Degraded GPS Signal Measurements

Degraded GPS Signal Measurements
With A Stand-Alone High Sensitivity
Receiver
G. MacGougan, G. Lachapelle, R. Klukas, K. Siu, Department of Geomatics Engineering
L. Garin, J. Shewfelt, G. Cox, SiRF Technology Inc.

Superieure des Telecommunications, France and BS in
BIOGRAPHIES
physics from Paris VI University.
Glenn MacGougan is a MSc. student in the Department
Geoffrey F. Cox holds a B.A. degree in Geology and
of Geomatics Engineering, University of Calgary. In
Chemistry at the University of Maine and a MEng. in
2000 he completed a BSc. in Geomatics Engineering at
Geomatics Engineering from the University of Calgary.
the same institution. He has previous experience in GPS
From 1996 to 2000, he worked in various areas of GPS.
related R&D at NovAtel Inc. and Trimble Navigation. He
Mr. Cox joined SiRF Technology, Inc. in the fall of 2000
expects to complete his MSc in September 2002.
as a Senior Applications Engineer.
Dr. Gérard Lachapelle is Professor and Head of the
John L. Shewfelt received a B.Sc. in Electrical
Department of Geomatics Engineering, University of
Engineering from the University of California Santa
Calgary. He has been involved with GPS developments
Barbara in 1981. He has since been involved in the
and applications since 1980. More information on site
design, development, test and integration of advanced
www.geomatics.ucalgary.ca/faculty/lachap/lachap.html
avionics and guidance systems . In 2000, he joined SiRF
Dr. Richard Klukas is an Assistant Professor in the Technology Inc. as Applications Engineering Manager to
facilitate the integration of GPS technology into
Department of Geomatics Engineering at the University
embedded products and platforms.
of Calgary. He holds BSc and MSc degrees in Electrical
Engineering and a PhD in Geomatics Engineering, all
from the University of Calgary. His research interests
include all aspects of wireless location. ABSTRACT
Lap Kee Siu is a BSc. student of Electrical Engineering The use of GPS for personal location using cellular
at the University of Calgary. He currently works as an telephones or other devices requires signal measurements
internship student in the Department of Geomatics under both outdoor and indoor situations. The outdoor
Engineering. He expects to complete his BSc. in May environment may range from clear to shaded/blocked
2003. signal measurements. The indoor environment may range
for single floor wooden constructions to high-rise
Lionel J. Garin, Lead GPS Architect, SiRF Technology
buildings and underground facilities.
Inc., has over 20 years of experience in GPS and
communications fields. Previous to that, he worked at In this paper, a high sensitivity receiver that operates in
Ashtech, SAGEM and Dassault Electronique. He is the unaided stand-alone mode is tested under a range of
inventor of the quot;Enhanced Strobe Correlatorquot; code and shaded signal environments, ranging from residential
carrier multipath mitigation technology. He holds an outdoor areas to urban canyons to residential houses. The
MSEE equivalent degree in digital communications measurement analysis is performed in both the
sciences and systems control theory from Ecole Nationale observation and position domains. The results show that
the receiver tested can yield measurements with C/N0

Presented at ION National Technical Meeting, San Diego, 28-30 January 2002.

degradations in excess of 20dB-Hz, as compared to line- degradation. Two identical HS SiRF units were used. The
of-sight measurements. Position results are a function of receivers are shown in Figure 1. Table 1 outlines the
the geometry of the remaining satellites, which in turn is a major similarities and differences between the SiRF
function of the environment. standard and high sensitivity (HS) receivers. The
receivers use the same basic hardware architecture;
however, the integrations times used are significantly
different, as the high sensitivity receiver uses long periods
INTRODUCTION
of non-coherent integration. In addition, the Kalman
GPS signal deterioration occurs through signal masking
filters used in the receivers are different. The filter
caused by natural (e.g. foliage) and man-made (e.g.
differences become evident in the position domain
buildings) obstructions, ionospheric scintillation, Doppler analysis. All receivers used herein output raw
shift, multipath, jamming, evil waveforms, and receiver
measurements. These were recorded for subsequent
and antenna effects. The impact of anyone of the above
analysis.
can result in partial to total loss of signal tracking and/or
tracking errors, depending on the severity of the effect
and the receiver tracking characteristics. These effects are
evident in a receiver's measure of the carrier to noise
density ratio C/N0 . Tracking errors, especially if
undetected by the receiver firmware, can result in large
position errors. Partial loss of tracking results in geometry
degradation, which in turn affects position accuracy.
The L1 C/A Code repeats every millisecond. This can be
used advantageously by the GPS receiver in that the
signal can be integrated for extended periods in order to
obtain a higher signal to noise ratio. Chansarkar & Garin
(2000) describes the use of GPS signals at very low power
levels using long dwell times. In terms of unaided GPS,
this integration can be performed coherently for up to
20ms. The maximum coherent integration time is due to
the navigation bit boundaries. Furthermore, non-coherent Figure 1: Receivers Used During Testing
integration, which is basically integration of the squared
signal, can be performed for long periods of time relative
Table 1: Comparison of Standard and High Sensitivity
to the coherent integration interval. Using the full
SiRF Receivers
coherent interval and long non-coherent integration times,
weak signal tracking in degraded environments is
possible.
Prior investigations into the use of low power GPS signals
using long dwell times have been performed by Peterson
et al. (1997), Moeglein & Krasner (1998), Garin et al
(1999), and van Diggelen & Abraham (2001). Testing at
the University of Calgary focussed on the use of long
integration times for the stand-alone case that is no
network aiding. There is a strong need to characterize
unaided receiver performance under GPS signal
deterioration to extend the use of GPS to a range of new
applications.
ANALYSIS CONVENTIONS
The following colour convention was used when
RECEIVER DESCRIPTION
comparing the results from the different receivers. The
The receiver type under test is a high sensitivity (HS) OEM4 receiver results are plotted in red, the standard
unaided SiRF receiver unit. For comparison purpose, a SiRF results in green and the HS SiRF receiver results in
standard model SiRF receiver and a NovAtel OEM4 blue and purple, respectively. In the figures shown in this
receiver are added. The latter is not expected to perform paper, the test warm-up period data is often shown in grey
well under signal degradation as its firmware is optimized or in a faded colour scheme. This data is not included in
for high accuracy performance but is used to show the the test statistics.
range of performance now possible under signal


Some figures show the testing period data only but from A test was designed, based on MacGougan et al. (2001),
multiple test runs. This data is not contiguous in time but using the simulator to assess the tracking threshold of the
is presented in a continuous fashion to better display the four test receivers. A constellation based on the real
similarities and differences between test runs. Black almanac for GPS week 1148 was used to produce
vertical lines delineate the different test runs. simulated GPS signals. A 20 minutes warm-up period
with undegraded signal tracking ensured that all receivers
In figures displaying the calculated fix density values for
had enough time to lock on all eight simulated satellites.
the receivers, an overlaid light blue dot is used to indicate
All the satellites used the same signal power, which was
2D fix versus 3D fix.
lowered from +10dB to -20dB relative channel power in
0.5 dB steps. A five-minute period with the lowest power
level possible (-20dB) finished off the test. The set-up for
GPS SIMULATION TESTING - TRACKING
this test is shown in Figure 3.
THRESHOLD TEST
In recent years, advances in simulation technology have
contributed to the development of state-of-the-art
hardware GPS simulators. Spirent Communications Inc.
makes the STR-4760 GPS simulator. The simulator is
described thoroughly in GSS, 2000. The simulation unit,
shown in Figure 2, used by the University of Calgary,
consists of a control computer (shown on the left) and two
16 channel L1 or 8 channel L1/L2 hardware simulator
boxes (shown on the right).
GPS signals are often attenuated by propagation through
different mediums (atmosphere, foliage, buildings, etc).
This results in an effective decrease in the carrier strength
component of C/N0 . In addition, broadband sources of RF
interference also decrease C/N0 by increasing the ambient
noise density. The simulator allows real-time control of
±
the signal level ( 20dB with respect to –160dBW) for
each satellite corresponding to one channel of signal
Figure 3: Tracking Threshold Test Set-up
output. This power level is referred to as the relative
channel power for the simulator as used in this paper. The
The tracking results, in terms of the average C/N0 , for all
signal level can be set equally and varied by the same
satellites tracked, and the associated simulator relative
amount for all channels. The C/N0 threshold for tracking
channel power are shown in Figure 4. The OEM4
weak signals can be determined by lowering the signal
receiver tracks to about –9dB relative channel power. The
level slowly until the receiver is no longer able to track
standard SiRF receiver does not fare much better as it
the satellite.
tracks down to -10dB relative channel power. The
tracking ability of the HS SiRF receivers is at or better
than the -20dB relative channel power. The number of
satellites tracked and the associated simulator relative
channel power is shown in Figure 5. The NovAtel and
standard SiRF receivers lose almost all satellites
simultaneously while the high sensitivity receivers lose
only 3 satellites gradually.
The 3D error in position for each receiver and the
associated simulator relative channel power is shown in
Figure 6. Strong correlation between low signal levels
and higher position error is evident. However, the HS
receivers are still able to provide position output; the five
satellites that they track provide a good geometry.
Figure 2: Spirent (GSS) STR-4760 Simulator


Figure 4: Signal Power Levels During Simulator
Tracking Threshold Test

Figure 6: 3D Position Error During Simulator
Tracking Threshold Test

FIELD TESTS
The aim of the field tests was to take the high sensitivity
receivers into increasingly difficult environments in terms
of GPS signal tracking. Thus, testing was first performed
under signal masking conditions and fast fading due to
roadside trees and foliage. Subsequent testing took place
under more severe signal masking and multipath
conditions in downtown Vancouver, B.C, Canada and
Calgary, Alberta, Canada. Finally, testing inside two
types of residential garages was performed to assess
conditions with no line of sight on any of the satellites.

FIELD TEST SET-UP AND DESCRIPTION
Figure 5: Number of Satellites Tracked During
Simulator Tracking Threshold Test The test set-up used for the base station is shown in
Figure 7. A high performance antenna, namely the
NovAtel 600 model, was mounted at a surveyed location
with a clear view of the sky. The test set-up used for the
rover receivers is depicted in Figure 8. The rover
receivers were always initialized under open sky
conditions for a 20-minute period. Test statistics only
refer to the data after this warm-up period.
The receivers are tested in parallel using a common
antenna (NovAtel 600 model) or an inline low noise
amplifier or LNA in the case of the simulation test. An
LNA (also part of the antenna) acts to set the signal


conditions (i.e. noise floor) to very similar values in each vehicle, when sufficient satellite coverage was available
receiver (Van Dierendonck, 1995). In order to ensure to update the INS with an accuracy level to be described
valid comparison, the signal conditions experienced by later. The antenna was mounted on the roof of the vehicle
each receiver must be very similar. In real conditions, this approximately one metre above the INS. In the case of the
is achieved by splitting the signal from a common active garage tests, the reference positions inside the garage
antenna. In simulation mode, this is achieved with the use were measured by extending the GPS position of a nearby
of an LNA prior to the signal splitter. point using a standard survey method.

FIELD TEST MEASURES
The analysis of results of the field-testing focuses on the
observation and position domains.
Position domain analysis is performed by comparison of
the test receiver’s positions with positions derived from
carrier phase differential GPS/INS using NovAtel's Black
Diamond system when available. Comparison is only
undertaken when the position errors estimated by the
GPS/INS system are better than 5 m in latitude and
longitude and 10 in height, at the 1 sigma level. This
ensures that the ensuing derived statistics are meaningful.
Analysis focuses on the receiver output position for the
Figure 7: Field Test - Base Station Set-up SiRF model receivers. The NovAtel OEM4 receiver is
used to collect raw data for post mission analysis. This
raw data is used in a least squares position solution using
height constraints (when the geometric dilution of
precision, GDOP, exceeds 5.0). This was done in order to
compare the 2D fix capability of the SiRF receivers with
that derived from a geodetic quality r eceiver. The SiRF
receivers employ a Kalman filtering and the software to
process the OEM4 data does not. The effects of the
Kalman filter will be noticeable during comparison of
kinematic testing periods with fewer instantaneous
outliers evident. The focus of the analysis in the position
domain is on horizontal and vertical errors. Vertical
performance is expected to be poor due to degraded
geometry and the typically poor performance of GPS in
terms of height. Errors are computed by subtracting the
reference values from the true values.
Signal quality, fading, availability, and dilution of
Figure 8: Field Test - Rover Set-up precision (DOP) are also be analyzed. Signal fading is
calculated by the difference of the C/N0 measurements
The modes of field-testing include: between like model receivers at the rover and base
• stations in order to eliminate receiver C/N0 estimation
Vehicular testing in a residential area of Calgary
biases. HDOP is assessed based on the receiver output
with foliage beside and overhanging much of the
HDOP for the SiRF receivers and the computed value
road
based on satellites tracked for the OEM4 receiver.
• Vehicular testing in downtown Calgary and
The measurement output of a GPS receiver typically
Vancouver
includes pseudorange, Doppler, and carrier phase
• Static test inside a wooden frame garage and a observations. The L1 pseudorange is the primary
concrete wall garage. measurement used in single point positioning and thus is
the focus of the measurement domain analysis. By
constraining the positions of the receiver to known
The vehicular tests have speeds of up to 50 km/h. A reference positions, an assessment of the errors in the
carrier phase differential GPS/INS system was used to pseudoranges can be made from the residuals of the
constrained least-squares solution. C3 NAVG2TM, a
provide the reference data for the positions of the test


software package developed at the University of Calgary, the estimated accuracy was better than 5 m in latitude and
is capable of this type of analysis . longitude and 10 m in height.
Fix density is also a useful measure of a receiver’s
capabilities in an environment with signal masking and
multipath. Fix density values can be determined directly
from the SiRF receiver output and from the processing of
the OEM4 data. However, a more pessimistic test
measure was used in the comparison of the receivers. The
2D fix density was calculated based on the number of
satellites used in epoch-to-epoch solutions during the test
period. If three satellites were used in a solution, a 2D fix
was assumed. Similarly, a 3D fix was assumed when four
or more satellites were present. Fix density is thus the
percentage of epochs with valid fixes based on a 1 Hz
data rate for the entire test period. This measure is
pessimistic as it is possible to obtain a 2D fix with two
satellites used in solution by using filtering, clock
coasting and height fixing (e.g., Lachapelle et al.,1997).In
summary the following measures are used:
Observation Domain:
• Residuals: as estimated from a least squares
solution constrained to known rover positions
• Fading: C/N0 (base) – C/N0 (rover) using like
type receivers
• HDOP: Horizontal Dilution of Precision
• Number of satellites used in solution
Position Domain: Figure 9: Vehicular Test in Residential Area
• 2D Fix Density: percentage of epochs with 3
The number of satellites used in the position solutions for
satellites used in solution
the receivers for all three test-runs is shown in Figure 10.
• 3D Fix Density: percentage of epochs with 4 or The associated fix densities for each receiver are shown in
more satellites used in solution Figure 11. The results indicate that satellite availability is
improved for the high sensitivity receivers, as they are
• 2D Error and height error.
capable of 3D fix during most of the test runs. The OEM4
reverts to 2D fix more frequently.
RESIDENTIAL TESTING UNDER TREES AND
FOLIAGE
A common problem for many GPS receivers is the fast
fading and tracking problems induced by trees and
roadside foliage. A vehicular data set was thus collected
in a residential area of Calgary with foliage beside and
overhanging much of the road. This environment is
shown in Figure 9. One to three level houses with trees
lining the roadside characterize this older area of Calgary
known as Mount Royal. Three test runs were performed
on September 27, 2001 beginning at approximately
16:10:00, 16:50:00, and 17:35:00 UTC time.
Unfortunately, the standard model SiRF receiver was not
available during these test runs and is not included in the
analysis. There were also problems with the GPS/INS
processing for test 1 so a differential code solution using
Figure 10: Residential Test - Number of Satellites
OEM4 receivers was used during the limited epochs when
Used In Solutions


The 2D errors for all three test runs and associated
statistics are shown in Figure 12. The associated HDOP
values are shown in Figure 13. The height errors during
the test runs are shown in Figure 14. More geometry
degradation is present due to fewer satellite observations
available for the OEM4 receiver. In addition, more
instantaneous outliers are present in the results from the
OEM4 receiver due to the use of unfiltered epoch-by-
epoch processing. The use of a Kalman filter for periods
with very poor geometry and a lack of quality
observations would reduce these outliers significantly, as
it has for the case of the SiRF receivers.

Figure 13: Residential Test - HDOPs

Figure 11: Residential Test - Fix Density Values

Figure 14: Residential Test - Height Errors

The run-to-run trajectories (based on the receivers’
output) for the two HS SiRF receivers are very similar
and that for one receiver is shown in Figure 15. The
NovAtel OEM4 receiver run-to-run trajectory is shown in
Figure 16. The instantaneous outliers in the OEM4
derived positions are also evident in this figure.

Figure 12: Residential Test - 2D Position Errors


Figure 17: Residential Test – Correlation Between
Fading and Pseudorange Residuals for PRN 11

Figure 15: Residential Test - HS66 Run-To-Run
Horizontal Positions
URBAN CANYON TESTING
The challenges for satellite-based navigation posed by the
downtown environment of many cities are numerous.
Large buildings tend to mask much of the sky and induce
large multipath and fading errors. The general problem
with using GPS in urban canyons translates into a lack of
satellite availability. However, high sensitivity GPS
receivers may be able to track the degraded signals of
satellites that are typically masked. This may be by
tracking a multipath signal or the combination of the
weakened direct signal and multipath signals. The
addition of these degraded observations could allow
position determination when otherwise impossible by
standard means.
In order to address the capabilities of the HS SiRF
receivers in urban canyons, tests were performed in
downtown Calgary and Vancouver. Both cities have
Figure 16: Residential Test - OEM4 Run-To-Run
concentrations of tall buildings.

In terms of signal fading and residuals, a low elevation
DOWNTOWN VANCOUVER
satellite, rising from 15° to 24°, was investigated during
run 2. Figure 17 shows the absolute value of the residuals Testing took place on October 13th and 14th , 2001, with
for PRN 11 along with the corresponding calculated five test runs and two test runs, respectively. The test
fading values for one of the HS receivers. Not only are trajectory used for all runs is shown in Figure 18 along
fast fading effects observable but also a strong correlation with some pictures of the testing environment. Each test
exists between the fading and the residual values. lasted approximately 30 minutes, following a twenty-
minute warm-up period under clear sky conditions.


Figure 18: Vancouver Test Trajectory

The number of satellites used in the position solutions for
the receivers for all seven test-runs are shown in Figure
19. The associated fix densities for each receiver are
shown in Figure 20. The results indicate that the
Figure 19: Vancouver Test - Number of Satellites
availability is again improved for the HS receivers, as
Tracked
they are capable of 3D fix for 91.9% and 93.6% of the test
runs. The standard SiRF receiver and the OEM4 unit both
revert to 2D fix more often with 11.2% and 15.2% 2D
fixes respectively versus 1.2% and 1.0% 2D fixes for the
high sensitivity receivers.
The 2D errors for all seven test runs and associated
statistics are shown in Figure 21. Major outliers are
present for all four receivers. In order to better understand
the performance of the receivers, outliers larger than three
standard deviations (3 ) were removed and the statistics
were recomputed. These results are shown in Figure 22.
The use of degraded measurements combined with HS
filtering effects leads to large errors and poor performance
in this environment. The associated HDOP values are
shown in Figure 23. The improvement in HDOP for the
HS receivers is again evident; although, this is not clearly
reflected in the 2D position error statistics. In terms of
vertical position errors, the performance of the receivers
is shown in Figure 24 along with corresponding statistics.
These results emphasize the need for proper filtering of
the observations in the position solution. Observations are
useful only when weighted appropriately.
Figure 20: Vancouver Test - Fix Density Values


Figure 21: Vancouver Test - 2D Position Errors
Figure 23: Vancouver Tests - HDOPs

Figure 24: Vancouver Test - Height Errors
Figure 22: Vancouver Test - 2D Position Errors With
Outliers Removed

DOWNTOWN CALGARY
Three successive tests were conducted in downtown
Calgary on September 27th . No reference position system
was used to provide a truth trajectory. However the
trajectory used is along straight East-West and North-
South streets and good accuracy performance can easily
be derived from plotting the test positions on an existing
map. Also, fix density information and the number of
satellites used in the solutions still provides useful
information in term of solution availability. These results
are shown in Figure 25 and Figure 26, respectively,


followed by Figure 27 depicting the HDOP during the
test runs. The results indicate that availability is again
improved for the HS SiRF receivers. The standard model
SiRF receiver and the OEM4 unit both revert to 2D fix
more often.
Figure 28, Figure 29, Figure 30, and Figure 31 show the
run-to-run trajectories for the HS receivers, the standard
model receiver, and the OEM4 receiver, respectively. The
estimated HS receiver trajectory degradation is likely due
to internal filtering problems.

Figure 27: Calgary Test - HDOPs

Figure 25: Calgary Test - Fix Density Values

Figure 28: Calgary Test HS66 SiRF Unit Run-To-Run

Figure 26: Calgary Test - Number of Satellites
Tracked


Figure 29: Calgary Test - HS14 SiRF Unit Run-To-
Run Horizontal Positions

Figure 32: Wood Frame and Concrete Wall Garage
Test Environments

The test in the concrete garage with the door closed began
with a 20-minute warm-up period followed by a test-
Figure 30: Calgary Test - ST30 SiRF Unit Run-To-
duration of 60 minutes. The antenna was moved from a
Run Horizontal Positions
surveyed point outside the garage to a surveyed point
inside the garage and the door was closed. The warm-up
period for the following figures will be shown using faded
colours.
The number of satellites tracked by the receivers is shown
in Figure 33. The associated fix density values are then
shown in Figure 34. The results clearly indicate the
failure of the standard GPS receivers to track signals
inside while the HS receivers maintain usable
observations of five to eight satellites during the test. In
addition, the fix density values indicate 3D availability for
99 percent of the test period.
The 2D errors for the receivers tested are shown in Figure
35 along with associated statistics. The HDOP values for
HS14 are shown in Figure 36 along with the number of
satellites used in the solutions. There is no significant
degradation of HDOP for the HS receivers. For a more
Figure 31: Calgary Test - OEM4 Unit Run-To-Run
intuitive representation of the horizontal errors a scatter
plot of the horizontal position is shown in Figure 37.

INDOOR TESTING
By taking a GPS receiver inside, the direct line-of-sight
component of the signals relied upon for effective
navigation is compromised. Attenuation of the direct
signal propagating through various types of materials is
expected as well as increased error due to multipath.
Testing was performed in two different types of
residential garages as shown in Figure 32. A wood frame
garage and then a garage with concrete walls located
under a living room were tested with and without the
garage door closed. For the purposes of this paper, the
worst-case results corresponding to the concrete
structured garage with the door closed are presented.


Figure 35: Concrete Garage Test - 2D Errors

Figure 33: Concrete Garage Test - Number of
Satellites Tracked

Figure 36: Concrete Garage Test - HDOPs

Figure 34: Concrete Garage Test - Fix Density Values

Figure 37: Concrete Garage Test - HS14 Horizontal
Positions


Stephen, J., and G. L achapelle (2001). Development and
Testing of a GPS-Augmented Multi-Sensor Vehicle
CONCLUSIONS
Navigation System. The Journal of Navigation, Royal
Institute of Navigation, 54, 297-319.
The use of high sensitivity GPS receivers in unaided
stand-alone mode results in higher availability of
MacGougan, G., G. Lachapelle, M. E. Cannon, J. Gee,
observations in residential areas, urban canyons, and
and M. Vinnins (2001) GPS Signal Degradation
some indoor environments. 3D Position fixes were
Analysis Using a Simulator. Proceedings of the
obtained more frequently with the HS receivers than the
Institute of Navigation ION Annual Meeting-2001
standard receivers tested under foliage and in urban
(June 10-13, 2001, Albuquerque, New Mexico).
canyons. In the indoor environment tested, the standard
receivers could not operate while the high sensitivity unit Moeglein, M. and N. Krasner (1998) An Introduction to
could still provide positions with accuracy better than SnapTrack Server-Aided GPS Technology. Proceedings
50m. The tracking threshold of the high sensitivity of the Institute of Navigation ION GPS-98 (September
receivers was tested using a GPS hardware simulator and 15-18, 1998, Nashville, Tennessee), 333–342.
found to be at least 9 to 10dB lower than the standard
Peterson, B., D. Bruckner, and S. Heye (1997)
mode GPS receivers tested. The ability to provide
Measureing GPS Signals Indoors. Proceedings of the
pseudorange measurements and positions, when otherwise
Institute of Navigation ION GPS-97 (September 16-19,
impossible using standard tracking, has clear advantages
1997, Kansas City, Missouri), 615–624.
for users in terms of availability. In general, better DOP
results from more usable observations. However, position van Diggelen, F. and C. Abraham (2001) Indoor GPS,
degradation due to increased noise and multipath on the The No-Chip Challange. GPS World , 12(9), 50–58.
measured pseudoranges results from the use of degraded
Van Dierendonck, A. J. (1995), GPS Receivers, Global
observations. More work remains to be done to improve
Positioning Systems: Theory and Applications, Vol I,
position reliability. In a vehicular case, the use of
ed. B.W. Parkinson and J.J. Spilker Jr. (1996),
additional sensors, such a low cost rate gyro, will more
American Institute of Aeronautics and Astronautics,
than likely improve availability and reliability (e.g.
Washington DC, pp. 344.
Stephen & Lachapelle 2001). Indoor, the use of MEMS
accelerometers and miniature direction finding sensors is Presented by G. MacGougan at ION National Technical
likely to be necessary to improve these characteristics. Meeting, San Diego, 28-30 January 2002.
The tests conducted herein dealt only with satellite
reacquisition in degraded environments. Acquisition in
such an environment is more difficult and requires
specific tests.

REFERENCES
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GSS, (2000), “STR Series Multichannel Satellite
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