finding gps coordinates on a map using pda

Applied Methods
FINDING GPS COORDINATES ON A MAP USING PDA
Nicolae-Iulian ENESCU 1
PhD Candidate, University Assistant, Computer Systems and Communications Department,
Faculty of Automation, Computers and Electronics
University of Craiova, Craiova, Romania
E-mail: nenescu@cs.ucv.ro
Abstract: The purpose is defining algorithms for the elaboration of GPS coordinates using the
PDA. The C++ and C# programming languages have been used for this. A software was
elaborated which: runs on PDAs that have Windows Mobile 2003 OS, assumes GPS
information and implements algorithms for establishing the coordinates on a map represented
as a TIF image. The precision for the coordinates is less than 10 meter. The program was
tested and used by a Japanese Company.
Key words: PDA; GPS; coordinates; map; algorithms
Requests
A software application was desired that should run on PDA with GPS antenna
attached. The application must locate the GPS coordinates on a map from Japan.
The coordinates known by longitude and latitude must be established over a given
image in TIF format which can represent a map with scale 1:200000 or 1:25000. The maps
contain information in their headers that refer to the map limits. This data includes the lefttop and right-bottom coordinates of the corners, the dimensions of width and height in pixels
and the lengths of the maps. It was necessary an algorithm that would also allow map
synchronization when the user goes beyond the limits of the current map.
Informatics solution
As a result of many PDAs (on the Japanese market) having Windows Mobile OS,
the informatics solution for implementing the requests was to use .NET CF C# with Microsoft
Visual Studio .NET 2003. Due to .NET CF having some limitations in loading a TIF image, a
dynamic library for the module that loads and saves a TIF image was built in C++ with
Microsoft eMbedded Visual C++ 4.
Because the program needed additional functions that are not in .NET CF there was
used an application framework named “Smart Device Framework 1.2” which enriches and
extends the .NET Compact Framework. There are many new class libraries and controls,
along with all the existing class libraries and controls, available free on www.opwnnetcf.org.
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Algorithms
Map algorithms
Algorithm for Map Index
200000 and 25000 maps structure is mesh type. It is called “Standard Area Mesh”.
This is the standard way to divide an area by constant intervals of longitude and latitude.
In the case of Japan, the latitude and longitude used since 2002 are WGS84 type,
but the map data stored in the application map is Tokyo Datum type. The “Standard Area
Mesh” has 3 levels.
1) The 1st Area Block (Primary Area Division or the First Mesh)
The Primary Area Division is made by dividing Japan into rectangles with the width
of 1-degree longitude and height of 2/3-degree latitude (1º X 40’).
The 1st Area Block has a correspondent area of 6,400km2, which is the area of 1
unit from 1:200000 map. Each unit has a correspondent 1:200000 map image.
The mesh code for the 1st area is 4 digits long. The value is composed from 2 parts
of 2 digits each.
The first 2 digits are obtained by multiplying the south edge latitude degrees with
1.5.
The next 2 digits are obtained by subtracting the west edge longitude degrees with
100.
2) The 2nd Area Block (Secondary Area Division or the Second Mesh)
Dividing the 1st area division vertically – horizontally by 8 makes the 2nd area. Each
unit is 7’ 30” X 5’ and corresponds to 1 unit of the 1:25000 map, which has an area of 100
km2.
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The mesh code for the 2nd area division is a 2-digit code. The first digit is numbered
from 0 for south edge to 7 for north, along longitude line. The second digit is numbered
from 0 to west edge to 7 for east edge, along latitude line.
3) The 3rd Area Block (Third Area Division or the Base Mesh)
Dividing the 2nd area division vertically – horizontally by 10 makes the 3rd area.
Each unit is 45” X 30” and it corresponds to an area of 1 km2.
The mesh code for the 3rd area division is a 2 digits code. The first digit is numbered
from 0 for south edge to 9 for north edge, along longitude line. The second digit is
numbered from 0 for west edge to 9 for east edge, along latitude line.
The 3rd Area Division is called the “Base Mesh”.
Algorithm for map management
The major concern is that there are irregular maps and the maps have a
trapezoidal shape instead of a rectangular one as we might expect. The first step towards a
good map joining quality is to adjust the maps’ shape.
The program must cut images exactly on the edges of the map. The problem was
that the images are not rectangles, and the information about the corners is not accurate. In
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order to solve those problems the application must find the real map corners and must
transform the maps in rectangles using affine transformation.
Real corners
(75, 75) – map info
(77, 77) – real corner
every
In
map
theory,
image
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should have a 2-pixel width boundary, gray colored. Unfortunately this is not always true.
There are many cases when the boundary line is missing, or is only 1-pixel width or the color
is not gray.
The program searches the real corner by checking the color of pixels.
The two arrows show the direction of searching. Every pixel, starting from the file
boundary, is checked to see if it has a color different than Black (text color), Brown (heights),
White (none).
There are 2 directions: one for finding the “X” coordinate and the other for finding
the “Y” coordinate. The same procedure is used for all the 4 corners.
There are a couple of conditions to be met in order to decide if a pixel contributes
to the real corner:
• the search is done separately on the line and on the column;
• if a pixel has the Gray color we assume that it is on the map border;
• if a pixel has a different color than Gray, Black and White, we assume that the
border is missing and that the pixel is where the map starts. The next 10 (10 is a
number experimentally issued) pixels’ color is checked to be sure they are image
pixels.
• if the distance between the default value and the found value is bigger than 15
pixels, than the default value is kept.
GPS Algorithms
This section provides guidance in the implementation of measurement processing
algorithms.
The discussions include:
− Mathematical constants used in GPS position determination computations.
− The GPS parity algorithm implementation to permit the user to detect demodulation
errors within the decoded navigation message.
− Interpretation of the satellite transmitting URA parameter.
− Satellite position determination using broadcast ephemeris parameters.
− Correction of the code phase time received from the satellite with respect to both
satellite code phase offset and relativistic effects.
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−
−
−
Compensation for the effects of satellite group delay differential.
Correction for ionospheric propagation delay.
Performing time transfer to UTC.
Mathematical Constants
The speed of light used for generating the data described in the above paragraphs
is:
c = 2.99792458 x 108 meters per second
which is the official WGS-84 speed of light. The user should use the same value for the
speed of light in computations. Other WGS-84 constants the user is required to use for
satellite ephemeris calculations are:
μ = 3.986005 x 1014 meters3 / sec2
WGS-84 value of the Earth's universal gravitational
parameter
Ωe= 7.2921151467 x 10-5 rad / sec
WGS-84 value of the Earth's rotation rate
The sensitivity of the satellite's antenna phase center position to small perturbations
in most ephemeris parameters is extreme. The sensitivity of position to the parameters (A)1/2,
Crc and Crs is about one meter/meter. The sensitivity of position to the angular rate
parameters is on the order of 108 meters/semicircle, and to the angular rate parameters is
on the order of 1012 meter/semicircle/second. Because of this extreme sensitivity to angular
perturbations, the value of π used in the curve fit is given here. π is a mathematical constant,
the ratio of a circle's circumference to its diameter. Here π is taken as
π= 3.1415926535898
Parity Algorithm
The user must perform error detection of the decoded navigation data using the
parity algorithm equations provided in Table 1. Figure 1 presents an example flow chart that
defines one way of recovering data (dn) and checking parity. The parity bit D*30 is used for
recovering raw data. The parity bits D*29 and D*30, along with the recovered raw data (dn)
are modulo-2 added in accordance with the equations appearing in Table 1 for D25 . . . D30,
which provide computed parity to compare with transmitted parity D25 . . . D30.
Table 1. Parity Encoding Equations
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where:
d1, d2, . . . . d24 are the source data bits
the symbol (*) is used to identify the last 2 bits of the previous word of the subframe,
D25, . . . . D30 are the computed parity bits
D1, D2, D3, . . . . D29, D30 are the bits transmitted by the satellite, and
⊕ is the "Modulo-2" or "Exclusive-Or" operation.
User Range Accuracy (URA)
The URA reported in the navigation message will correspond to the maximum value
anticipated during each subframe fit interval with uniform SA levels invoked. Referring to the
decimal equivalent of the transmitted four-bit binary number as N -- with N a positive
integer in the range of 0 through 15 -- the accuracy value is defined to mean "no better than
X meters", in accordance with the following relationships:
− If the value of N is 6 or less, X = 2(1 + N/2),
− If the value of N is 6 or more, but less than 15, X = 2(N-2),
− N = 15 will indicate the absence of an accuracy prediction and will advise the SPS
user to use that satellite at the user's own risk.
For N = 1, 3, and 5, X is rounded to 2.8, 5.7, and 11.3 meters respectively; the
above relationships yield integer values of X for all other values of N. Using these values of X
the user may utilize a look-up table approach for interpreting the URA message.
Figure 1. Example Flow Chart for User Implementation of Parity Algorithm
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User Algorithm for Ephemeris Determination
The user will compute the ECEF coordinates of position for the phase center of each
satellite's L-Band antenna utilizing a variation of the equations shown in Table 2. Subframes
2 and 3 parameters are Keplerian in appearance; the values of these parameters, however,
are obtained via a least squares curve fit of the predicted ephemeris for the phase center of
the satellite's antenna (time-position quadruples; t, x, y, z).
a. Coordinate System
The equations given in Table 2 provide the satellite's antenna phase center position
in the WGS-84 Earth-Centered Earth-Fixed reference frame defined as follows:
ORIGIN = Earth's center of mass*
Z-AXIS = Parallel to the direction of the CONVENTIONAL INTERNATIONAL ORIGIN (CIO)
for polar motion, as defined by the BUREAU INTERNATIONAL DE L'HEURE (BIH) on
the basis of the latitudes adopted for the BIH stations**
X-AXIS = Intersection of the WGS-84 reference meridian plane and the plane of the mean
astronomic equator, the reference meridian being parallel to the zero meridian
defined by the BUREAU INTERNATIONAL DE L'HEURE (BIH) on the basis of the
longitudes adopted for the BIH stations***
Y-AXIS = Completes a right-handed Earth-Centered, Earth-Fixed orthogonal coordinate
system, measured in the plan of the mean astronomic equator 90 degrees east of
the X-axis***
* Geometric center of WGS-84 ellipsoid
** Rotation axis of WGS-84 ellipsoid
*** X, Y axis of WGS-84 ellipsoid
b. Geometric Range Correction
When computing the geometric range, the user will account for the effects due to
earth rotation rate (reference Table 2) during the time of signal propagation so as to
evaluate the path delay in an inertially stable coordinate system. Specifically, if the user
works in Earth-fixed coordinates the user should add (−ΩeyΔt ,ΩexΔt,0) to the position
estimate (x, y, z).
Application of Correction Parameters
In order to properly account for satellite clock bias and propagation delays, the user
receiver must perform corrections to observed pseudo range measurements. The pseudo
range is defined as:
PRmeasured = c(treceived - ttransmitted)
where
PRmeasured = measured pseudo range
treceived = time that ranging measurement was received at the user location
ttransmitted = time that ranging signal was transmitted from the satellite
Group Delay Application
The SPS user who utilizes the L1 frequency will modify the code phase offset with
the equation:
where TGD is provided to the user as subframe 1 data.
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Table 2. Elements of Coordinate Systems
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Satellite Clock Correction
The polynomial defined in the following allows the user to determine the effective
satellite PRN code phase offset referenced to the phase center of the satellite antennas (Δtsv)
with respect to GPS system time (t) at the time of data transmission.
The coefficients transmitted in the subframe 1 describe the offset apparent to the
control segment two-frequency receivers for the interval of time in which the parameters are
transmitted. This estimated correction accounts for the deterministic satellite clock error
characteristics of bias, drift and aging, as well as for the satellite implementation
characteristics of group delay bias and mean differential group delay. Since these
coefficients do not include corrections for relativistic effects, the user's equipment must
determine the requisite relativistic correction. Accordingly, the offset given below includes a
term to perform this function.
The user will correct the time received from the satellite with the equation (in
seconds)
t = tsv − (Δt sv)L1
(1)
where
t = GPS system time (seconds),
t sv = effective SV PRN code phase time at message transmission time (seconds),
(Δt sv) L1 = SV PRN code phase time offset (seconds).
The satellite PRN code phase offset is given by
(Δt sv )L1 = af0 + af1(t - toc) + af2(t - toc)2 + Δtr - TGD
(2)
where
af0, af1, and af2 are the polynomial coefficients given in subframe 1, toc is the clock data
reference time in seconds, and Δtr is the relativistic correction term (seconds) which is given
by
Δtr = F e (A)1/2 sin Ek
The orbit parameters (e, A, Ek) used here are described in discussions of data
contained in subframes 2 and 3, while F is a constant whose value is
F = -2 (μ)1/2 / c2 = - 4.442807633 (10)-10 sec / (meter)1/2
Note that equations (1) and (2), as written, are coupled. While the coefficients af0,
af1, and af2 are generated by using GPS time as indicated in equation (2), sensitivity of tsv to t
is negligible. This negligible sensitivity will allow the user to approximate t by tsv in equation
(2). The value of t must account for beginning or end of week crossovers. That is, if the
quantity t - toc is greater than 302,400 seconds, subtract 604,800 seconds from t. If the
quantity t - toc is less than -302,400 seconds, add 604,800 seconds to t.
Ionospheric Model
The SPS user should correct the time received from the satellite for ionospheric
effect by utilizing parameters contained in page 18 of subframe 4 in the model given below.
It is estimated that the use of this model will provide at least a 50 percent reduction in the
SPS user's RMS error due to ionospheric propagation effects.
The ionospheric correction model is given by
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F = 1.0 + 16.0[0.53 - E]3 and
αn and βn are the satellite transmitted data words with n = 0, 1, 2, and 3.
Other equations that must be solved are
The terms used in computation of ionospheric delay are as follows:
- Satellite Transmitted Terms
αn the coefficients of a cubic equation representing the amplitude of the vertical
delay
(4 coefficients = 8 bits each)
βn the coefficients of a cubic equation representing the period of the model
(4 coefficients = 8 bits each)
- Receiver Generated Terms
E elevation angle between the user and satellite (semi-circles)
A azimuth angle between the user and satellite, measured clockwise positive
from the true North (semi-circles)
φu user geodetic latitude (semi-circles) WGS-84
λu user geodetic longitude (semi-circles) WGS-84
GPS time receiver computed system time
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- Computed Terms
x phase (radians)
F obliquity factor (dimensionless)
t local time (sec)
φm geomagnetic latitude of the earth projection of the ionospheric intersection
point (mean ionospheric height assumed 350 km) (semicircles)
λi geomagnetic latitude of the earth projection of the ionospheric intersection
point (semi-circles)
φi geomagnetic latitude of the earth projection of the ionospheric intersection
point (semi-circles)
ψ earth's central angle between user position and earth projection of ionospheric
intersection point (semi-circles)
Universal Coordinated Time (UTC)
Depending on the relationship of the effectivity date to the user's current GPS time,
the following three different UTC/GPS-time relationships exist:
a. Whenever the effectivity time indicated by the WNLSF and the DN values is not in
the past (relative to the user's present time), and the user's present time does not fall in the
timespan which starts at DN + 3/4 and ends at DN + 5/4, the UTC/GPS-time relationship is
given by
tUTC = (tE - ΔtUTC) {Modulo 86400 seconds}
where tUTC is in seconds and
ΔtUTC = ΔtLS + A0 + A1 (tE - tot + 604800 (WN - WNt)), (seconds);
tE = GPS time as estimated by the user on the basis of correcting tsv for factors described in
previous paragraph as well as for ionospheric and SA (dither) effects;
ΔtLS = delta time due to leap seconds;
A0 and A1 = constant and first order terms of polynomial;
tot = reference time for UTC data;
WN = current week number (derived from subframe 1);
WNt = UTC reference week number.
The estimated GPS time (tE) is in seconds relative to end/start of week. The
reference time for UTC data (tot) is referenced to the start of that week whose number
(WNt). The WNt value consists of the eight LSBs of the full week number. The user must
account for the truncated nature of this parameter as well as truncation of WN, WNt, and
WLSF due to rollover of the full week number. The absolute value of the difference between
the untruncated WN and WNt values will not exceed 127.
b. Whenever the user's current time falls within the timespan of DN + 3/4 to DN +
5/4, proper accommodation of the leap second event with a possible week number
transition is provided by the following expression for UTC:
tUTC = W [Modulo (86400 + ΔtLSF − ΔtLS)] , (seconds);
where
W = (tE − ΔtUTC − 43200) [Modulo 86400] + 43200 , (seconds);
and the definition of ΔtUTC (as given in "a" above) applies throughout the transition period.
Note that when a leap second is added, unconventional time values of the form 23: 59:
60.xxx are encountered. Some user equipment may be designed to approximate UTC by
decrementing the running count of time within several seconds after the event, thereby
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promptly returning to a proper time indication. Whenever a leap second event is
encountered, the user equipment must consistently implement carries or borrows into any
year/week/day counts.
c. Whenever the effectivity time of the leap second event, as indicated by the
WNLSF and DN values, is in the "past" (relative to the user's current time), the relationship
previously given for tUTC in "a" above is valid except that the value of ΔtLSF is substituted for
ΔtLS. The CS will coordinate the update of UTC parameters at a future upload so as to
maintain a proper continuity of the tUTC time scale.
Implementation for the solution
The application is built around the Multiple Document Interface (MDI) specification.
The purpose of this application is providing the user a tool to interact with map images. The
images are TIF files and represent maps of all regions in Japan. There are two (2) kinds of
maps the application can work with. The classification is based on the map scale
specification and these are 1:200000 and 1:25000.
The information in the TIF file can have 2 types of organization:
- by strips (see figure 2)
- by tiles (see figure 3)
Both types are supported.
Figure 2. TIF Strip Organization
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Because the PDA memory is limited, not the entire TIF file is loaded in the main
memory. Only an image of 900x900 pixels is kept in the main memory. This image is named
“Working data”.
When the user drags the map outside of the “Working data” image, then this
image is updated with a new sub-zone of the TIF map.
When this happens, the hour glass cursor appears on the screen for a short time.
For each TIF file organization (strips or tiles) there is an algorithm to extract a
“Working data” sub-zone.
In the above picture a “Cij” element is referred to as the color element (pixel) at the
row i and column j in a certain strip. Li is referred to as the line(row) i in a strip.
A strip contains one or more lines of pixels. When a “Working data” covers a strip,
the entire strip is loaded from the file and then only the necessary zone is extracted from the
strip. A “Working data” zone can cover one or more strips – all of them are read from the
file. This leads to slow performance if the TIF file has a big width. For this reason, the TIF tile
organization is preferred because it is much faster.
Figure 3. TIF Tile Organization
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The tile number increases from left to right and from top to bottom.
Each tile has a fixed number of rows and columns. These parameters are
established when the map is exported. The “Cij” elements are referred to as color elements
(pixels) in a tile.
A working data can cover one or more tiles. Only the covered tiles are loaded from
the file and after that only the area of interest is kept in the main memory. The tile
organization is faster than the strip organization because less information has to be loaded
from the file at each “Working data” update.
In the program, for configuring the GPS and receiving GPS sentences it was used
the framework “Smart Device Framework 1.2”.
The next three figures represent the main screens of the program used for: set GPS
characteristics, display GPS information and find GPS coordinates on a TIF map.
Figure 4. Screen from application
used for GPS Settings
Figure 5. Screen from application used
for displaying GPS Information
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Figure 6. Screen for finding GPS coordinates on map
About PDA
Personal digital assistants (PDAs) are handheld devices that were originally
designed as personal organisers, but became much more versatile over the years. PDAs have
many uses: calculating, use as a clock and calendar, playing computer games, accessing the
Internet, sending and receiving e-mail, use as a radio or stereo, video recording, recording
notes, use as an address book, GPS and use as a spreadsheet. Newer PDAs also have both
color screens and audio capabilities, enabling them to be used as mobile phones
(smartphone), web browsers or media players. Many PDAs can access the Internet, intranets
or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). One of the most
significant PDA characteristic is the presence of a touch screen.
Touch screen PDAs, including Windows Pocket PC devices, usually have a
detachable stylus that can be used on the touch screen. Interaction is then done by tapping
the screen to activate buttons or menu choices, and dragging the stylus to, for example,
highlight text.
Text input is usually done in one of two ways:
•
Using a virtual keyboard, where a keyboard is shown on the touch screen. Input
is done by tapping the letters.
•
Using letter or word recognition, where letters or words are written on the touch
screen, and then "translated" to letters in the currently activated text field.
Despite rigorous research and development projects, this data input method still
requires much patience from the user since it tends to be rather inaccurate.
The currently major PDA operating systems are:
•
Palm OS - owned by PalmSource
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•
Windows Mobile (Pocket PC), (based on the Windows CE kernel) - owned by
Microsoft
•
RIM for the BlackBerry - owned by Research In Motion
•
Many operating systems based on the Linux kernel - free (not owned by any
company). These include:
• GPE - Based on GTK+/X11
• OPIE user interface/Qtopia - based on Qt/E Qtopia is developed by
Trolltech, OPIE is a fork of Qtopia developed by volunteers
•
Symbian OS (formerly EPOC) owned by Ericsson, Motorola, Panasonic, Nokia,
Samsung, Siemens and Sony Ericsson
Many PDAs run using a variation of the ARM architecture (usually denoted by the
Intel XScale trademark). This encompasses a class of RISC microprocessors that are widely
used in mobile devices and embedded systems, and its design was influenced strongly by a
popular 1970s/1980s CPU, the MOS Technology 6502.
About GPS
The Global Positioning System, usually called GPS, is the only fully-functional
satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts
precise timing signals by radio, allowing any GPS receiver (abbreviated to GPSr) to
accurately determine its location (longitude, latitude, and altitude) in any weather, day or
night, anywhere on Earth.
GPS has become a vital global utility, indispensable for modern navigation on land,
sea, and air around the world, as well as an important tool for map-making and land
surveying. GPS also provides an extremely precise time reference, required for
telecommunications and some scientific research, including the study of earthquakes. GPS
receivers can also gauge altitude and speed with a very high degree of accuracy.
Calculating positions
To calculate its position, a receiver first needs to know the precise time. To do this,
it uses an internal crystal oscillator-based clock that is continually updated by the signals
being sent in L1 from various satellites. At that point the receiver identifies the visible
satellites by the distinct pattern in their C/A codes. It then looks up the ephemeris data for
each satellite, which was captured from the NM and stored in memory. This data is used in a
formula that calculates the precise location of the satellites at that point in time.
Finally the receiver must calculate the time delay to each satellite. To do this, it
produces an identical C/A sequence from a known seed number. The time delay is
calculated by increasingly delaying the local signal and comparing it to the one received
from the satellite; at some point the two signals will match up, and that delay is the time
needed for the signal to reach the receiver. The delay is generally between 65 and 85
milliseconds. The distance to that satellite can then be calculated directly, the so-called
pseudorange.
The receiver now has two key pieces of information: an accurate estimate of the
position of the satellite, and an accurate measurement of the distance to that satellite. This
tells the receiver that it lies on the surface of an imaginary sphere whose radius is that
distance. To calculate the precise position, at least four such measurements are taken
simultaneously. This places the receiver at the intersection of the four imaginary spheres.
Since the C/A pattern repeats every millisecond, it can only be used to place the user within
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300 kilometers (180 mi). Thus the multiple measurements are also needed to determine
whether the receiver has lined up its internal C/A code properly, or is "one off".
The calculation of the position of the satellite, and thus the time delay and range to
it, all depend on the accuracy of the local clock. The satellites themselves are equipped with
extremely accurate atomic clocks, but this is not economically feasible for a receiver. Instead,
the system takes redundant measurements to re-capture the correct clock information.
To understand how this works, consider a local clock that is off by 0.1
microseconds, or about 30 meters (100 ft) when converted to distance. When the position is
calculated using this clock, the range measurements to each of the satellites will read 30
meters too long. In this case the four spheres will not overlap at a point, instead each sphere
will intersect at a different point, resulting in several potential positions about 30 meters
apart. The receiver then uses a mathematical technique to calculate the clock error that
would produce this offset, in this case 0.1 microseconds, adjusts the range measurements by
this amount, and then updates the internal clock to make it more accurate.
This technique can be applied with any four satellites. Commercial receivers
therefore attempt to "tune in" to as many satellites as possible, and repeatedly make this
correction. In doing so, clock errors can be reduced almost to zero. In practice, anywhere
from six to ten measurements are taken in order to round out errors, and civilian receivers
generally have 10 to 12 channels in total.
Calculating a position with the P(Y) signal is generally similar in concept, assuming
one can decrypt it. The encryption is essentially a safety mechanism; if a signal can be
successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS
satellite. In comparison, civil receivers are highly vulnerable to spoofing since a set of
navigationally consistent C/A signals can be generated using readily available off the shelf
signal generators. Even if the victim receiver incorporates RAIM features it will still "buy in' to
the spoofing signals since RAIM only checks to make sure the signals make sense from a
navigational perspective.
Accuracy
Best case. The position calculated by a GPS receiver relies on three accurate
measurements: the current time, the position of the satellite, and the time delay for the
signal. Errors in the clock signal can be reduced using the method above, meaning that the
overall accuracy of the system is generally based on the accuracy of the position and delay.
The measurement of the delay requires the receiver to "lock onto" the same
sequence of bits being sent from the satellite. This can be made relatively accurate by timing
comparing the rising or trailing edges of the bits. Modern electronics can lock the two signals
to about 1% of a bit time, or in this case about 1% of a microsecond. Since light travels at
299,792,458 m/s, this represents an error of about 3 meters (10 ft), the minimum error
possible given the timing of the C/A signal.
This can be improved by using the higher-speed P(Y) signal, assuming the same 1%
accuracy in locking the retrieved P-code to the internally generated version. In this case the
same calculation results in an accuracy of about 30 centimeters (1 ft). Since the P-code
repeats at 10.23 MHz, it has a "repeat range" of about 30 kilometers (20 mi). This explains
the terminology; when using the P-code, it was first necessary to calculate a coarse position
with the C/A code in order to determine how to line up the P-code with the internally
generated copy.
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Atmospheric effects. One of the biggest problems for GPS accuracy is that changing
atmospheric conditions change the speed of the GPS signals unpredictably as they pass
through the ionosphere. The effect is minimized when the satellite is directly overhead and
becomes greater toward the horizon, since the satellite signals must travel through the
greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough
location is known, an internal mathematical model can be used to estimate and correct for
the error.
Because ionospheric delay affects the speed of radio waves differently based on
their frequencies, the second frequency band (L2) can be used to help eliminate this type of
error. Some military and expensive survey-grade civilian receivers can compare the
difference between the P(Y) signal carried in the L1 and L2 frequencies to measure
atmospheric delay and apply precise corrections. This correction can be applied even without
decrypting the P(Y) signal, as long as the encryption key is the same on both channels. In
order to make this easier, the U.S. Government has added a new civilian signal on L2, called
L2C, starting with the Block IIR-M satellites. The first Block IIR-M was launched in 2005. It
allows a direct comparison of the L1 and L2 signals for ionospheric correction.
The effects of the ionosphere are generally slow-moving and can easily be tracked.
The effects for any particular geographical area can be easily calculated by comparing the
GPS-measured position to a known surveyed location. This correction, say, "10 meters to the
east" is also valid for other receivers in the same general location. Several systems send this
information over radio or other links to the receivers, allowing them to make better
corrections than a comparison of L1 and L2 alone could.
The amount of humidity in the air also has a delaying effect on the signal, resulting
in errors similar to those generated in the ionosphere but located much closer to the ground
in the troposphere. The areas affected by these problems tend to be smaller in area and
faster moving than the billows in the ionosphere, making accurate correction for these
effects more difficult.
Multipath effects. GPS signals can also be affected by multipath issues, where the
radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. This
delay in reaching the receiver causes inaccuracy. A variety of receiver techniques, most
notably narrow correlator spacing, have been developed to mitigate multipath errors. For
long delay multipath, the receiver itself can recognize the wayward signal and discard it. To
address shorter delay multipath from the signal reflecting off the ground, specialized
antennas may be used. This form of multipath is harder to filter out since it is only slightly
delayed as compared to the direct signal, causing effects almost indistinguishable from
routine fluctuations in atmospheric delay.
Multipath effects are much less severe in dynamic applications such as cars and
planes. When the GPS antenna is moving, the false solutions using reflected signals quickly
fail to converge and only the direct signals result in stable solutions.
Ephemeris and clock errors. The navigation message from a satellite is sent out only
every 12.5 minutes. In reality, the data contained in these messages tends to be "out of date"
by an even larger amount. Consider the case when a GPS satellite is boosted back into a
proper orbit; for some time following the maneuver, the receiver’s calculation of the
satellite's position will be incorrect until it receives another ephemeris update. Additionally,
the amount of accuracy sent in the ephemeris is limited by the bandwidth; using the data
from the satellites alone limits its accuracy.
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Further, while it is true that the onboard clocks are extremely accurate, they do
suffer from clock drift. This problem tends to be very small, but may add up to 2 meters (6 ft)
of inaccuracy.
These sorts of errors are even more "stable" than ionospheric problems and tend to
change on the order of days or weeks, as opposed to minutes. This makes correcting for
these errors fairly simple by sending out a more accurate almanac on a separate channel.
Conclusions
GPS allows receivers to accurately calculate their distance from the GPS satellites.
The receivers do this by measuring the time delay between when the satellite sent the signal
and the local time when the signal was received. This delay, multiplied by the speed of light,
gives the distance to that satellite. The receiver also calculates the position of the satellite
based on information periodically sent in the same signal. By comparing the two, position
and range, the receiver can discover its own location.
Several "real world" effects intrude and degrade the accuracy of the system. These
are outlined in the table below (Table 3), with descriptions following. When all of these
effects are added up, GPS is typically accurate to about 15 meters (50 ft).
Table 3. Sources of GPS error
Source
Effect
Ionospheric effects
± 5 meter
Ephemeris errors
± 2.5 meter
Satellite clock errors
± 2 meter
Multipath distortion
± 1 meter
Tropospheric effects
± 0.5 meter
Numerical errors
± 1 meter or less
Differential GPS (DGPS) helps correct these errors. The basic idea is to gauge GPS
inaccuracy at a stationary receiver station with a known location. Since the DGPS hardware
at the station already knows its own position, it can easily calculate its receiver's inaccuracy.
The station then broadcasts a radio signal to all DGPS-equipped receivers in the area,
providing signal correction information for that area. In general, access to this correction
information makes DGPS receivers much more accurate than ordinary receivers.
The most essential function of a GPS receiver is to pick up the transmissions of at
least four satellites and combine the information in those transmissions with information in
an electronic almanac, all in order to figure out the receiver's position on Earth.
Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude
(or some similar measurement) of its current position.
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Applied Methods
References
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Federal Aviation Administration, FAA WAAS fact-sheet, EGNOS, the European equivalent, went
on-line in 2005.
2.
Hydrographic Journal. Developments in Global Navigation Satellite Systems. April 2002.
3.
United States Department of Defense. Announcement of Initial Operational Capability.
December 8, 1993.
4.
National Archives and Records Administration. U.S. Global positioning system policy. March
29, 1996.
5.
Federal Highway Administration. Nationwide DGPS Program Fact Sheet.
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Office of Science and Technology Policy. Presidential statement to stop degrading GPS. May
1, 2000.
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HowStuffWorks. How GPS Receivers Work.
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Physics Today. Relativity and GPS. May 2002.
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Space Environment Center. SEC Navigation Systems GPS Page. August 26, 1996.
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Peter H. Dana: Global Positioning System Overview — Large amount of technical information
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Simplified explanation of GPS at howstuffworks.com
GPS SPS Signal Specification, 2nd Edition — The official (civilian) signal specification.
Satellite Navigation: GPS & Galileo (PDF) — 16-page paper about the history and working of
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History of GPS, including information about each satellite's configuration and launch.
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system on behalf of the US Government.
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1
Nicolae - Iulian Enescu is Teaching Assistant at the Computer Systems and Communications Department at the
Faculty of Automation, Computers and Electronics, University of Craiova, Romania.
In July 2000 he graduated the Faculty of Automation, Computers and Electronics at the University of Craiova.
From 2001 he is PhD student at the Academy of Economic Studies, Faculty of Cybernetics, Statistics and Economic
Informatics.
He is author and coauthor of more than 20 journal articles and scientific presentations at conferences, and coauthor
of 3 books.
His work focuses on the data communication, assembler, programming techniques, high level programming
languages, databases and software testing.
He collaborate with many IT&C Companies for developing some software projects.
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