Miniature Space GPS Receiver by means of Automobile

[SSC07-VIII-1]
Miniature Space GPS Receiver by means of Automobile-Navigation Technology
Hirobumi Saito
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agenc
3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 Japan; 81-42-759-8363
koubun@isas.jaxa/jp
Takahide Mizuno*, Kousuke Kawahara*, Kenji Shinkai*, Takanao Saiki*, Yousuke Fukushima*, Yusuke
Hamada**, Hiroyuki Sasaki***, Sachiko Katumoto*** and Yasuhiro Kajikawa****
*Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 Japan
**Musashi Institute of Technology, 1-28-1 Tamazutsumi, Setahaya-ku, Tokyo, 158-8557 Japan
***Soka University, 1-236 Tangi-cho, Hachioji-city, Tokyo, 192-8577 Japan
****Tokyo Denki University, 2-2 kanda, Nishikicyou, chiyoda-ku, Tokyo, 101-8457 Japan
ABSTRACT
Miniature space GPS receivers have been developed by means of automobile-navigation technology. We expanded
the frequency sweep range in order to cover large Doppler shift on orbit. The GPS receiver was modified to output
pseudorange data with accurate time tag. We tested the performance in low earth orbits by means of a GPS simulator.
The range error caused by the receiver is measured to be 0.9 meter in RMS. Receiver was on-boarded on INDEX
(“REIMEI”) satellite, which was launched in 2005. Cold start positioning was confirmed repeatedly to finish within
30 minutes on orbit. The orbit determination was performed to evaluate the random position error of GPS receiver
by means of the residual error. The random error of GPS position is as large as 2 meter for PDDP=2.5 on orbit. The
RMS value of range error is evaluated to be 0.6m from the flight data. These results on orbit are consistent with the
simulation results in use of a GPS simulator. This miniature space GPS receiver is at present in commercial market.
modification6 and the performance of the modified
receiver on orbit.
INTRODUCTION
Recently miniature GPS (Global Positioning System)
receivers have been utilized for automobile- navigation
equipments as well as cellular phones. Their weight is
several tens grams and their power consumption is less
than 1 watt. The key technologies for their
miniaturization are highly integrated circuits for
receiver functions, based upon mass productions.1,2
The next chapter describes the performance of the
commercial GPS receiver for automobile-navigation.
Then we explain the modification for space application.
Evaluation with a GPS simulator was performed to
predict the performance on orbit. The following
chapter describes the results of the simulation. The
flight results are qualitatively described in the last
chapter. The conclusion is finally presented.
On the other hand, space-borne GPS receivers are as
heavy as several kg and power consumption is as much
as 10 W.3-5 In general, the space-borne GPS receivers
are manufactured with use of space-qualified parts
dedicated for space application, separately from
commercial GPS receiver manufacturing. This leads to
expensive development cost and large size of
instruments.
PERFORMANCE OF GPS RECEIVER FOR
AUTOMOBILE ̿ NAVIGATION
Specification of GPS Receiver for AutomobileNavigation
Model CCA-370HJ of Japan Radio Corporation is
selected to be modified for space application in this
research. Specification of CCA-370HJ is shown in
Table 1.2 Its weight is 35 grams and its size is 58.7 x
In this research, we modified a model of GPS receiver
for automobile-navigation to space application
successfully. This paper describes the issues of such
Saito
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The CCA-370HJ receiver has 8 channels for tracking,
one of which is utilized for search and acquisition.
Therefore the search and the acquisition operation is a
sequential operation in CCA-370HJ receiver. Time to
first fix (TTFF) for cold start is modeled as
Table 1: Specification of the GPS Receiver
CCA-370HJ for Automobile-Navigation
Receiving system
8 Channels (Tracking)
1 Channel(Acquisition)
RF input
Frequency
1575.42MHz (L1) C/A code
Sensitivity
-126dBm
Geodetic system
WGS-84
Positioning accuracy
30 m 2DRMS
Maximum velocity for tracking
200km/h (56m/sec)
Output data rate
1Hz
TTFF
Hot start
8.5sec to 52sec
Warm start
25sec to 88sec
Cold start (Spec)
95sec to 11min
TTFF = 4 Tacq + (N–4) Tfail + 30 [sec].
The first term is the total time of the acquisition for four
NAVSTRAR satellites. The time Tacq is the average
time of the acquisition for one NAVSTAR satellite
when the acquisition successfully finishes. The second
term is the total time of searches when the searches do
not succeed well. The time Tfail is the time of search for
one NAVSTAR satellite, when the acquisition fails. In
the case of automobile-navigation, approximately
Tacq=5 seconds and Tfail=16 seconds. The third term is
the receiving time of the ephemeris data from the fourth
NAVSTAR satellite that is locked on.
Cold start (Actual) 50sec to 5min
Power supply Main voltage
Current
DC +5.0V±0.25V
180mA typ. 270mA max
Preamplifier power supply
DC +4 to 5V 10mA to 30mA
Weight
35g
Size
58.7×36.3×11.0 mm3
The search of the frequency direction is performed
repeatedly by scanning the frequency input to the
correlator. The search time of the frequency direction
increases proportionally to the scanning range of
frequency.1 The frequency range fs of the scanning
for the frequency search is supposed to be determined
such that fs envelops the maximum frequency range
fd of the Doppler shift and the frequency drift fo of
the local oscillator ( temperature compensated crystal
oscillator, TCXO). Thus
fs = fd + fo .
Fig.1: Left : Automobile-Navigation GPS
Receiver CCA-370HJ,
Right : Flight Model with RF Hybrid for
INDEX Satellite
(2)
The maximum Doppler shift fd in automobilenavigation is calculated to be about ±5 kHz. The
frequency drift fo of the TCXO in CCA-370HJ is
estimated to be ±12kHz. Thus the scanning range fs
of frequency is about ±17kHz for CCA-370HJ for
automobile-navigation application.
36.3 x 11.0 mm3. The receiver has 8 channels for
NAVSTAR satellite tracking, one of which is utilized
for search and acquisition. This model is applied to
many instruments for automobile-navigation. The left
side of Figure 1 is the photograph of CCA-370HJ. The
right side of Figure 1 is the flight model receiver for
INDEX satellite. The flight model contains a RF
hybrid for all-sky GPS antenna.
Radiation Test
We performed total doze radiation tests of the GPS
receiver with Co60. In the radiation test, GPS radio
signal received outside of the radiation facility is guided
by a coaxial cable and irradiates the GPS receiver under
radiation test. The receiver survives for 20 krad during
GPS positioning.
Cold Start Acquisition on Ground
We performed simulation tests of the commercial GPS
receiver for automobile navigation. The GPS simulator
Spilent 476 with 12 channels was used for the test.
Forty-four cases of cold start simulation at the fixed
position on earth surface indicate that the minimum
time to first fix (TTFF) is 49 seconds (N=4) and the
maximum TTFF is 4 minutes 19 seconds (N=17), where
the receiver tries to search N satellites of NAVSTAR in
total until 4 NAVSTAR satellites are locked on.
Saito
(1)
Also radiation tests with proton of 30 MeV and 200
MeV have been carried out. No single event latch-ups
are observed for 30 MeV and 200 MeV proton. Protons
of 200 MeV induce single event upsets at the GPS
receivers. It is estimated that single event upset may
occur once per several days at sun-synchronized orbit of
1200 km altitude.
MODIFICATION FOR SPACE APPLICATION
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Expansion of Frequency Scanning Range
within tolerance for automobile navigation.
We calculate distribution of the Doppler shift that is
received by a GPS receiver on the low earth satellite.
The user satellite is assumed to be amateur radio
satellite JAS2, where the altitude of about 690 km, the
inclination of 98.6º, the right ascension of 102.3º, the
eccentricity of 0.035, argument of perigee of 154.7º, the
mean anomaly of 107.2º, and the epoch time of 0:00:00
UTC, the first of July, 1999. The duration of simulation
is two months from the epoch time. Figure 2 shows the
cumulative time as a function of the Doppler shift. The
maximum Doppler shift is calculated about ±45 kHz.
However, it is not the case for space application. The
GPS receiver for automobile-navigation generates
position data with time tag which is not accurate
enough to space application. Therefore, as a minimum
amount of modification, the GPS receiver was modifies
to output pseudorange data with an accurate ( less than
a micro second ) time tag6. The pseudorange data is
transmitted to a ground station through satellite
telemetry system. The accurate position is calculated on
ground with a conventional GPS algorithm. This
version of GPS receiver modified for space application
is evaluated in this paper and was on-board on INDEX
satellite.
In order to modify the automobile GPS receiver CCA370HJ for space application, the frequency range fs of
the scanning is required to be expanded up to ±57 kHz
by substituting fd=±45kHz and fo=±12kHz into Eq.
(2). Since the original automobile receiver has the
scanning range of 17 kHz, the scanning range for
space application is 3.3 times as wide as one of
automobile receivers. The manufacturer of CCA370HJ modified the scanning range of frequency in
their embedded ROM program based upon our analysis
for space application.
As the second version, we have already developed the
further improved version of the space GPS receiver,
which can output accurate position data with accurate
time tag. The improved version is scheduled to be tested
on orbit in 2008.
PERFORMANCE
SIMULATOR
EVALUATION
WITH
GPS
Cold Start Acquisition on Orbit
Time Synchronization
We perform simulation tests by means of Spilant 476
simulator with 12 channel signals, where the GPS
receiver is on a satellite in a low earth orbit.
We perform simulation tests by means of Spilent 476
simulator, where the GPS receiver is on a satellite in a
low earth orbit. The simulation shows that the
automobile GPS receiver outputs inaccurate time tag
with a fluctuation of less than 0.3 second. The
manufacturer of CCA-370HJ explains that accurate
determination of the position and the receiver clock bias
is performed in CCA-370HJ. However, only the
accurate position data are output with inaccurate time
tag. The position data are output in terms of earth-fixed
coordinate WGS-84 and velocity of automobile is less
than 56m/sec (200km/h).The fluctuation of time tag is
The satellite orbit assumed to be one of INDEX
(a=7009.939km, e=0.0039, i=97.829 deg, !㸻165.908
deg, "=196.661 deg, epoch time=2005 Aug 23rd
21:09:58.8UTC ). The number of the NAVSTAR
satellites that geometrically visible from INDEX
satellite is temporarily distributed between 8-13. The
average number of the visible NAVSTAR satellites is
10.7. The visible satellite number is equal to or less
than 12 for 93% of time. The antenna is assumed to
cover all the sky, which is an appropriate model for
INDEX satellite. We tested 44 simulation cases where
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Simple
Model
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Simulations
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Fig.2: Cumulative Time of Doppler
Shift of GPS Signal Received by LowEarth-Orbit Satellite
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Fig.3: Cumulative Probability of TTFF
Based on GPS Simulation
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(a) Satellite
(a) PDOP and Latitude
(b) Fixed Position
on Earth
(b) Position Error
Fig. 5: Pseudorange Rate and Range Error Based
on GPS Simulation #1
(a) Satellite in 690km Sun Sync. Orbit
(b) Fixed Position on Earth
Fig. 4: Position Error on Orbit Based on GPS
Simulation㸡1
(a)PDOP and Latitude, (b)Position Error
PN=NC4p4(1-p)N-4.
the GPS receiver powers on every 20 minutes from the
epoch time with a cold start mode. Figure 3 indicates
the cumulative probability of time to first fix (TTFF).
The minimum TTFF is 8 minutes and the maximum
TTFF is 28 minutes. These TTFF values are 7-10 times
longer than the original GPS receiver for automobilenavigation on ground. For the receiver modified for
space, approximately Tacq=32sec and Tstop=84sec (see
Eq.(1)). The reason is that the frequency scanning range
is expanded to cover large Doppler frequency on orbit.
Note it is confirmed that a GPS receiver for automobile
navigation can not start positioning in the case of
satellite orbiting condition.
Then we obtain the probability that the receiver
acquires four NAVSTAR satellites at time t, combining
Eqs.(1) and (4). The result of this simple model is also
shown in Figure 3 and is qualitatively consistent with
the simulation result in Figure 3.
Position Accuracy
In order to predict the accuracy of GPS positioning on
orbit we perform simulation tests with a GPS simulator.
The position data are calculated based on the
pseudorange data from the receiver by an external
computer. The orbit is assumed to be a sun-synchronous
orbit with altitude of 690km (a=7068.137km, e=0.0,
i=98.19r, Ȑ=0, Ȱ=0). Figure 4 shows the result of
the simulation #1 without ionospheric effect.
The orbits of NAVSTAR satellites are circular orbits
with altitude of about 20,000km and period of about 12
hours. The orbit of INDEX is the circular orbit with
altitude of 640km and period of 97 minutes.
Approximately speaking, the combination of the
NAVSTAR satellites visible from INDEX satellite
remains almost the same during a relatively short time
(let say 25min) compared with the orbit period (97 min).
We propose the following simple model on the GPS
cold start TTFF in a short time region. The GPS
receiver searches the Mth NAVSTAR satellite at a
certain time. The probability p that the Mth NAVSTAR
satellite is visible is given as
p!
Figure 4(a) is the time history of the position dilution of
precision (PDOP) and the latitude of the user satellite.
PDOP value, namely, visibility of NAVSTAR satellite
changes due to the orbit motion of the user satellite. On
average 6~8 NAVSTAR satellites are tracked by the
receiver and the average value of PDOP is 2.7. At a
certain moment when the user satellite is at high
latitude, the number of visible NAVSTAR satellites
decreases to 4~5, and PDOP value degrades to higher
than 10. Figure 4(b) shows time history of the position
error. When the PDOP value degrades to higher than
10, the position error becomes more than 10 m.
However, RMS value of position error for eight hours is
2.0 m.
average number of visible NAVSTAR
total number of NAVSTAR
= 10.7 / 32 = 0.33.
(3)
The probability PN that receiver searches totally N
NAVSTAR satellites and then locks on four
NAVSTAR satellites for positioning is given by a
binominal distribution as
Saito
(4)
Range Accuracy
Performance of GPS receiver is essentially determined
by range accuracy. Combining with PDOP values,
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NATO, STANAG is applied to the simulation.7,8 This
model provides with the ionospheric delay Di as Eq.(5)
Table 2: Estimated Error on Orbit ( RMS, [m] )
Error source
Estimated error
Ephemeris data
2.1
GPS satellite clock
2.1
Ionosphere
Di !
in orbit
0.9
Total range error
6.2
Position error ( PDOP = 2.7 )
16.7
" sin E $ 0.076 $ sin E # [m] .
2
(5)
The quantity TEC denotes the total electron content [m]. The typical value of TEC=1.0 x10 7[m-2] is assumed
in the simulation#2. There, Fc and E denote the carrier
frequency of GPS signal [Hz] and the elevation angle of
the NAVSTAR satellite with respect to the local
horizon of the user satellite, respectively. This model
provides with the ionospheric delay of 1.6~12 m
according to the elevation angle of 90º ~ 0º.
range accuracy results in position accuracy. We obtain
the range error on orbit with a GPS simulator in the
following way. Based upon the pseudorange data from
the GPS receiver, we calculate the time error of the
receiver clock with an external computer. The range
between the user and the NAVSTAR satellite is
calculated from the pseudorange data and the clock
error. This range is compared with the true range data
from the GPS simulator to evaluate the range accuracy
of the receiver.
In the simulation #2, we add the ionosperic effect to the
simulation #1. The simulation #2 gives the range error
of 5.5m in RMS. Based upon the simulation #1 without
the ionopheric effect and the simulation #2 with the
ionospheric effect, the RMS value of range error due to
the ionosheric effect for the above-mentioned model
and the parameter is estimated to be 5.4m.
Prediction of Position Accuracy on Orbit
Attention is mainly paid to the large pseudorange rate in
the orbit. If pseudorange rate is too large compared
with the loop frequency response, tracking loop for the
pseudorange could not follow the rapid change of the
pseudorange. The orbit parameters are the same as ones
in the section of Position Accuracy. Figure 5(a) shows
time history of the pseudorange rate and the range error
of NAVSTAR satellite of PRN=28 for the 690km sunsynchronous orbit. The ionospheric delay is not
included in this simulation (simulation #1). The
pseudorange rate changes rapidly from +8 km/sec to –8
km/sec during the visible time of 30 minutes due to the
orbit motion of the user satellite. However, the range
error remains almost constant as much as 0.9m in RMS.
The purpose of the simulation in this research is to
evaluate the range error from the receiver itself.
Therefore these simulations #1,#2 do not include the
ephemeris data error and the clock error of the
NAVSTAR satellites. Total position errors on orbit
including the ephemeris data and the clock error of the
NAVSTAR satellites are estimated. Table 2 shows the
total range error and the error budget as well as the
position error on orbit. The receiver range error is 0.9
m from the simulation #1 in the previous section. The
error due to the ionospheric delay is 5.4 m in RMS from
the previous section. The typical values of the
ephemeris error and the clock error of the NAVSTAR
satellites are obtained from ref 8. The PDOP value is
2.7 in average which is observed in the simulation #1.
The total position error is estimated to be 16.7 m (RMS)
by multiplying the range error by PDOP.
Figure 5(b) shows the data obtained from the same
receiver for the fix position on earth surface. The
NAVSTAR satellite of PRN=24 remains visible for
several hours on the earth surface and the pseudorange
rate is very small. The range error has drifting bias
error of about 2 m and random error of about 2 m.
Preamplifier
These results indicate that CCA-370HJ with the
modification for space application keeps almost the
same range accuracy in orbit as on the ground. No
degradation due to the large pseudorange rate is
observed. The RMS value of the range error of receiver
measurement is 0.9 m in orbit without the ionospheric
effect.
Wilkinson
Divider
GPS
Receiver
Figure 6: Configuration of All-Sky Antenna
Next, the simulation #2 where ionospheric effect is
included is performed. The ionospheric model of
Saito
Fc %
2
5.4
Receiver measurement
82.1 % TEC
2
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FLIGHT TEST OF MINIATURE SPACE GPS
RECEIVERS IN INDEX SATELLITE
The GPS receiver modified for space application was
onboard in INDEX satellite. INDEX has mass of 72 kg
and three axial attitude control function with accuracy
of 0.05r.
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We developed a new type of all-sky GPS antenna
combined with two GPS antennas. Most of GPS
receivers for automobile-navigation equip their
preamplifier inside the antenna. Their RF coaxial cable
between the antenna and the receiver works also as a
power feeder for the preamplifier circuit. A RF hybrid
has to work not only as RF combiner for two antennas,
but also as dc power feeder for two preamplifiers
without resistance. A Wilkinson divider meets this
requirement for RF performance and dc performance.
Figure 6 shows the configuration of the all-sky antenna.
Figure 7 shows the two GPS antennas in the INDEX
satellite and the measurement result of the antenna
pattern with INDEX satellite body. For coverage of
95 %, the antenna gain is measured to be higher than –5
dBi, which is the requirement for the GPS receivers.
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Fig.8: Cumulative Probability of Cold Start
TTFF of Flight Results in INDEX
above Japan. It started positioning successfully 7
minutes later.
We measured the TTFF on orbit by cold-starting every
12hours 40 minutes from July 25th 2006. Figure 8
indicates the cumulative probability of TTFF for cold
start mode. Cold start positioning succeeded within 30
minutes, which is consistent with the GPS simulations
shown in Figure 3.
We evaluated the position accuracy of the GPS receiver
on orbit. Based upon the GPS simulation, the position
accuracy on orbit is estimated to be 16.7m in Table 2.
Unfortunately we do not have any other positioning
data which is more accurate than the GPS data.
Therefore, orbit determination is performed in use of
GPS position measurement. Then the residual error,
which is defined as a difference between the GPS
measurement and the orbit determination, is considered
a random error of GPS receiver if orbit determination
duration is short enough. The pseudorange data which
the onboard GPS receiver measures every second is
transmitted to ground. The position of INDEX satellite
is calculated on ground in use of the pseudorange.
Then the orbit determination is performed with a
gravity model (WGS-84 gravity model with 12
harmonics in this research). Finally the residual errors
are calculated as a difference between the GPS position
data and the orbit determination data. Figure 9 is the
residual errors, the PDOP values, and the number of
visible GPS satellites as functions of time. The orbit
determination is performed in use of GPS data for 60
seconds. The behavior of these values are very
consistent with the GPS simulation shown in Figure 4.
The number of visible GPS satellites is seven in average.
The number of visible NAVSTAR satellites is more
than six for 97% of time. PDOP value is less than 4.0
for 90% time. The average value of PDOP is 2.5.
INDEX satellite was launched by Dnpre rocket from
Baikonuar base on Aug. 24th 2005. The GPS receiver
powered on in cold start mode on Aug. 27 16:02 (UTC)
GPS antenna
REF: 10 dBi
10 dB / div
Table 3 shows the average and the standard deviation of
the residual error in the along-track direction, the radial
direction, the out-of-plane direction and the total
position error. The residual error follows to be a random
distribution without bias. The standard deviation of the
GPS antennas
Figure 7: All-Sky Antenna Configuration and
Pattern of INDEX Satellite
Saito
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total position error is 1.46 m in RMS, which is
consistent with the result of the GPS simulation #1
(2.0m in RMS). The standard deviation of the position
error in the radial direction is almost twice as large as
the ones of along-track and the out-of-plane
components. This is because the NAVSTAR satellites
distribute in the outward direction and the “vertical”
DOP is larger than the “horizontal” DOP.
Table 3: Residual Error of GPS Positioning on Orbit
Component
In Figure 10, the values of PDOP are divided into
several bins and the RMS values are calculated in each
bin. The RMS value of the residual error in each bin is
almost proportional to the PDOP value as shown by
circles in Fig.10. In general,
RMS position error = Ȫrange™PDOP,
6
15
4
10
Residual Error [m]
5
0
2
PDOP
22:55
23:05
23:15
23:25
1.23
Along-Track
0.03
0.42
Out-of-Plane
-0.08
0.67
-
1.46
Miniature space GPS receivers have been developed by
means of automobile-navigation technology. We
expanded the frequency sweep range in order to cover
large Doppler shift on orbit. The GPS receiver was
modified to output pseudorange data with a accurate
time tag. First the performance in low earth orbits by
means of a GPS simulator. The range error caused by
the receiver is measured to be 0.9 meter in RMS. The
position accuracy is estimated to as much as 16.7
meters (RMS) in the low earth orbits. Mainly the
ionospheric effect degrades the accuracy of GPS
positioning.
Visible GPS No.
Residual Error [m], PDOP
8
0.09
CONCLUSION
30
Visible GPS No.
Radial
with the RMS value of range error in the GPS
simulation #1 in Figure 5 (0.9m).
whereȪrange is the RMS value of range error. The slop
of the fitted line of the RMS value in Figure 10
corresponds toȪrange, which is about 0.6 m. The RMS
value of range error observed in the orbit is consistent
25
Standard Deviation[m]
Total
(6)
20
Average[m]
0
U T C
This GPS receiver was on-boarded on INDEX satellite,
which was launched in 2005 into a circular orbit with
640km altitude. Cold start positioning was confirmed
repeatedly to finish within 30 minutes on orbit. The
orbit determination was performed to evaluate the
random position error of GPS receiver by means of the
residual error. The random error of GPS position is as
much as 2m (RMS) for PDOP=2.5 on orbit. The RMS
value of range error is evaluated to be 0.6 m from the
flight data. These results on orbit are consistent with the
simulation results in use of a GPS simulator.
Fig.9: Residual Position Error (Random
Position Error), PDOP and Number of
Locked Satellite in INDEX GPS Flight Data.
Interval of Orbit Determination is 60 second.
This miniature Space GPS receiver is at present in
commercial market9.
References
1. Hata, M., Ogasawara, Y., Shoji, M., Itoh, W.,Kume,
A., Mino, A., Kojima, S., Okada, Y.and Ohga, T.,
“GPS-SOC
for
AutomobileNavigation,”
Proceedings of GPS Symposium 2002, in Japanese,
Fig.10: Relation between Residual Position Errors
(Random Position Error) and PDOP in INDEX GPS
Flight Data. Open Circles show RMS Values
Residual Position Errors in Each Bin of PDOP.
Saito
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Japanese Institute of Navigation, pp.113-122, Nov.
2002.
2. Japan Radio Corporation, “Speceification of GPS
Receiver CCA-370HJ,” in Japanese, 1998.
3. Unwin, M.J, Oldfield, M.K., Purivigraipong, S. and
Kitching, I., “Preliminary Orbital Results from the
SGR Space GPS receiver,” Proc. of ION GPS-1999,
pp.849-855, 1999.
4. Mehlen, C. and Laurichese, D., “Real-Time GEO
Orbit Determination Using TOPSTAR 3000 GPS
Receiver,” Proc. of ION GPS-2000, 2000.
5. Kawano, I., Mokuno, M. and Kasai, T., “Relative
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8
21st Annual AIAA/USU
Conference on Small Satellites
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