field test assessment of assisted gps and high sensitivity gps receivers under weak/ degraded signal...

Field Test Assessment of Assisted GPS and High Sensitivity GPS Receivers under Weak/

Degraded Signal Conditions

Sanjeet Singh, and M.E Cannon Position Location And Navigation (PLAN) Group

Department of Geomatics Engineering, Schulich School of Engineering, University of Calgary, Canada

R. Klukas, University of British Columbia-Okanagan, Canada

G Cox, SiRF Technology Inc, San Jose, California
BIOGRAPHY Mr. Sanjeet Singh is an M.Sc. candidate in the Department of Geomatics Engineering, Schulich School of Engineering, University of Calgary. He was born in Fiji and obtained a B.Sc. in Electrical Engineering at the University of Calgary. He did his internship as a software engineer at Nortel Networks. His current research focuses on High Sensitivity and Assisted GPS. Dr. M. Elizabeth Cannon is Professor and Head of Geomatics Engineering, Schulich School of Engineering at the University of Calgary. She has been involved in GPS research and development since 1984, and has published numerous papers on static and kinematic positioning. She is a Past President of the ION, current Chair of the ION satellite Division, and the recipient of the 2001 Kepler Award. Dr. Richard Klukas is an Assistant Professor in the Faculty of Applied Science at UBC – Okanagan. He has developed his research expertise in wireless location in both industry and academia and holds a number of patents. Mr. Geoffrey F. Cox is currently Staff Engineer at SiRF Technology, Inc. where he has been since 2000. Before joining SiRF, Mr. Cox worked in various areas of GPS application development for Satloc Inc., Nikon Inc. and NovAtel Inc. from 1996 to 2000. He holds an M.Eng. in Geomatics Engineering from the University of Calgary. ABSTRACT The Federal Communications Commissions (FCC) Enhanced 911 (E-911) phase II mandate requires the
29ION GNSS 18th International Technical Meeting of theSatellite Division, 13-16 September 2005, Long Beach, CA

wireless operators to locate mobile users with an accuracy of 50 m 67% of the time and an accuracy of 150 m 95% of the time. Some wireless carriers are currently using positioning technologies such as Assisted GPS (AGPS) and High Sensitivity GPS (HSGPS) in order to meet the E-911 challenge. A wireless network can aid a AGPS receiver enabling it to acquire and obtain a position fix under weak signal conditions (e.g. inside a building), while the HSGPS receiver may require initialization under open sky conditions before it can be brought inside to continue tracking GPS signals. The purpose of this paper is to investigate the effects of aiding parameters such as satellite ephemeris or almanac, and varying timing or horizontal positioning uncertainties on AGPS signal acquisition. A SiRFLocTM evaluation kit is used to investigate the performance of the AGPS receiver. The other objective is to compare the tracking performance of HSGPS and AGPS receivers. Field tests were carried out in a sub-urban environment, a residential garage and a concrete basement under static receiver conditions. Acquisition tests show the importance of aiding data such as satellite ephemeris and good timing or position accuracy under significantly weak signal conditions, e.g. the concrete basement. Tracking tests show that AGPS and HSGPS receivers have similar results in terms of position solution accuracy, availability and number of satellites tracked. INTRODUCTION The FCC-E911 mandate and Location Based Services (LBS) for mobile users have been driving for accurate positioning solutions, which should be able to work anywhere and all the time. The FCC-E911 phase II mandate requires an accuracy of 50 m 67% of the time and an accuracy of 150 m for 95% of the time for handset

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based positioning technologies (FCC, 2000). Cell phone operators are required to comply with phase II by the end of 2005. There has been an increase in demand for LBS such as mobile gaming, personal navigation; locating a place such as nearby restaurant, hotel or friends. GPS is already used by many cell phone operators because of its superior positioning accuracy when compared to existing positioning technologies such as Enhanced Observed Time Difference of arrival (E-OTD) (Syrjärinne, 2001). GPS signals can undergo signal masking or blockage due to natural or man-made structures. Materials such as glass, steel, wood or concrete can also cause multipath effects, thus degrading the positioning accuracy. Tall glass buildings in an urban or sub-urban environment can block satellites from certain directions, therefore, reducing availability. Some GPS signals may be reflected off these surfaces, which would require a GPS receiver to acquire and or track the reflected signals. An indoor environment presents severe challenges to GPS receivers in terms of signal acquisition or tracking. GPS signals can be attenuated by as much as 20 to 25 dB inside a building compared to outside Line-of-Sight (LOS) conditions (van Diggelen, 2001). Satellite availability is a major problem due to signal blockage and the only source of GPS signals may be reflected multipath signals or highly attenuated LOS signals. The inability of conventional receivers to work in these types of challenging conditions has led to the development of High Sensitivity GPS (HSGPS) and Assisted GPS (AGPS) receivers. In recent years, field tests have been carried out using AGPS or HSGPS receivers under many different field conditions, e.g. Garin et al., (2002), Krasner et al., (2002) and MacGougan (2003). Simulation tests have also been carried out to investigate the effects of different aiding parameters on AGPS (Karunanayake et al., 2004). However, no field tests have been carried out to compare the tracking performance of HSGPS and AGPS or investigate the effects of different aiding data on AGPS which have been published. Field tests represent real life scenarios and consequently it would be valuable to investigate the acquisition or tracking performance of AGPS and HSGPS receivers. The purpose of this paper is to investigate the effects of aiding parameters such as satellite ephemeris or almanac, and varying timing or horizontal positioning uncertainties on AGPS signal acquisition. The other purpose this paper is to compare the tracking performance of AGPS and HSGPS. HIGH SENSITIVITY AND ASSISTED GPS A GPS receiver has to acquire signals to obtain coarse estimates of satellite Doppler or code phase before it is able to commence tracking. Longer coherent and non-


coherent integration is required to acquire and or track signals less than –150 dBm (Chansarkar and Garin, 2000). The coherent integration time is limited to 20 ms due to the navigation data bits. Coherent integration can be carried beyond 20 ms if the navigation data bits could be predicted, however coherent integration is further limited due to residual frequency errors such as satellite motion, receiver, clock instability and receiver induced motion. Integration can be carried out non-coherently to further enhance the receiver sensitivity (Shewfelt et al., 2001). HSGPS receivers use a combination of longer integration time as well as massive parallel correlation to acquire and track weak signals (Rounds and Norman, 2000). Longer integration times could have an adverse effect on the Time to First Fix (TTFF), but parallel searching of different C/A code phases would ensure that this is not the case. Longer integration times enhance sensitivity, while smaller bandwidths of the code or carrier tracking loops reduce the thermal noise, which therefore enables the receiver to track weaker GPS signals (Sundhir et al., 2001). There are a number of factors that could affect the performance of an HSGPS receiver. Thermal noise has to be reduced to minimize the code and carrier tracking errors so that that the receiver can continually track the GPS signals. Clock jitter becomes a considerable error source at low signal levels, thus a stable clock is required to reduce this jitter (McGougnan, 2003). An HSGPS receiver in an unaided mode requires an initialization of 15 to 20 minutes before it can be brought indoors to track GPS signals. The bit error rate (BER) is high in an indoor environment making it impossible for HSGPS receivers to demodulate the navigation data; hence acquisition is generally not possible (Syrjärinne, 2001). The receiver has to download the satellite ephemeris from a cold start resulting in longer TTFF (>30 s, open sky conditions), therefore is not viable for commercial applications such as emergency situations, which have more stringent requirements on the TTFF. Assisted GPS, as shown in Figure 1, gets aiding data such as timing, approximate user position, frequency assistance, satellite ephemeris and almanac from a server via a wireless network, for example. Assistance data is required to aid in the acquisition process, thus enabling a quicker position fix resulting in a shorter TTFF. Assistance data also enhances the acquisition sensitivity of the receiver and several investigations have shown that AGPS can reach acquisition sensitivities of –150 dBm or –155 dBm (van Diggelen, 2001; Bryant et al., 2001). AGPS methods are divided into two categories. The position can be computed at the mobile station (MS), which is called MS-Based or at the network, MS-Assisted. In the MS-Based implementation, the network would send

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all of the assistance data enabling the handset to acquire and obtain a position fix. In the MS-Assisted mode, the handset receives assistance such as visible satellites and satellite Doppler data which enables the receiver to quickly acquire the satellites and to send pseudorange information to the server, which then computes the user position (Pietilä and Williams, 2002). AGPS is like a receiver in hot start mode, whereby it knows the approximate time, has approximate position, ephemeris and frequency thus reducing the C/A-code or Doppler bins, hence lowering the acquisition search time. Satellite ephemeris combined with an approximate user position can be used to predict the approximate satellite Doppler for the user location. Frequency assistance will further reduce the Doppler search space. Timing and approximate position assistance can be used to predict the approximate C/A-code phase. If the combined timing and position assistance is less than one millisecond or 300 km then the approximate code phase can be predicted, therefore an entire sweep of 1023 code chips is not required (Kinnari, 2002). The approximate user position can also be used to determine the visible satellites, while the almanac can be used to predict the approximate location of the satellite constellation.

Figure 1: The AGPS System TEST SETUP AND RECEIVERS USED As shown in Figure 2, field tests were carried out in three different environments whereby the receiver was at a fixed point throughout the test (i.e. static). The three areas included a sub-urban environment, a residential garage and a concrete basement. The receivers used for the tests were SiRFLocTM AGPS, SiRFXTracTM HSGPS and the SiRF standard unit (www.sirf.com). The SiRFLocTM AGPS is a multimode receiver capable of receiving aiding data from various wireless networks such as CDMA or GSM. The SiRF receivers are based on the StarII architecture and will be referred to AGPS, HSGPS and Standard from herein. Tracking and acquisition thresholds for the three receivers are given in Table 1 (Karunanayake et al., 2004). The receivers were all connected to a


microstrip patch antenna, which was located at the surveyed test point, and a splitter was used to connect to each receiver. The tracking tests were conducted using all three receivers, while only the AGPS receiver was used to carry out the acquisition tests. The AGPS receiver received aiding data from the reference receiver, known as the Time Transfer Board (TTBTM). Aiding data would include such things as satellite ephemeris, satellite almanac, frequency assistance, timing uncertainties and approximate user position (broken down into both horizontal and vertical uncertainties). The reference receiver was connected to the NovAtel 700 antenna (referred to as the reference antenna) via a 30 m cable. The reference antenna was located in an open area with clear LOS signals.

Figure 2: Field Tracking Setup Diagram

Table 1: Acquisition and Tracking Sensitivities of

SiRF Receivers (Karunanayake et al., 2004).

Receiver Acquisition Sensitivity

(dBm)

Tracking Sensitivity

(dBm) SiRF AGPS -153 -155 SiRF HSGPS -140 -155 SiRF Standard -133 -141

ACQUISITION TESTS Acquisition tests were carried in each of the three locations. The field sites were chosen to enable the evaluation of the acquisition performance of the AGPS receiver under many different field conditions with varying degrees of signal attenuation and satellite blockage. Various aiding scenarios were tested to determine the effects of different aiding data on AGPS signal acquisition. During each test, more than twenty trials were carried out and 30 position fixes were obtained for each trial. After 30 position fixes, the AGPS receiver would restart from a cold start to obtain the position fix for the next trial. During a cold start, the receiver has no

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prior knowledge of satellite ephemeris, almanac or GPS time. The receiver may fix or may not obtain a position fix (Non-Fix) within the required time of five minutes. Acquisition results were quantified by using factors such as TTFF (normalized with respect to the longest TTFF), positional accuracy, number of satellites used in the position solution and carrier-to-noise ratio (C/N0). The single-point position results were obtained by processing the raw pseudorange data from the receiver using the C3NAVG2 software (Petovello et al., 2000). The C3NAVG2 software, developed by the PLAN group at the University of Calgary uses the least squares algorithm to estimate the epoch-to-epoch position, which is very suitable for our analysis, since no filtering is performed. Aiding Scenarios

1) Timing Uncertainty of: 10, 50, 125, 250 or 500 μs

2) Horizontal Position Uncertainty of: 5, 20, 50, 100 or 350 km

3) No ephemeris 4) No almanac

During the acquisition tests, the approximate user position of the receiver was set to one of the surveyed points at the University of Calgary campus. The horizontal position uncertainty was kept fixed to 5 km for scenario one, while the timing uncertainty was kept the same 125 μs for scenario two. The AGPS receiver would continue to receive almanac and ephemeris for scenarios one and two. The timing and horizontal position uncertainty was set to 125 μs and 5 km for scenarios three and four. The vertical position uncertainty was kept fixed at 150 m for all the scenarios. Sub-Urban Test The sub-urban test was carried out at surveyed point at the University of Calgary campus on October 9, 2004. The test site is shown in Figures 3 and 4. There was a tall glass building on the east side, a smaller concrete building on the west and a glass walkway to the north, which could cause some satellite blockages. The southern side is unhindered with some coniferous trees in the south west side. The test results with varying time or position uncertainties are shown in Figures 5 and 6.


Figure 3: Test Area of the Sub-urban Environment

Figure 4: Surrounding Area for the Sub-urban Environment

Figure 5: Normalized TTFF and Position Fixes for Time Aiding with Ephemeris, Almanac and

Horizontal Position Uncertainty of 5 km

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Figure 6: Normalized TTFF and position Fix

for Horizontal Position Aiding with Ephemeris, Almanac and Timing Uncertainty of 125 μs

Table 2: Position results and Number of Satellites used for Precise Time Aiding

First Five Fixes Thirty Fixes Precise Time

Aiding: (μs) 2D RMS Error (m)

Mean # of SVs

2D RMS (m)

Mean # of SVs

10 40.4 5.1 35.4 6.3 50 47.6 5.2 39.3 6.2

125 44.4 5.3 38.4 6.1 250 44.3 5.3 47.3 7.0 500 57.5 5.3 54.5 6.5

Table 3: Position Results and Number of Satellites used for No Almanac and No Ephemeris Aiding

First Five Fixes Thirty Fixes

Type of Aiding 2D RMS Error (m)

Mean # of SVs

2D Error RMS

Mean # of SVs

No Ephemeris 49.4 7.1 40.4 6.7 No Almanac 41.4 6.7 35.9 7.2

Table 4: Position Results and Number of Satellites used for Horizontal Position Aiding

First Five Fixes Thirty Fixes Horizontal

Position Aiding: (km)

2D RMS Error (m)

Mean # of SVs

2D RMS Error (m)

Mean# of SVs

5 44.4 5.3 38.4 6.1 20 42.6 5.4 38.8 7.6 50 41.2 5.1 31.7 8.1

100 46.4 5.2 34.8 8.5 350 41.2 5.3 36.2 7.9


The sub-urban environment reflected nominal signal conditions as shown by Figure 7 with higher elevation satellites having mean C/N0 of 43 dB-Hz. Field tests under nominal signal conditions showed that changing the timing or position uncertainty does not have any effect on the TTFF. The TTFF was ten times longer without ephemeris and same without almanac when compared to 125 μs time aiding. Field tests in the sub-urban environment showed the importance of satellite ephemeris to reduce the TTFF. The AGPS receiver was able to obtain a position fix 100% of the time and used an average of six satellites to compute a position solution. The 2D position error showed that first five position fixes had worse position accuracy compared to thirty position fixes because fewer satellites were used initially resulting in poor geometry, therefore, degrading the position accuracy for first five fixes.

Figure 7: C/N0 PDF for the Sub-urban Test

Residential Garage Test The acquisition tests were carried out in a residential garage (located within 5 km from the university) on December 9, 2004. The garage setup as shown in Figures 8 and 9 was underneath the living room of a house and the walls are made up of wood and concrete. The door facing the East side was made up of wood, while the wall facing the South side was partially constructed of wood and the remaining two walls facing West and North were made of concrete. The garage door was closed during all acquisition tests and the reference antenna, which was connected to the reference receiver, was located outside the garage. The AGPS receiver was connected to a microstrip patch antenna, which was placed at a surveyed point inside the garage. Results with varying time or position uncertainties are shown in Figures 10 and 11.

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Figure 8: Test Setup for the Garage Test

Figure 9: Surrounding Area for the Garage Test

Figure 10: Normalized TTFF and Position Fix for Time Aiding with Ephemeris, Almanac and



Figure 11: Normalized TTFF and Position Fix

For Horizontal Position Aiding with Ephemeris, Almanac and Timing Uncertainty of 125 μs


First Five Fixes Thirty Fixes Precise

Time Aiding:

(μs)

2D Error (RMS) (m)

Mean # of SVs

2D Error RMS (m)

Mean# of SVs

10 16.8 5.3 23.1 5.2 50 27.2 5.3 23.0 5.2

125 19.0 5.1 37.4 5.0 250 15.4 5.0 29.7 5.4 500 16.5 5.2 21.9 5.3

Table 6: Position Results and Number of Satellites used for No Almanac and No Ephemeris Aiding


Type of Aiding

2D RMS Error (m)

Mean # of SVs

2D Error (RMS)

Mean # of SVs

No Ephemeris 21.3 5.2 18.5 6.4 No Almanac 25.9 5.3 25.2 6.2




2D Error RMS (m)

Mean # of SVs

2D Error RMS (m)

Mean# of SVs

5 19.0 5.1 37.4 5.0 20 56.3 5.4 37.1 4.1 50 45.1 5.4 56.7 4.8

100 25.2 5.2 40.4 3.8 350 43.1 5.3 50.7 3.1

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The residential garage is a typical environment where many end users may live. GPS signals were heavily attenuated from certain directions (concrete walls) with a mean CN0 of 23 dB-Hz and stronger satellites from some directions (wooden walls or front door), which had a mean C/N0 of 33 dB-Hz (see Figure 12). The garage test showed that when the timing uncertainty or the position uncertainty was increased so did the TTTF; for example when the timing uncertainty was changed from 50 μs to 125 μs, the TTFF increased by 50%, similarly when the position uncertainty was changed from 20 km to 100 km, the TTFF increased by 100%. The TTFF without ephemeris was six times longer than time aiding of 125 μs, while the test without the almanac had the same TTFF as the 125 μs case. The AGPS receiver used an average of five satellites to compute the position solution and obtained a position fix 100% of the time during all the acquisition tests. The position accuracy for the five first position fixes was better than for thirty position fixes because occasionally during the thirty fix case the receiver would lose lock of the weaker signals which resulted in poor geometry.

Figure 12: C/N0 PDF for the Garage Test

Concrete Basement Test The acquisition test as shown in Figures 13 and 14 was carried out in a concrete basement which is located at the University of Calgary campus on November 30, 2004 The basement represented an extremely difficult environment with a lot of satellite blockages and highly attenuated signals. There is a door located on the North side, which was made of wood and two small windows were located on the North West side. The remaining three side walls were made of reinforced steel concrete. Similar to the previous tests, the AGPS receiver was connected to a microstrip patch antenna which was placed at a surveyed point inside the concrete basement, while the reference antenna was kept outside. The results with varying time or position uncertainty are shown in Figures 15 and 16.

ION GNSS 18th International Technical Meeting of the
29Satellite Division, 13-16 September 2005, Long Beach, CA
Figure 13: Test Setup for the Concrete Basement Test

Figure 14: Surrounding for the Concrete Basement

Test

Figure 15: Normalized TTFF and Position Fix for Time Aiding with Ephemeris, Almanac and


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Figure 16: Normalized TTFF and Position Fix

for Horizontal Position Aiding with Ephemeris, Almanac and Timing Uncertainty of 125 μs


First Five Fixes Thirty Fixes Precise

Time Aiding:

(μs)

2D Error RMS (m)

Mean # of SVs

2D Error RMS (m)

Mean # of SVs

10 60.1 5.2 49.6 4.7 50 61.0 5.1 78.2 4.0

125 61.7 5.3 68.4 3.6 250 54.5 5.4 55.0 3.6 500 43.0 5.3 77.0 3.2

Table 9: Position Results and Number of Satellites used No Almanac Aiding


Type of Aiding

2D Error RMS (m)

Mean # of SVs

2D Error RMS (m)

Mean # of SVs

No Almanac

63.2 4.1 56.1 3.4




2D Error RMS (m)

Mean # of SVs

2D Error RMS (m)

Mean# of SVs

5 61.7 5.3 68.4 3.6 20 97.5 5.1 58.5 3.2 50 48.7 5.4 86.4 3.3

100 86.0 5.2 104.1 3.2 350 62.6 5.3 62.3 2.7


The concrete walls severely attenuated the GPS signals (see Figure 17), and satellites had mean C/N0 of 20 dB-Hz. When the timing or position uncertainty increased so did the TTFF, for example when the timing uncertainty was changed from 50 μs to 125 μs, the TTFF increased by 100%. Similarly, when the position uncertainty was changed from 50 to 100 km, the TTFF increased by 40%. AGPS was unable to acquire without ephemeris assistance because of really weak signals, which resulted in higher BER making it impossible for the AGPS receiver to demodulate the navigation data. The TTTF without almanac was similar to the time aiding of 125 μs. The number of satellites used in the position solution was anywhere from three to five illustrating significant satellite blockage. The percentage of successful position fixes decreased with increasing time or position uncertainty, for example when the timing uncertainty was changed from 50 to 500 μs the percentage of position fixes decreased from 95% to 50%. Similarly when the position uncertainty was changed from 20 to 100 km the percentage of position fixes decreased from 85% to 72%. The position accuracy for the thirty position fixes was worse than five fixes because fewer satellites were used which would result in poor geometry thus degrading the position accuracy for thirty fixes.

Figure 17: C/N0 PDF for the Concrete Basement Test

Comparison of the Three Environments A comparison of three environments is required to illustrate the difference in terms of position accuracy and TTFF, which would be explained by using factors like C/N0 and number of satellites tracked. The results with varying time uncertainty are shown in Figure 18, Similar trends (TTFF and number of satellites tracked\) were observed with varying position uncertainty, so is shown here. The acquisition tests in the concrete basement had the longest TTFF, used to lowest number of satellites and had the worst position accuracy because the signals were

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severely attenuated (20 dB-Hz) and had significant satellite blockage, when compared to the other two environments. The acquisition tests illustrate the importance of good timing or positioning uncertainties under weak signal conditions. Accurate time or position aiding gives a better estimate of the code phase and Doppler, thus reducing the acquisition search time. When the position or timing uncertainties are increased under weak signal conditions, the C/A-code phase and Doppler search space is increased resulting in a longer search time or TTFF. Acquisition tests showed that the almanac is not required for signal acquisition because it gives coarse estimates of the satellite orbital parameters, but the receiver requires much finer estimates, which can be provided by the satellite ephemeris. When the receiver is not given the ephemeris, then it has to download it, which can take up to 30 s under open sky conditions, thus lengthening the acquisition search time. Furthermore, it is impossible to demodulate the navigation data or ephemeris under weak signal conditions found in the concrete basement, for example.

Figure 18: Normalized TTFF and Number of Satellites Tracked for the Three Environments for Time Aiding TRACKING TEST Tracking tests were carried out at the same test sites using the AGPS, HSGPS and standard receivers. Tracking tests that were carried out inside a building required an initialization of twenty minutes under open sky conditions to ensure that the receivers had the complete satellite almanac and ephemeris (MacGougan, 2003). The AGPS

receiver is able to acquire the signals inside the garage, however initialization was carried out on both receivers to keep the methodology consistent. The reference receiver provided aiding data to the AGPS such as satellite ephemeris and almanac. Time and position uncertainty was kept at 125 μs and 5 km respectively. The tests were carried out to illustrate such factors as multipath effects, signal blockage, signal attenuation and solution availability under different field test conditions. Availability is defined as the percentage of the time that a successful position fixes may be ascertained. The tracking tests were analyzed through the horizontal and vertical errors, number of satellites used in the position solution, C/N0 and measurement residuals. The residual errors (obtained in post-mission using the C3NAVG2 software) and CN0 were obtained from the AGPS receiver. Sub-Urban Test The tracking test was carried out for two hours near a known point at the University of Calgary campus on October 9, 2004. The test setup (shown in Figures 3 and 4) was similar to the acquisition test setup, except the AGPS, HSGPS and standard receivers were all used testing this case. The azimuth/elevation of satellites tracked is given in Figure 19, while the horizontal and vertical errors of the three receivers are shown in Figures 20 and 21.

Figure 19: Azimuth Elevation for Satellites Tracked

During the Sub-urban Test.

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Figure 20: Horizontal Errors for the AGPS, HSGPS

and Standard Receivers in the Sub-urban Test

Figure 21: Vertical Errors for the AGPS, HSGPS and

Standard Receivers in Sub-urban Test

The tracking test in the sub-urban environment showed that the AGPS and HSGPS receivers had similar performance in terms of horizontal and vertical accuracy, number of satellites used and solution availability. AGPS used an average of six satellites with 99% availability, had horizontal and vertical accuracies of 43.4 and 52.5 m, while the HSGPS used an average of seven satellites with 9%% availability, and horizontal and vertical accuracies of 37.9 m and 55.5 m. The standard receiver used an average of six satellites with 97% availability with horizontal and vertical accuracies of 31.6 m and 37.0 m. The close proximity of the test site to the glass buildings made the receivers vulnerable to multipath effects. Further residual and C/N0 analyses were carried out to determine the effects of multipath. The location of PRNs 26 and 29 could cause some reflected signals from the tall glass building on the East side. When the C/N0 is plotted in Figure 22, it shows that two satellites had deep fades when compared to PRN 31 (higher elevation), suggesting that the two satellites (PRN 26 and PRN 29) were affected


by reflected signals. This was further reflected in the residual plot as shown in Figure 23 where PRNs 26 and 29 had larger residual errors compared to PRN 31. After PRN 26 was rejected, the AGPS receiver had a horizontal accuracy of 35.4 m. Similar results were observed when PRN 29 was rejected, however, when PRNs 26 or 29 were rejected for the standard receiver, the horizontal position accuracy degraded suggesting that the AGPS receiver was tracking multipath signals.

Figure 22: Time Series Showing the C/N0 for Satellites

26, 29 and 31 in the Sub-urban Test

Figure 23: Time Series Showing Residual Errors for

Satellites 26, 29 and 31 in the Sub-urban Test

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Residential Garage Test The tracking test in the garage was carried out on December 9, 2004 for 1.5 hours. The standard receiver was unable to track signals indoor, therefore only the AGPS and HSGPS receivers were used for the test. An initialization of twenty minutes was carried outside before the receivers were brought inside the garage to carry out the tracking test. The AGPS receiver is able to acquire the signals inside the garage, however initialization was carried out on both receivers to keep the methodology consistent. The tracking tests were carried out inside the garage with the closed door. The azimuths/elevations of the satellites are shown in Figure 24, while the horizontal and vertical errors are shown in Figures 25 and 26.

Figure 24: Azimuth and Elevation for Satellites

Tracked During the Garage Test

Figure 25: Horizontal Errors for AGPS and HSGPS Receivers During the Garage Test


Figure 26: Vertical Errors for AGPS and HSGPS Receiver for the Garage Test

The AGPS and HSGPS receivers gave similar results in terms of position accuracy, average number of satellites used in the position solution and availability. The AGPS receiver used an average of seven satellites, with 99% availability, horizontal and vertical position accuracies of 20.8 m and 30.9 m, while the HSGPS used seven satellites with 98% availability, and horizontal and vertical accuracies of 18.9 m and 31.0 m. The garage illustrates varying degrees of signal attenuation. For example PRN 7 had stronger signals compared to PRN 26 because it was entering from the wooden wall on the south side, while PRN 26 signals entered from concrete walls as shown in the C/N0 plot in Figure 27. Weaker signals such as PRN 26, means higher thermal noise resulting in larger code or tracking errors which are illustrated in Figure 28, where PRN 26 has larger residual errors when compared to PRN 7.


7 and 29 in the Garage Test

0

Figure 28: Time Series Showing Residual Errors for

Satellites 7 and 26 During the Garage Test Concrete Basement Test The AGPS and the HSGPS receivers were used to carry out tracking tests in the concrete basement on December 6, 2004. Similar to the garage test, the receivers were initialized outside for twenty minutes and then brought indoor, where the tracking tests were carried out for another two hours. The azimuths/elevations for the satellites tracked are shown in Figure 29, while the horizontal and vertical errors are shown in Figures 30 and 31.

Figure 29: Azimuth and Elevation for Satellites

Tracked During the Concrete Basement Test

Figure 30: Horizontal Errors for AGPS and HSGPS

Receivers in the Concrete Basement Test


Figure 31: Vertical Errors for AGPS and HSGPS Receivers During the Concrete Basement Test

The AGPS and the HSGPS gave similar results in terms of position accuracy, number of satellites used and availability. The AGPS receiver used an average of six satellites, with 95% availability, horizontal vertical position accuracies of 47.8 m and 57.2 m, while the HSGPS receiver tracked an average of six satellites, with 97% availability, and horizontal and vertical position accuracies of 45.3 m and 54.6 m. The GPS signals were highly attenuated as shown in Figure 35, where PRN 24 and PRN 31 had mean C/N0 values of 24.6 and 20.0 dB-Hz. The receivers were tracking weak signals (less than 20.0 dB-Hz) that were close to the tracking threshold. Therefore, they would frequently lose lock on the satellites resulting in large code tracking errors, which are shown in the residual plots (see Figure 33).


24 and 31 During the Concrete Basement Test

Figure 33: Time Series Showing Residual Errors for Satellites 24 and 31 During the Concrete Basement

Test Comparison of the Three Environments The AGPS and HSGPS receivers had better positioning accuracy in the residential garage test compared to the sub-urban test, although it had had lower signal strengths. The sub-urban test site was located close to a tall glass building which caused strong specular reflections or multipath effects, introducing pseudorange errors, hence degraded the position accuracy. The residential garage on the other hand was subject to diffuse reflections (due to the material such as wood or concrete), which is not as strong as specular reflections. The concrete basement had really weak signals (20 dB-Hz), higher thermal noise, which led to larger code tracking errors, resulting in a lower positioning accuracy compared to the other two environments. The positioning accuracy was better for the tracking tests because there were more position fixes (>> 30 fixes). As more positioning fixes were obtained, the tracking loops (carrier phase or code tracking loops) reduces the Doppler or code-phase errors, hence the tracking errors are minimized which were illustrated in the results. Similar position accuracy results for the three environments for the HSGPS and AGPS receiver suggest that aiding data does not enhance the tracking performance because it provides coarse estimates of the C/A code or Doppler, however, the tracking loops require much finer estimates of these parameters. CONCLUSIONS The acquisition tests showed that changing the timing or horizontal position uncertainty had no effect on TTFF under nominal signal conditions such as the sub-urban environment but had really long TTFFs under weak signal conditions such as the concrete basement. The aiding data that is essential for the AGPS receiver during signal acquisition is the satellite ephemeris, a timing accuracy of 125 μs and the approximate location with accuracy of 5 km. The SiRF AGPS was able to obtain a position fix in

a reasonable amount of time (<30 s) under different field conditions making it suitable for LBS and E911. Field tracking tests in different environments showed the ability of AGPS and HSGPS to track signals in many different signal conditions. Both receivers had similar results in terms of position accuracy, availability and number of satellites used in the position solution. All the field tests were carried out under static conditions thus in future further field tests needs to done under kinematic conditions (Karunanayake et. al, 2005). The tests also showed that AGPS had very long TTFFs under really weak signal conditions such as the concrete basement, requiring further investigation on the possibility of hybrid technologies such as AGPS with Wireless Local Area Network (WLAN) (Eisfeller et al., 2004). ACKNOWLEDGMENTS We would like to thank SiRF Technology Inc. and their staff for providing the AGPS and reference receivers and their technical support, which was very valuable for this research. We would also like to thank Merlin Keillor, Judy Smith and staff at the Disability Resource Centre for their continued support. Dharshaka Karunanayake (my friend and AGPS colleague) and fellow colleagues in the PLAN group are thanked for their encouragement and support throughout this research. REFERENCES Bryant, R, S. Dougan and E. Glennon (2001), GPS Receiver Algorithms and Systems for Weak Signal Operation, Proceedings of ION GPS-2001, September 11-14, Salt Lake, UT, pp 1500-1510 Chansarkar, M and L. Garin (2000), Acquisition of GPS Signals at Very Low Signal TO Noise Ratio, Proceedings of ION NTM 2000, Anaheim CA January 26-28, pp. 731-737. Diggelen, F (2001), Global Locate Indoor GPS Chipset & Services, Proceedings ION GPS-2001, September 11-14, Salt Lake City UT, pp. 1515 1521. Eisfeller, B. (2004), D. Gänsch, S. Müller, and A. Teuber, Indoor Positioning using Wireless LAN Radio Signals, Proceedings of ION GPS-2004 September 21-24, Long Beach, CA, pp. 1936-1947. Federal Communications Commission – Enhanced 911 (FCC-E911) Mandate (2000), http://www.fcc.gov/Bureaus/Engineering_Technology/Public_Notices/1999/da992130.html, (Accessed on August 8, 2005)

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Garin, L. J, M. S. Phatak, and H. Gehue (2002), Indoor GPS Test Methodology and Indoor Performance Tests, Proceedings of ION GPS-2002, September 24-27, Portland, OR, pp. 192-199. Karunanayake, M. D M.E Cannon, G. Lachapelle and G. Cox (2004),, Evaluation of Assisted GPS (AGPS) in Weak Signal Environments Using a Hardware Simulator, Proceedings of ION GPS-2004 September 21-24, Long Beach, CA, pp. 2416-2426. Karunanayake, M.D., M.E. Cannon, G. Lachapelle and G. Cox (2005), Effect of Kinematics and Interference on Assisted GPS (AGPS), Proceedings of ION NTM 2005, San Diego, CA, January 24-26, pp. 1071-1081. Kinnari, T. (2001), Accurate Time Transfer in Assisted GPS, MSc Thesis, Tempere University of Technology, Finland. Krasner, N.F., G. Marshall and W. Riley (2002), Position Determination Using Hybrid GPS/Cellphone Ranging, Proceedings of ION GPS-2002, September 24-27, Portland, OR, pp. 165-176. MacGougan, G. (2003), High-Sensitivity GPS Performance Analysis in Degraded Signal Environments, M.Sc. Thesis, The University of Calgary www.geomatics.ucalgary.ca/links/GradTheses.html Petovello, M., M.E. Cannon and G. Lachapelle (2000), C3NAVG2 Operating Manual, Department of Geomatics Engineering, University of Calgary, May. Pietilä, S and M. Williams (2002), Mobile Location Application and Enabling Technologies, Proceedings of ION GPS-2002 September 24-27, Portland, OR, pp, 2416-2426. Rounds, S. F and C. Norman (2000), Combined Parallel and Sequential Detection for Improved GPS Detection, Proceedings of ION NTM 2000, San Diego CA January 26-28, pp. 368-372. Shewfelt, J.L, R. Nishikawa, C. Norman and G.F. Cox (2001). Enhanced Sensitivity for Acquisition in weak Signal Environments through the use of Extended Dwell Times, Proceedings ION GPS-2001, September 11-14, Salt Lake City, UT, pp.155-162. Sudhir, N S, C. Vimala and J.K. Ray (2001), Receiver Sensitivity Analysis and Results, Proceedings ION GPS-2001, September 11-14, Salt Lake City, UT, pp. 1420- 1426.


Syrjärinne, J. (2001), Studies of Modern Techniques for Personal Positioning PhD Thesis, Tempere University of Technology, Finland. Syrjärinne, J. and T. Kinnari (2002), Analysis of GPS Time-Transfer Accuracy in GSM and UMTS Networks and Possibilities to Improve Sensitivity, Proceedings of ION GPS-2002, September 24-27, Portland, OR, pp. 184-191.

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