llites as an Accurate
Fre quency / Time Standa rd";
(pages 186· 190 ). Scriptum del'
vonraege, 37 We inhe imer UK W
Tagung , 19/20 Se ptember 1992
- - -- -- - - -----""--==='-'="-==
Matjaz Vidmar, S53MV
A DIY Receiver for GPS and
GLONASS Satellites
Part-7 (conclusion)
In l his the Final part of th is project
the Receiver Software is desc ribed,
d<'1 l1il ing t he Real-Time 11lSk", Main
Progra m-Loop tasks and th e Sortware Menus and Comma nds.
GPS/( ; I, O ;-.JASS Receiver
Software Overview
Satel lite nav igation is one of the first
applications that totally depends on the
ava ilability of suitable com puters and
the corresponding softwa re. Although
initially the digita l co mpute r was only
intended to solv e the nav igation equation s, other tasks were being gradually
adde d to simplify the hardware in front
of and behi nd the comput er itself. In
the GPSjGLO NASS receiver described
even mo st o f the signa! pruccssi ng is
perform ed in soft ware, just to keep the
analogue front-e nd and dedicated DSP
hardware as simple as pos siblc .
The softw are running in a GPS or
GLO :'-lASS receiver is therefore very
comp lex and includes a variety of very
different functions . For exam ple. dig ital
signa l processing requires qu ick hut
simpl e integer urlthrnctic, while so lving
the navigation equat ions requir es high
a(TUraey floating-point arith metic . The
latter docs nut need to he as quick as
the forme r signa! processing, hUI a
con siderab le num ber of ope rations still
nee d to he performed in a limited
amount of time.
To ma ke a fair comparis on one shou ld
consider the dev elopment time for the
hardw are and for the softwa re.
In the cas e o f the navi gati on rec eiver
described her e, th e software re quired
between tw ice and three time s as much
time to develop than the hardwa re!
t .ntortu narel y. it Is much more diffi cult
10 descri be the software down to the
smallest detai l than it is the hardware ,
For the hardw are one can draw the
ci rcu it diagram s and prepare det ailed
parts lists, On th c other hand, detailed
de scriptions of the software tend to
become boring and min or detail s tend
to hide the real problem being. solved .
Therefore, only the major functions
perform ed by the so ftware will be
described in th is article. The se include
signa l acquie ui on and proce ss ing. almanac and precision ephem eris data co llccnon, time and frequency measnrc mcnts, so Jvi n ~ the nuvig arion eq uations
and data display in a suita ble format for
the m er. At the e nd the user inter face .
displ ay me nus and use r commands will
he desc ri bed in det ail.
The ove rall software is writte n in
diffe rent langu age s due to the differing
functi ons to be performed : M('MW \ O
avwmbly languag e, DSP comput er
high-level langua ge an d even directl y
in m achine rod c.
Th c digi tal signal pron' ~ sm g so ftwa re
is wriucn in the MC68 0 10 assembly
language . Till' COlTcspe nding fill' has
th e ex tcnsiou .ASM. This file is fir st
co m piled into mac hine code and then
into hex adecimal forma t, <;,0 t hat it can
be eas jly insert ed in the DSP com pute r
hig h-le vcl Ianguugc.
The orbital m ech anics and nav igation
equat ion pan i ~ written in the ])S P
co mp ute I high-l evel lang uage . The latter sup ponc :I floating -point forma t with
a 32- t'li t mantissa and l o-bit exponent.
A 1 2-hit mantissa is gene rally sufficient
considering the accu racy of the da ta
obtained from the GPS or G1.0~ A SS
sate llites . The corresponding file has an
ex tensio n ,SRC an d ca n be compiled
into a .EXE file and executed on a I1SP
co mp ute r equip/X'd with the described
dedicat ed DSP hard ware board, hut
with an un mod ified CPU board!
In a portable G PS/G I,O NASS rece iver
all of the software is stored in a
27C25 6 EPRO :..1. T he latter incl udes a
start ing program . the high- level tan gnage compiler and a ve rsion of the
,SRe file with all of t he comments an d
Ulilll'cl's sary symbols remov ed .
Whe n the portable rece iver is turn ed 0 11
the pro gram is compil ed in the RA M,
Th is ope ration takes around 10 seco nds
and is necessary to save EP ROM s pace,
since the compiled program in 1I11'
RA \.1 take s around JOO kbyt cs.
The presen t discu ssion app lies to the
cu rren t software vers ions v l n (Gl'S)
or V39 (GLONASS) , Wh en r unni ng th e
so ftware on a DSP computer the type o f
d isp lay m ay he sel ected by the
·r U r A I. /P A RT IA I, RI ~SI ~T switc h
while sta rti ng the program : switch 0 (lC ll
(TOTAL RE SE T) selec ts the LC D,
while switch d osed (PAR T IAL R J ~ ­
SET) selects the C RT display . Of
course, the LC D can on ly be se lected in
a stand-alone portable receiver. and the
corresponding input 011 the CPU hoard
RCHI-Ti mc Tasks: Signal
Acq uisitio n lind Processing
The sign al acquisition and process ing
tasks run under the I "Hz interrupts
requested by the dedicated DSP hardware module .
(~ --------------'-"-"-"~~~~~
T he ta sks inclu de :
- Multiplex ing of the single-cha nnel
hard ware among four different satcllites,
- C/A -rode synchronisation acqu isition and track ing,
- Carr ier lock acquisition and track ing,
- T he 50bps navigati on-dat a dem odu lation.
Bit synchronisa tion and frame synchro nisat ion with purity check
- Averagtng of the me asured code
phase, code rate and carrier Irequent'y,
Aft er au interru pt req uest is received
fro m the ded icated DSP hardwa re. the
inrcrrupr-scrv icing routine will first
latch the contents of all the counters in
the ded icated hard ware . It will then
read th e latched cont ent and reset and
arm the interru pt request flip. flop, The
ha rd wa re counters are never re set. The
act ual integ rated value is computed
from the differe nce bet wee n [he act ual
co un ter content and the previous ....mpic content. The four difference s arc
furth er normalised us ing the result from
the re ference I ms div ider, Fi nally, the
inte rrupt -servicing routi ne also incrcrnem s :I 32-bit mil lisecond counter.
whi ch is later used ro re late the meas urements to the 50 bps navigation data .
Th e single-channel hardware is multiplexed tlmong all of the sate llites
recei ved. However, due to th e lim ita-_
no ns of the hardw are. switching. to
an other sate llite will co rrupt one m illiseco nd of da ta . T herefore. the basic
m ultiplex ing perio d incl udes one m illisecond to sw itch the hardware fo llo wed
hy 8 milli seconds to co llect the da ta
from a given sate llite , A fter th is l)
milli second period the hardware is
switched to another satelli te. The multi plexing rate is therefore 111 hops per
seco nd
The mul tiplexing rate and especia lly
the mu ltip lexing sequence have to be
chosen carefully. The navigation data is
tran smitt ed at a speed of 5Obps. so one
bit is 20Ins long and lasts exactly 20
interrupt peri ods, If the navigation data
is to be co llected from a given sate llite,
thcn this sa tellite should get at least a
few l rns samples o f data from each
20ms bit period. Further, the mu ltip lexing period shou ld not he an integer
subm ultiple of the hit per iod, so that
the hit transit ions can be detected .
Conside ring the limitations o f the singlc-channel hard ware, the mult iplex ing
sequenc e can not allow the collecting
of navigati on dat a Irom more th an two
satelli tes at a time. In practice, since
four satelli tes need to he received for a
navigation so lution. the navigation da ta
can o nly be co llected from a single
sate llite at a time usinp: half of the
single-c hannel hardwar e time. The re maining hardwa re time is sp lit among
the remai ning three satellite s. The
priv ileged satellite that gets more hardware time of cours e needs to he
periodicall y exchanged to allo w co llecting o f th e navi gation data from all four
sate llites.
There is yet another constraint on the
mu ltiplex ing seque nce. If a ce rtain
sate llite on ly pets a few sam pling
periods. then fa lse loc ks of the carrier
recov ery loop become vel)' likely . In
order to avoid this, the fo llowing
mu ltiplexin g sequenc e is used in the
C;PSjGLO NASS rc cc rvcrs llJ this
project (1 = privi legc d satellite; 2, 3 &
4 = others):
1212 131414 12131314
12121314 14 12131314
T he complete multiplexing sequence
th erefore repe at s afte r I S mult iplexing
per iods or 162 m illiseco nds .
Thc CjA -code synch ronisation is always obtained from the signa l magnitude obta ined from the ded icated hard ware . T he signal phase information is
inte ntionally not used for this purpos e
since ~h e ca rrier phase lock is a much
more cr itical oper at ion . Therefore, for
the CI A-code synchro nisat ion, the early
and late magnitudes arc co mputed from
the related I an d Q sum s for every lITI S
acc umu lation period. T hese sums arc
then averaged over the R milliseco nds
con tainin g valid data in a 9 m illisec ond
mu ltiplexing period .
The initial state of the re ceiver is
unlocked and the CjA ~co d e synch ro nisat ion has to be obtained first. The
hardwa re variab le de lay will therefore
be sca nned through all possible C/
A -code phases (1023 for GPS and 511
for GLONASS) by increment ing the
variahie -delay counter in suit able steps
(6 for GPS and 9 for GLOKASS).
W hen a sig nal m agnitu de abo ve
threshol d is de tec ted, the so ftware
switc hes to the synchronisation m ain197
(~ - --
following multiplex period. accounti ng
for all of the time spent by the
hardware processing the signals from
the other satellites. In a navigation
receiver the code phase and the carr ier
frequency are the main parameters to
be measured , and these arc supplied by
the corresponding phased-locked loops.
In addition to this, the code rate is also
used by the software to comp ute a
rou gh approximation for the carrier
frequency and eliminate the ambiguity
caused by the I kfIz signal samp ling
rate. Before further processing, the
cod e phase. code rate and carrier
freque ncy arc averaged over 16 multiplexing per iods correspond ing to a time
span of 2XKms (privileged satellite) or
S64ms (other satellites) . The ave rage d
measur ements are placed in a FIFO
mcmory together with I ms time tags to
be rea d hy the main program.
The last task performed by the interrupt
rout ine is navigation data processing.
The latter includes yet another I'LL for
hit synchronisatio n. Th is I'LL locks on
the transiti ons in thc data stream . The
demodu lated l ms samples containing
the tran sitions arc rejected, while all of
the other availab le samples for a given
sate llite arc acc umulated into hits
(GPS) or half-bits (the GLONASS
Manchester phase is nor known yet).
The following navigation data processing depends on the data format and this
is slightly differe nt between GPS and
GLONASS . The GpS data is formatted
into 30-bit words containing 24 true
data bits and 6 parity-check bits. The
word synchronisation is obtained by
checking the parit y bits, including the
last two bits of the previous word, for
any possible word phase. The BPSK
polarity am biguity is also resolved by
the parity bits. The synchron ised and
checked GPS data words are placed in
another FIFO memory together with
l ms time tags, to be read by the main
The GLONASS data is formatted into
lines with 85 data bits in Manc hester
form at and a non -Manchester sync
pattern, for a total dura tion corresponding to 100 bits. The sync pattern is not
used in the GLONASS receiver in this
project. The synchronisation is obtained
by checking the 8 parity bits for any
possible half-bit phase (200 possible
phases), to resolve the Manchester
phase ambiguity as well. Since the data
bits are different ially encoded, there is
no polarity ambiguity to be resolved.
Like in the GPS receiver, the correctly
received data lines are placed in an-,
other FIFO memory, together with 1ms
time tags, to be read by the main
Ma in Progr a m Loo p Tasks
Since most of the functions performe d
by the main program loop require
high-accuracy floating-point arithmetic,
the main program is mainly written in
the DSP compu ter high-level language.
Of course, all of the interfaces to the
interrup t routine and ( 0 the various
peripherals (initialisa tion of the dedicated DSP hardware, LCD drive and
the real-time dock chip) arc at least
part ially written directl y In the
MC6ROIO machi ne code and arc inserted in hexadecimal format in the
main program source code,
The ma in prog ram loop executes once
for every new set of avera ged meas ured
data. The latter is available every R64
milliseconds for the three satell ites that
gel less hardware time. The privileged
channel supplies three separate set s of
ave raged data in the same time period,
b ut the excess data is not used by the
main program loop.
T he ma in program loop also updates
the l.CD or writes a new line on a CRT
disp lay. The internal operation of the
program is howev er independent of the
selected menu on the display . The
men u on ly affects the keyboar d functions and some comp uta tions closely
related to the format of the displayed
data, like coord inate conversions.
T he Fi rst task o f the main program loop
is to write the look-up tables in the
dedicated DSP hardware mem ory. This
operation is done at rece iver po wer-lip,
whe n changing satellites. when adjust mg the carri er frequency (in 1 kHz
steps) or when switching the privileg ed
satell ite . The satellites can he selected
ma nually, but usua lly the software is
set to autom atically selec t v isible satellites.
When a given satellite is selected, the
receiver requires some time to lock on
its signal. The software will first look
for all possible CIA-code phases . If the
lock is not achi eved, the main program
loop will change the hardware look-up
table frequency in 1 kHz steps in a
given frequency range (20 kH z in the
GPS rece iver, or 25 kH z in the GLO NASS receiver). Of course, the look-up
table freque ncies for all four sate llites
can also be preset manually.
(f'.- -- - -- - -- - --""--"""=
= = =
sample to relate all of the data 10 a
sing le time point using linear interpo lation . T his simplifies the following computations, since the positions and veloci ties of all four satellites need to he
ca lcula ted for a single point in time.
velocity does not require a numerical
iterative method, since the equations
result linear for this unknown . The
computed ve locity vecto r is converted
into magnitude. azimuth and elevation
on the display .
'Inc measured data and the satellite
positions and velociti es (comp uted from
the precision epheme ris data) are the n
assemb led into the navigation equation s. A set of three time-difference
navigation equations is obtaine d from
code-phase d iffere nces and another set
of three Doppler-difference navigation
equations is obtained from carrierfrequency differences.
The accuracy of the navigation solution
depe nds on the geometry of the satellites. In place o f the GOOP the software only computes the determinant of
the linea rised system of equations at the
calcu lated position. Th is dete rminant is
a dimensionless quantity. The higher
the de terminan t, the more accura te the
solution. If the determi nant is too low,
an error conditi on is signalle d . The
Dopple r-difference equations have the
same determinant if solved for velocity.
The time-d ifference navigation equa tions arc solved first, using the Newtons
met hod . The starting poi nt is taken in
the Ea rths ce ntre (x=y=z=O). f rom this
starting point the Ne wtons met hod
n..squire s octween three and fou r iterations to converge to the final result for
a user located on the Earths surface.
The result in Cartesian coor dinates
x ,y,z is then converted to longitude,
latitude and height.
The ma in program loop also performs
data averaging. Roth position and velocity arc averaged. Only good data
with no erro r I signalled is added to the
average . Th e determi nant of the system
of equations is used as a weight for
each new data set added ( 0 the average .
Of course, the averaging buffer may be
manually reset if desired.
Th e position obtained may now he
co rrected for the propagation anoma lies
in the ionosphere and troposphere. The
prese nt softwa re does not app ly any
correction for the ionosphere . T he navigation equations arc only corrected for
the troposphere at the calculated heigh t
and the Ne wtons me thod is iterated
once aga in to obtain the final result.
The display incl udes several different
menus and I W O of them arc devoted to
the immediate and averaged data. The
position may be displayed in different
formats: degrees on ly, deg rees, min utes
and secon ds or Gauss-Krueger rectangu lar grid . Other menus are used to
show the recei ver status and the almanac data.
Since the pos ition is already avail able
from the time-d ifference navigation
eq uations, the Doppler-d ifference navigation equations are solved ' to obtain
the velocity of the user. Solving the
Doppler-difference equations for tbe
Finally, the main program loop usually
also performs an automa tic satellite
select ion. This function is triggered if
an error condition is signalled continuous ly for a certa in pcriod of time (100
main loop s). The soft ware then uses the
almanac data and the real-time clock to
find the visible sate llites at the ave raged user location. The rece iver is then
programmed for the four visible sarellites with the highest elevations in the
sky. Although this procedure docs not
yield the best GooP its operation is
SohW3Tt" Menu.s a nd
Co mma nds
Since a portable GPS/GI.ONASS receiver only has a small keyboard with a
few key s and a smal! alphanumeric
display. the various user commands
need to be arranged into several different men us.
Th e keyboard has eight 'd ifferent keys,
four o f them (eorrcspondmg to ASC II
c haracters 4, 5, 6 and 7) arc used to
se lect the four main menu s. Depre ssing
these keys only c hanges the content of
the di splay and the functions of the
other keys, but does not affect the
interna l operation of the (;I 'S /(jLO NASS rec ei ver. Some of these keys
have additional functions if de pres sed
more than once. Depressing key 4
cyclica lly shows a ll four virtual reccivcr c hannels (satellites) on the d isplay. Depressi ng key 7 cyclica lly shows
the genera! receiver status and the
almanac data for all currently visible
sate llites,
The remai ning keys (ASCn 0, I , 2 and
3) arc ca lled paramet er keys. Depressing these keys affects the interna l
ope ration of the G PS/GLO NASS re ce ive r as a function of the current
menu. The mode of operation of these
keys is a lso dependent on the actua l
In order to und erstan d (he commands of
a GPS/GLONASS receiver it is necessary to understand the internal opera tion of the latter. 1\ GPS/CiJ.ONASS
receiver includes a non-volatile RAM
to store the almanac data and a realtime clock that arc a lways powered by
a small internal Ni(:d battery. The
non-volatile RAM is used to store the
almanac data and the ap proximate user
position 3 !O a ' result of a previous
receiver ope ration. At power-up this
data is used together with the rea l-rime
clock data to find a ll v isible satellites
and speed-up the acquisition of four
usable satellite signal s.
When a GPS Of GLONASS rece iver is
first powered lip, all of the non-volati le
RA M conta ins random data ami a tota l
reset is required. The total reset erases
all a lmanac data and pUIS the receiver
in the mannaI satellite select mode. All
receiver virtual channels are set to a
centra l carrier frequency and a GPS
PRN# 16 or GI.ONA SS ClI N#13. The
menu 4 is selected to show the virtual
channel # 1 data ,
The initial satell ite signal acquisition
without any almanac data may take a
large amount of time. especially in a
single-channel rece iver. l b c rece iver
20 1
(~ ------------""--"""""""="""'~
doc s not know which satellite to look
for not its frequency offset caused by
th e Doppler shift and by the unknown
frequency drift of the receiver itself.
The current so ftware for the UPSj
GL ON ASS receiver in this project is
not abl e to select dif ferent satellites
a utomatically witho ut any almanac
data, so this has (0 he do ne manually .
After manuall y selecti ng the satc llitc(s).
the software is goin g to try to achieve
CIA-code lock. If the latter docs not
occur on the given IF frequency, the
receiver is going to scan the expected
IF frequency range in 1 kl-lz steps by
writing the corre sponding look -up tabl e.
On ly the privileged satellite IF is
scanned in the range from 23 10 kHz to
2330 kllz (C PS) or from 1675 Id17- to
1700 k Hz (GLONASS).
W hile searching for the initial signal
acquisition there is a small di fference
between the errs and ( il.O NASS reccivcrs. The errs constel lation is now
complete and more than four visi ble
satellites can he found at any time, so
the ( ;PS rec eiver is only goin g to
switch the privileged channel after a
sate llite signal is acquired . On the other
han d, the (;LONASS co nstella tion is
not complete and someti mes there is
just one visible satellit e, so the GLONASS receive r is going to try a
diffe rent virtual channel with a di ffer cut satell ite if the current priv ileged
satellite was not ac quire d.
After a sate llite signal has been ac quired. the key 4 m enu shows th e most
impo rtant receiver parameters :
virt ual channe l num ber
GPS satellite PRN code
GLONASS satellite RF channel
look-up ta ble preset centra l IF
frequency (UII-)
mea sured IF frequency (kIT:!""
from code rate)
signal level (S -meter)
satellite health flag (O=OK)
GPS mer range accuracy (m)
GPS anti-spoofing flag
GI .ONASS ephemeris upload
»sc (days)
GI.ONASS ephemeris age (S)
If the rec eiver has been turned on for
the first lime, or has not been used for a
considerable period of tim e (more than
one week), the best thing to do is to
collect the complete almanac da ta first.
To speed-up the almanac data collection it is recommended thai the remaining three receiver virtua l channels are
set to the same satellite and to the sam e
IF frequency as the channel that already
acquired a satellite. After all four
virtual channels achi eve data lock, the
a lmanac data collection takes 12.5
minutes for (iPS or ;) minutes for
GLONASS, since the (jUJNASS rece iver doe s not make use of half o f the
almanac frames .
The accumulated almanac data can he
checked by repeatedly pressing key 7.
The almanac data includes the satellite
name/number pl us the following inform ation:
C H#:
AZ :
GLONASS Rf> channel number
elevation (degree s)
azimu th (degr ees)
Doppler frequency shift (lIz,
po larity as in the IF)
G PS almanac/precision
ephemeris data
satellite almanac health flag
(G PS O=O K;
GPS anti-spoofing and
configuration flag
UTe da le and time
The elevat ion, az imu th and Doppler
shift arc comp uted for th e reference
user po sition as obtained from the
averaged data , Afte r a tota l reset this
reference pos ition is set in ce ntra l
Europe. Checking the almanac it is
poss ible to lind out whe n the receiver
can be switched to automat ic satellite
selectio n. In the automatic sate llite
selec tion mode , the program selects the
four sate llites with the highest el evations and sets the corresponding cen tral
IF values in th e hardware look-up
table s. This selection ca n be done
imm edi atel y hy pressing key I) in me nu
7, or hy switch ing th e prog ram to do
th is automatically after a period of had
data (key 3 in menu 7).
After four satellites have been acquired
the dis play m ay be switched 10 men u 5
or 6 to show respectively the immediate
or averaged data.
These menus show the fo llowing:
determ inant of the system of
equ ations (menu 5 only)
A VG: averaging weight (menu 6
velocity vector ma gnitude
(kmJh), azimuth and elevat ion
LAT, latitude (degrees or n. north)
LO'<, longitude (deg rees or m, east)
height abov e elli psoid (m)
The GPS and GLONI\SS systems are
m ain ly inten ded for navi gat ion, but
there are m any ot her less ad verti sed but
not less impo rt ant, not les s interesting
ap plica tions of these system s. Since
these systems arc known and the technology 10 use them is available to
anyone, we rad io amateurs should consider our own applications of these
systems (131.
Althoug h the nav igation itself is not of
much interest to radio amate urs, it
would probably make more sense to
trans mit GPS or GLONASS coord inates
o f a contest location, rat her than the
inaccurate EU or WW locato r, which is
already not accurate enough for serious
microw ave or laser comm unications.
By the way. (iPS and GLO NASS usc
almost th e same co ordinate system and
a long time avera ge shows differences
of the order of only 10m between the
A side prod uct of both UPS and
GLO NA SS is accurate time and frequency broadcast. In order to achieve
the spe cified nav igation accuracy the
liming measurements have to be performed to an accuracy of about 10m .
The same requiremen t applies to the
on-board satellite atomi c clocks . The
final user time transfer ,lCt'uracy ranges
bet ween 30ns and lOOns, dependin g
also on the knowledge of the exact user
loca tion. Th us the user shou ld also
com pute his position even if he ouly
needs accurate tim e
Radio amateurs could use this time
transfer capability of both (iPS and
GLO NASS every time when acc urate
synch ronisation is required. Coherent
comm unications are j ust an examp le,
the accuracy of GPS or GIDNASS
o ffers more tha n thi s. f or exam ple, the
actual propag ation path of the radio
signal and the propagation mechanism
could be inve stigated in this way.
Th e frequency broadcast accura cy of
both GPS and GLON ASS is in the
range of 10-12, far better than can be
achieved with HF or LF standard
frequency transmitt ers. The accuracy of
the latter is limited to around 10-7 by
the propagation effects alone, and this
is not enough for serious microwave
work. GPS and GLO~ASS arc also
availab le globally 24 hours per day and
arc not limited by the transmitter range.
Fina lly, GPS and ULONASS represent
a step away from being just an operator
of black-box amate ur radio equipment.
Alt hough there arc several ready -made
GPS receivers on the market we will
probably have to develop our own
rece ivers for om exper iments, both the
hardware and the software . Building
such a receiver may be an intere sting
challenge as well. The receiver shown
in this article is perha ps ju st the first
step, other related projects or better
receivers will follow soon.
Thi s completes this constructio n
project . although the editor [eels certain
that furthe r articles on the subject in
general and this project in panicluor
may appear in futur e issues.
11 complete list of all literature references fottows on the next page.
11 complete list of kits and printed
circuit boards fo r this project appears
on page-255 oj this issue.
Matjaz Vidmar: "Digital Signal
Processing Techniques for Radio
Amateurs, Part-2: Design of a
DSP Computer for Radio Amateur
VHf Communications 1/91.
pp 2-24
(2J Matjaz Vidmar: "Digital Signal
Processing Techniques for Radio
Amateurs, Part-S: Construction
and use of the l) SP Computer";
VIIF Communications 2/89,
pp 74 ~94
(3J Jonathan S. Abel, James W.
Chaffee: "Existence and
Uniqueness of GPS Solutions";
pages 952·956/6.91. Vol.27 IEEE
Trans. on Aerospace and
Electronic Systems
f41 "Interface Control Document
MHOR-00002·400, rev.E ", (84
pages), August 7th, 1975,
Rockwell Internat ional Corpora.
tion, Space Division, 12214
Lakewood Boulevard, Downey,
California 90241, USA
"Interface Control Document
GPS·200", (102 pages), August
7th, 1975.
Rockwell Internation al Corporation, Space Operations and
Satellite Systems Division, 12214
Lakewood Boulevard, Downey,
California 90241, USA
"Global Satellite Navigation
System GLONASS Interfac e
Control Document" (46 pages),
Tips. Improvements and Corrections
DIY Construction of a Receiver
Satellites by Matjaz Vidmar
S53MV; Issues 1/94 to 4/95
For those having d ifficulty obtain ing a
suitable display modu le, the Philips
LH ~ 244F-90 (with LED rea r illumination) can be used.
In diagrams 20 and 28 (1/9 5) some
minor errors crept in and are corrected
in the d iagrams overleaf.
In Fig.38 (2/95) the identification of L2
is missing on the IOOJlII ind uctor next
to rc CA30IN .
In FigA2 (2/95) the identification o f the
diode HSCG l OO l is missing, it leads
tlway from pin- t v of the I>TACK Ie
74 HC245 .
10 Fig's.a l and 42 (2/95) and 46 and 47
(3/95) the log ic gales are incorrectly
drawn. Corrected diagrams are shown
Also in Fig,46 (3/95) the crystal is
shown as 10 MHz instead of 12 \i llz.
Vt.CI O'Ir""
~HI . , ,,,- .« ,..t u. ,..~
~' 01.
..,.. e,
Flg .26 GLO:'llASS Rf Module
l!Jrv r
Below :
, )
Fig's:41 and 42
, ) , GLONASS nSf
Hardware par
FigA6: CPU Roam Circuit () i:.lgrdm {pa r t-I)
. . .. ....,
<...t.-.u..HU i 1JO
.. .... w
~ !ttQ
" , •
HD ~~
n (lllli)
- .'.
!I,v ..r
Fi2.47: CPU Hoard Circuit Diagram (part-2)
(f' - - - --
- --""'--""""=
= """-"=
Motjaz Vidmar, S53..\ IV
Hardware and Software Update
Since the publication o r the series of
articles about the Gl' S!(;!.ONASS re-
ceiver in UKW-RerichteIYHF-Communlcatio ns there have been a few modifications of the hard ware and softwar e of
these receivers. The original articles
descri be the opera tion of the G PS
softwa re Yi n and GLON ASS software
V39. The current update describes the
new DPS software V 125 and 0 1.0NASS software
V42. The new (, PS V1 25 and (lLONASS V42 inc lude th e follo wing mod ifications:
(1) Improved interna l ope ration of the
software. The new software is able to
handle the overflows of the hardware
counters co rrectly thus almost elim inating the occurrence of the "T" error.
(2) Additions to the comma nd set:
(2.1) In menu #5, key #0 will shift the
privileged RX chann el.
(2.2) In menu #7, key #0 has a new
function: in AUT mode it operates as
before while in :MAN mode this com252
mand sets the carrier frequencies o f the
curre nt ly sele cted satellites.
(2.3 ) In menu #7, key #1 has an
addi tiona l function : the receiver \vilJ
display the Kcplcr ian elements of the
GPSiG LONASS satellites as decod ed
from the almanac data before entering
the total RESE T sequence.
(3) 1\ simple b i-directio na l R. S-232 interface is included. requ iri ng only a few
add itional hardware components to be
installed in the recei ver.
(3.1) The RS-232 interface output circu it is shown on r ig . I. Because of
hardware limitations, the bit rate ca n
on ly be set to 1000b ps. The output data
format is a serial async hrono us tran smission inc luding a start bit, 8 data bits, no
parity and one stop h it. The output
signal level ranges from OV to +5V
only, although these levels are usually
accepted by most RS-232 receive rs. The
signa l polarity is inverted as usual in
RS-23 2: OV represents a logical " 1"
while +5V represents a logical "0" . The
data output matches the LCD disp lay
content, the display cle ar co mmand
being replaced by a CR 'L F combin atio n.
(3,2) The RS-232 interface input circuit
is shown on Fig,2, Because o f hardwa re
limitations, the hit rate can only he set
to IOObps or 10 limes slower than the
output rate. The RS-232 input can be
used to issue com mands ide ntical to
those co ming fro m the 8-key keybo ard.
Only A SCII characters " 0" , " I", "2" ,
"3", "4" , "Y', " 6" and " 1" are therefore
accepted as valid commands. All othe r
codes are simply igno red. The data
format is 8 b its. no parity. one or more
stop bits. The signal po lar ity is inverted
as usual in RS·232.
Since the PC.'6 input is now used for the
RS·2 32 interface input, it can no longer
be used to select the disp lay type, C RT
Fi~, 1 :
RS-H ! In terrace
Ou tput Ci rc u it
' ('1~ "~11 U
0011 )
GPs, / G LO N "S ~
__iJ:<:' ~ ~:~~!t. ~o~ ,_<t _J
.... - - - - , - - - - - - ,
, "
-- - ---- Fi:.:.2:
n.S-HZ Inte rface
Input C ircuit
6P5/ GL O NA~~
CPU bOo.rJ.
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