Effect of Kinematics and Interference on Assisted GPS (AGPS)

M.D. Karunanayake, M.E. Cannon, G. Lachapelle Position, Location and Navigation (PLAN) Group

Department of Geomatics Engineering, University of Calgary, Canada

G. Cox, SiRF Technology Inc.

BIOGRAPHIES M. Dharshaka Karunanayake is an M. Sc. candidate in the Department of Geomatics Engineering at the University of Calgary. He received a B.Sc. in Electrical Engineering from the University of Calgary in 2002. His current research focuses on High Sensitivity GPS and Assisted GPS. Dr. M. Elizabeth Cannon is Professor and Head of Geomatics 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 and the recipient of the 2001 Kepler Award. Dr. Gérard Lachapelle holds a CRC/iCORE Chair in Wireless Location in the Department of Geomatics Engineering. He has been involved with GPS developments and applications since 1980 and has authored/co-authored numerous related publications and software. More information is available on http://PLAN.geomatics.ucalgary.ca 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 E-911 mandate from the Federal Communications Commission (FCC) requires a mobile 911 caller location to be established for 67 percent of calls to within 50 meters, and 95 percent of calls to within 150 meters for wireless handset-based solutions. To meet the E-911 requirement, a GPS receiver can be incorporated into mobile phones which use frequency modulated (FM) signals for communications. The incorporation of GPS into a cellular handset means that a potential jammer will be operating nearby at the cellular frequency.

A second concern is that mobile phone users will be using their phones while they are driving, during which user dynamics increase the Doppler search range in the acquisition process. They also limit the duration of the pre-detection integration time since the Doppler varies quickly for high user dynamics. The focus of this paper is to investigate Assisted GPS (AGPS) receiver performance from two perspectives. The first is to evaluate the acquisition capabilities of an AGPS receiver in vehicular dynamic conditions. The second is to investigate the effect of various Radio Frequency Interference (RFI) effects on AGPS acquisition and tracking performance. A SiRFLocTM evaluation kit is used to investigate AGPS receiver performance. Tests are conducted using a hardware simulator and results are analyzed in terms of Time-To-First-Fix (TTFF) and position accuracy. In dynamic mode, it was found that with increasing velocity, a decrease in position accuracy was seen for both AGPS and HS receivers. Three types of in-band interference were analyzed: Continuous Wave (CW), Amplitude Modulated (AM) and FM. It was found that an AGPS receiver was able to tolerate 5 to 10 dB more interference power than a High Sensitivity (HS) receiver and 10 to 15 dB more than a conventional receiver in acquisition mode. Both AGPS and HS had similar performance while tracking. In all RFI cases, all the receivers were able to tolerate more interference while tracking than in acquisition. INTRODUCTION The Enhanced-911 (E-911) mandate from the Federal Communications Commission (FCC) in the United States requires that a mobile 911 caller location must be established for 67 percent of calls to within 50 meters, and 95 percent of calls to within 150 meters for wireless handset-based solutions [FCC, 2003]. Many wireless carriers are looking at Assisted GPS (AGPS) as the solution for meeting the FCC criteria since the position accuracy from GPS-based solutions is higher than

network-based technologies such as Time of Arrival/Time Difference of Arrival (TOA/TDOA) and Angle of Arrival (AOA) [LaMance et al., 2002]. The E-911 mandate has also led to an emerging market known as location-based services (LBS). Wireless network operators are now deploying services such as driving directions, location of restaurants, banks or movie theatres and tracking of people for safety purposes. Previous research has investigated the effect of timing and assistance data on AGPS receiver acquisition performance using a hardware signal simulator [Karunanayake et al., 2004]. This research was done under static conditions whereas user dynamics increase the Doppler search range in the acquisition process. They also limit the duration of the pre-detection integration time since the Doppler varies quickly for high user dynamics. Aiding data provided by the cellular network server helps to reduce the search space and may improve the acquisition performance under dynamic conditions of an AGPS receiver compared to other GPS technologies such as High Sensitivity (HS) receivers. Mobile phones use frequency modulated (FM) signals for communications and the incorporation of GPS into a cellular handset means that a jammer will be operating nearby at the cellular frequency. For example, Global System for Mobile Communications (GSM) phones used in Europe work either on the 900 MHz or 1800 MHz frequency bands while North American GSM phones primarily use the 1900 MHz band. Code Division Multiple Access (CDMA) technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800-MHz and 1900-MHz frequency bands in the US. The major US carriers using CDMA are Air Touch, Bell Atlantic/Nynex, GTE and Primeco. In 1994, the FCC announced it was allocating spectrum specifically for Personal Communication Services (PCS) technologies at the 1900 MHz band [About.com, 2005]. A summary of handset frequencies and power levels that will manifest themselves as out-of-band GPS jammers is provided in Table 1.

Table 1: Mobile Operating Frequencies and Power Levels [Paddan et al., 2003]

Cellular Standard

Transmit Freq (MHz)

Max. Handset Output Power

GSM 880-915 and 1710-1785 +33 dBm

IS-95 824-849 +23 dBm PCS 1850-1910 +24 dBm

A high jammer power level can cause the generation of unwanted mixing products (spurs) if the level exceeds the

linear range of the circuit blocks. An out-of-band jammer mixes with spectral components to create spurs in the same frequency band as the desired signal. If the power levels are high enough, the resulting spurs may possibly exceed the linear range of the circuit, resulting in the receiver's inability to retain GPS signal lock. There is a possibility of these signals interfering with GPS signals and causing problems for acquisition. Therefore, care must be taken to isolate the mobile signals from GPS to avoid interference and jamming of the GPS signals [Deshpande, 2004]. AGPS-capable cell phones will be subjected to Radio Frequency Interference (RFI) from other sources which can cause degradation of GPS accuracy and reliability. High order harmonics of amplitude modulated (AM) and FM radiobroadcast transmitters emissions fall close to the GPS L1 frequency (1575.42 MHz), which can potentially cause interference. With an AM broadcast, the harmonic order is very high (985) and the likelihood of RFI is minimal. However, for an FM broadcast, the harmonic order is lower (15 to 18) and the maximum effective isotropic radiated power (EIRP) is higher (50 to 60 dBW). Analog TV broadcast maximum EIRP limits are higher than for FM, while harmonic orders are lower (2 to 9 for RFI signals within 2 MHz of GPS L1) and will cause more interference [Erlandson and Frazier, 2002]. Previously, research has been done on the effect of interference on GPS performance, e.g. Johnston [1999], Burns et al. [2002], Deshpande and Cannon [2004] and Deshpande [2004]. Most of that research was performed using a software receiver and focused on acquisition. However, little research has been conducted to investigate the effect of interference on AGPS receiver acquisition and tracking performance. The purpose of this paper is to investigate AGPS from two perspectives. The first is to evaluate the acquisition capabilities of an AGPS receiver in vehicle dynamic conditions. The second is to investigate the effect of various in-band RFI on AGPS acquisition and tracking performance. Results from hardware simulation tests will be quantified in terms of Time-To-First-Fix (TTFF) as well as position accuracy. Results will be compared to other GPS technologies that do not use assistance data. The concept of AGPS is briefly described in the next section since the paper focuses on the performance of AGPS. ASSISTED GPS Cellular phones are mostly used in dense urban areas and indoors, which are the areas where conventional GPS positioning becomes unstable or does not work at all. The

reason for this is that the GPS signal is not powerful enough to penetrate buildings and other obstacles without losing some essential information. The underlying principle behind AGPS is to compensate for the missing information and to improve acquisition sensitivity by providing vital GPS data along with accurate time and/or location from a reference GPS receiver that has a clear line-of-sight (LOS) view of the GPS satellites. The server collects satellite almanac, ephemeris, approximate user position (which is defined by the initial reference position and its associated uncertainty) and timing assistance data from the reference receiver, computes the assistance information and sends the assistance information to the rover receiver, normally via a cellular link. The receiver, which typically resides in the handset of a mobile cellular phone user, uses this information to speed up the acquisition process. In practice, with AGPS, the TTFF can be reduced from 40-60 seconds for stand-alone GPS receivers to 1-10 seconds in LOS conditions [Syrjärinne, 2001]. The basic principle of AGPS is illustrated in Figure 1.

Figure 1: AGPS System The network can provide various types of assistance data including ephemeris, almanac and approximate user position. The ephemeris is valid for up to four hours and can take up to 30 seconds to download a satellite’s ephemeris in a conventional GPS receiver. By providing the ephemeris from an external source, an AGPS-capable mobile handset can focus on acquisition and position computation earlier, which leads to a much lower TTFF. The almanac can be used to predict the approximate location of a satellite. Another parameter the network can provide to an AGPS receiver is the position of the reference station receiver. It is assumed that the rover/user receiver is within 100 km of the reference station and therefore, both receivers have roughly the same visible satellites. From this information, an AGPS receiver (i.e. a

mobile cellular phone user) can determine the satellites in view [Garin et al., 1999]. The accuracy of timing has a significant effect on TTFF. At weak signal conditions (GPS power level less than -140 dBm), one can expect approximately a 30% improvement in TTFF with 125 µs timing accuracy over 250 µs timing accuracy and about a 45% improvement over 500 µs. In case of aiding with initial position, the greater the initial position offset from the reference receiver, the higher the TTFF. Larger offsets also result in larger errors in the position domain. As for the horizontal uncertainty, an effect on TTFF can be seen but no significant effect on the position domain. For more information on the effect of timing and assistance data on AGPS, see Karunanayake et al. [2004]. For the AGPS tests conducted in this paper, the reference receiver will provide ephemeris, almanac as well as 125 µs timing accuracy. As for the initial approximated user position, the location of the reference station will be given with a horizontal uncertainty of 30 km. This is the typical value used in many AGPS field tests conducted in the past. The vertical uncertainty was set to 50 m. TEST SETUP AND RECEIVERS USED All of the tests in this paper were done using a hardware simulator. The GPS RF simulation system at the University of Calgary is the Spirent GSS 6560, which is comprised of a control computer and two synchronous 12-channel L1 Coarse/Acquisition (C/A) code hardware signal simulation units. These units will be referred to as simulator vehicles throughout this paper. The simulator allows real-time control of the signal level (±20 dB with respect to -130 dBm) for each satellite corresponding to one channel of signal [Boulton, 2002]. Interference signals were generated using the Agilent signal generator (E 4431B). This signal generator is capable of generating various types of signals such as continuous wave, swept wave, AM, FM, pulsed signals and broadband noise. The signals from the simulator and the signal generator were combined using an interference combiner (GSS 4766). The four receivers tested in this paper are described below: • SiRFLocTM – This is a 12-channel AGPS receiver.

The aiding information is provided via another receiver, referred to as the Time Transfer Board (TTB). The TTB acts as the reference receiver and can provide assistance data including timing and reference position and associated uncertainties with the initial position provided.

• SiRFXTracTM – This is a 12-channel L1-only C/A code High Sensitivity (HS) receiver which is similar to the SiRFStarII but with improvements made in acquisition and tracking sensitivity.

• SiRF Standard – This is a conventional 12-channel L1-only C/A code GPS receiver based on the SiRFStarIIe LP chipset.

• NovAtel OEM4 – This is a 24-channel dual frequency geodetic receiver, which is optimized for high accuracy performance under normal operating conditions.

The setup used to test the effect of different RF interference during tracking in this paper is shown in Figures 2 and 3. It should be noted that the simulator vehicle 2 is always connected to the TTB, which acts as the reference receiver. Typically, in a real-world AGPS architecture, this reference receiver resides at the base station (see Figure 1).

Figure 2: Interference Tracking Test Setup Diagram

Figure 3: Tracking Test Setup The setup for both acquisition and tracking tests is similar. The only difference is that for the acquisition tests, all the receivers are set to a cold start mode at each trial. In cold start mode, it is assumed that the receiver has

no acquisition aiding information available, meaning it has no information about the current time, the orbits of the satellites or its current position; therefore, it has to perform a full search. However, for tracking, all receivers are initialized at -130 dBm and kept at that power level for the remainder of the test. No cold starts are performed during the test. All hardware simulation tests were designed with 10 to 12 satellites in simulation with no orbital, atmospheric or any other errors. Only the effect of noise on receiver performance will be seen in order to isolate the dynamic and RFI effects from other error sources. The Low Noise Amplifier (LNA) provides a 30 dB gain while also setting the effective noise floor for the receivers under test. The 5 dB attenuator provides the LNA with burnout protection. It should be noted that although the AGPS receiver is cold started for each acquisition test trial, the TTB provides assistance data to the AGPS receiver at the beginning of each trial, after the cold start has been performed. The cold start is done so that the procedure is consistent for all of the receivers tested. DYNAMICS TESTING An acquisition test was performed to investigate the effect of different velocities on receiver acquisition performance. For this test, the power level of each satellite in simulator vehicles 1 and 2 was kept at -130 dBm. As shown in Figure 4 which illustrates the test setup, simulator vehicle 2 was connected to the TTB (which is the reference receiver that provides assistance to the AGPS receiver) and this vehicle was stationary throughout the test.

Figure 4: Dynamics Test Setup A 20-km trajectory as shown in Figure 5 was created. Initially, the vehicle accelerated to a prescribed velocity over a 15 second interval and then maintained this velocity. At this velocity, the simulated car followed the

20-km trajectory for the remainder of the test. The acquisition test was started after the prescribed velocity was reached. The velocity of vehicle 1 was set to one of five levels: 36 km/h, 72 km/h, 108 km/h, 180 km/h and 360 km/h. These dynamic scenarios represent different velocities that may be experienced while driving. The 360 km/h is used to test the sensitivity of the receiver as this would require a really fast automobile!

Figure 5: Dynamic Test Trajectory For the AGPS receiver, the precise timing accuracy provided by the TTB was set to 125 µs. The horizontal uncertainty was set to 30 km. This uncertainty is suitable since the entire trajectory is only 20 km. The vertical uncertainty was only set to 50 m for all of the velocities tested since the height was not varied during the test (i.e. the test trajectory was purely horizontal). The position of the simulated automobile was taken as the truth trajectory. At each velocity, at least 30 trials were conducted. For the dynamic test, a trial is considered to be an acquisition from a cold start followed by 60 position fixes to give a sufficient samples for analysis. The receiver is then cold started to start a new trial. It is assumed that with a cold start, ephemeris, almanac and an initial position is not known prior to acquiring the GPS signal. The raw pseudorange data was extracted at 1 Hz from each receiver and post-processed using the University of Calgary’s C3NAVG2TM software [Petovello et al., 2000]. C3NAVG2 uses a least-squares algorithm to estimate epoch-to-epoch positions, which is suited to analyze the impact of errors on positions and velocities since no filtering is performed. Figure 6 shows the TTFF values for different velocities for both the AGPS and HS receivers. All TTFF values have been normalized, meaning all values were divided by the maximum TTFF value of all the trials in this particular test. The TTFF is at least four times lower with the AGPS receiver over the HS receiver. With the HS receiver, an increase in TTFF is seen as the velocity increases but the TTFF for the AGPS remains constant. As expected, the aiding data provided to the AGPS receiver helps to reduce the search space in acquisition and consequently, a TTFF improvement is seen with

AGPS over the HS. The 2D position error as a function of velocity for both receivers is shown in Figures 7 and 8.

Figure 6: Average TTFFs for Different Velocities

Figure 7: 2D Position Error for AGPS

Figure 8: 2D Position Error for HS

From Figures 7 and 8, it is clear that as the velocity increases, the position accuracy decreases. The AGPS and HS receivers have similar performance. In Figure 9, the AGPS position errors (latitude, longitude and height) for different velocities are shown. From this figure, it is clear that the longitude errors are larger for 360 km/h compared to 180 km/h test. The height errors remain relatively the same since the test trajectory was purely horizontal.

Figure 9: AGPS Position Errors for 72, 180 and 360 km/h

INTERFERENCE

In the following sections, the effect of CW, AM and FM interference on AGPS acquisition and tracking performance is investigated. CW Interference Continuous wave interference was tested in a narrow in-band range of the GPS L1 frequency (Doppler search range of the acquisition process). The interference frequency is within the Intermediate Frequency defined bandwidth of the receiver. The correlation process spreads the continuous wave signal over the pre-detection integration bandwidth and decreases the signal power to

reduce the effect of the interference signal [Johnston, 1999]. The GPS L1 frequency was used as the carrier frequency for the CW signals analyzed. The interference power levels were varied from 0 to +50 dB relative to the GPS signal power, which was set to -130 dBm. Table 2 shows the amount of CW interference power (relative to -130 dBm) that each of the receivers is able to tolerate in acquisition and tracking. In acquisition, the AGPS was able to tolerate 10 dB more CW interference than HS and standard receivers. Better performance from AGPS is expected since the AGPS receiver has a higher acquisition sensitivity than the HS receiver. In tracking, the AGPS and HS receivers were able to track up to 45 dB relative CW interference power. It is not surprising that the AGPS performance in tracking is similar to the HS since aiding provides “coarse” estimates intended to assist acquisition. This “coarse” assistance is not useful in improving tracking performance because when the receiver is in tracking mode, it has a much more precise GPS time and location. The Standard SiRF receiver was only able to track with 30 dB relative CW interference power. However, the conventional geodetic OEM4 receiver was able to track up to 45 dB, the same level as the AGPS and HS receivers. In all cases, the tracking threshold was at least 5 dB higher than the acquisition threshold. In the case of the HS receiver, a 20 dB improvement in tracking over acquisition was seen.

Table 2: Acquisition and Tracking Threshold under CW Interference

Receiver Acquisition (dB) Tracking (dB)

AGPS 35 45 HS 25 45

Standard 25 30 OEM4 N/A 45

The 2D error as a function of relative CW interference power is shown in Figures 10 and 11. The 2D RMS error for different interference power levels during acquisition and tracking is shown in Figure 12. It should be noted that the position results were obtained by post-processing using the C3NAVG2TM software with an HDOP limit of 5 and a VDOP limit of 5. The elevation mask was set to 5°. All of the position results in the interference section are obtained using the above criteria.

Figure 10: 2D Error vs. Relative CW Interference Power (Acquisition)

Figure 11: 2D Error vs. Relative CW Interference Power (Tracking)

Figure 12: 2D RMS Error for each CW Interference Power Interval for Acquisition and Tracking Tests

During acquisition, all of the receivers were able to give a position within less than 4 m (RMS) until 25 dB of interference power. The AGPS, which had the most tolerance under CW interference, was able to acquire up to 35 dB but the position accuracy degrades at the upper limits. This shows a trade-off between interference tolerance and position accuracy. In tracking mode, the OEM4 receiver provided the best position accuracy. Once again, at the upper limits of each receiver, severe performance degradation was seen. In the case of the AGPS receiver, the 2D RMS error was greater than 30 m. AM Interference An AM signal is a CW signal whose amplitude varies as a function of the modulating signal. AM is used widely for radio communications. The AM signal was tested with the carrier frequency at GPS L1 frequency and the modulating signal was set to 10 Hz while the modulation depth of the AM signal was set to 50%. The modulation depth determines the amount of modulation present in the signal. The interference power levels were varied from 0 to +50 dB relative to the GPS signal power, which was set to -130 dBm. Table 3 shows the acquisition and tracking thresholds of each of the receivers used.

Table 3: Acquisition and Tracking Threshold under AM Interference

Receiver Acquisition (dB) Tracking (dB)

AGPS 35 45 HS 30 45

Standard 25 30 OEM4 N/A 45

In acquisition, the AGPS was able to tolerate 5 dB more AM interference than an HS receiver and 10 dB more than a conventional receiver. As in the case with CW interference, three of the receivers (AGPS, HS and geodetic conventional receivers) are able to track up to 45 dB relative AM interference power. The SiRF Standard tracking performance was comparably lower; it was only able to tolerate 30 dB of AM interference. The better performance from OEM4 compared to SiRF Standard even though they are both conventional GPS receivers with similar tracking performance in normal GPS conditions is due to the fact that the OEM4 uses narrow chip spacing and its tracking loops have a lower bandwidth. In all cases, the tracking threshold was higher than acquisition threshold. The 2D error as a function of relative AM interference power is shown in Figures 13 and 14. The 2D RMS error for different interference power levels during acquisition and tracking is shown in Figure 15. From Figure 14, one can clearly see that at the upper limits of each receiver, the position accuracy degrades significantly. For the SiRF Standard receiver at 30 dB of AM interference power, the 2D errors exceed 50 m.

Figure 13: 2D Error vs. Relative AM Interference Power (Acquisition)

Figure 14: 2D Error vs. Relative AM Interference Power (Tracking)

Figure 15: 2D RMS Error for each AM Interference Power Interval for Acquisition and Tracking Tests

During acquisition under AM interference, all of the receivers were able to give a position within less than 5 metres (RMS) up to 25 dB of AM interference power. As in the case of CW interference, the AGPS receiver was able to acquire with up to 35 dB relative interference power but the position accuracy degraded at the upper limits, close to 25 m of 2D RMS error. In the case of tracking with the AGPS receiver, the 2D RMS error at 40 dB appears to be greater than that of 45

dB. This is due to 4 epochs that biased the results. When these 4 epochs are removed from the solution, the 2D RMS error for AGPS at 40 dB is about 2.75 m, which is much lower than the 2D RMS error at 45 dB. FM Interference An FM signal is a continuous wave signal whose frequency varies as a function of the modulating signal. FM signals are used for radio broadcasts in the 88-108 MHz frequency range and for cellular transmission at various frequencies listed in Table 1. The signal level of these FM signals is very high and the high order harmonics of FM signals in the GPS frequency band will have considerable power as compared to GPS signal levels. This will result in interference from the FM signal that needs to be mitigated. The GPS L1 frequency was used as the carrier frequency for the FM signals analyzed. The modulating frequency was kept at 10 Hz while the frequency deviation was set to 2 MHz. The interference power levels were varied from 0 to +50 dB relative to the GPS signal power, which was set to –130 dBm. The acquisition and tracking threshold of each of the receivers used is shown in Table 4. In acquisition, the AGPS was able to tolerate 10 dB more FM interference than an HS receiver and 15 dB more than a conventional receiver. In tracking, AGPS and HS receivers were able to tolerate up to 45 dB relative AM interference power, the same level as CW and AM interference. The Standard receiver was only able to tolerate up to 35 dB relative power while the OEM4 was able to tolerate up to 40 dB of relative FM interference power. Once again, in all cases, the tracking threshold was higher than the acquisition threshold.

Table 4: Acquisition and Tracking Threshold under

FM Interference

Receiver Acquisition (dB) Tracking (dB) AGPS 40 45

HS 30 45 Standard 25 35 OEM4 N/A 40

Carrie-to-noise ratio, C/N0, is the best measurable value of the signal quality present at the input to a GPS receiver. C/N0 is an instantaneous measure of the ratio of the carrier power present to noise power density measured per Hertz of bandwidth. With a minimum guaranteed LOS GPS signal power at -130 dBm, the nominal C/N0 level is 44 dB-Hz (Lachapelle, 2003). Figures 16 and 17 show the average C/N0 for all satellites and the relative FM interference power during acquisition and tracking tests.

All of the receivers are able to tolerate up to 25 dB of relative interference power before the C/N0 decreased.

Figure 16: Average C/N0 vs. the Relative FM Interference Power (Acquisition)

Figure 17: Average C/N0 vs. the Relative FM Interference Power (Tracking)

The 2D error as a function of relative FM interference power is shown in Figures 18 and 19. The 2D RMS error for different interference power levels during acquisition and tracking is shown in Figure 20.

Figure 18: 2D Error vs. Relative FM Interference Power (Acquisition)

Figure 19: 2D Error vs. Relative FM Interference Power (Tracking)

Figure 20: 2D RMS Error for each FM Interference Power Interval for Acquisition and Tracking Tests

During acquisition under FM interference, all of the receivers were able to give a position within less than 5 m (RMS) up to 25 dB of FM interference power. Once again, the position accuracy degraded at the upper limits but the degradation was not as severe. For example, the AGPS receiver was able to provide position within less than 9 metres at 45 dB relative FM interference power. In tracking mode, the position accuracy with FM interference was much better than under CW or AM interference. All of the receivers provided position within an accuracy of less than 5 m (RMS), even at the upper limits. The OEM4 receiver provided the best position accuracy in tracking under FM interference. CONCLUSIONS In this paper, hardware simulation tests were performed using the Spirent GSS 6560 simulator to determine the effect of kinematics and interference on AGPS receiver acquisition and tracking performance. The results were compared to HS and conventional GPS receivers. In dynamic mode, it was found that with increasing velocity, a decrease in position accuracy was seen for both AGPS and HS receivers. Both receivers showed similar results in position accuracy. For HS, an increase in velocity resulted in an increase in TTFF. The effect of CW, AM and FM in-band interference (close to the GPS L1 frequency) on receiver acquisition and tracking performance was analyzed. It was found that the AGPS receiver was able to tolerate 5 to 10 dB more interference power than an HS receiver and 10 to 15 dB more than a conventional receiver in acquisition mode. Both AGPS and HS had similar performance while

tracking. In all RFI cases, all the receivers were able to tolerate more interference while tracking than in acquisition. ACKNOWLEDGMENTS We would like to acknowledge SiRF Technologies Inc. and their staff for providing us the hardware (AGPS receiver and TTB) and their expertise that was invaluable for completing this research. The authors would also like to thank Sameet Deshpande and Nyunook Kim for their expertise and assistance. REFERENCES About.com (2005), Selling the Cell Phone, http://inventors.about.com/library/weekly/aa070899.htm, Accessed January 21, 2005 Boulton, P. (2002), GSS6550 Multi-Channel GPS/SBAS Simulator Product Specification, Issue 1.00, May 21. Burns J., C. Cutright and M. Braasch (2002), Investigation of GPS Software Radio Performance in Combating Narrow Band Interference, Proceedings of ION AM 2002, Albuquerque, NM, June 24-26, pp. 523- 530. Deshpande, S. (2004), Modulated Signal Interference in GPS Acquisition, Proceedings of ION GNSS 2004, Long Beach, CA, September 22-24, pp. 76-86. Deshpande S. and M.E. Cannon (2004), Interference Effects on the GPS Signal Acquisition, Proceedings of ION NTM 2004, San Diego, CA, January 26-28, pp. 1026-1036. Erlandson, R. and R. Frazier (2002), An Updated Assessment of the GNSS L1 Radio Frequency Interference Environment, Proceedings of ION GPS 2002, Portland, OR, September 24-27, pp. 591-599. FCC Report (2003), Report and Order and Second Further Notice of Proposed Rule Making, Report No. FCC 03-290, Washington, DC. http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-290A1.pdf Garin, L. J., M. Chansarkar, S. Miocinovic, C. Norman and Hilgenberg (1999), Wireless Assisted GPS-SiRF Architecture and Field Test Results, Proceedings of ION GPS 1999, Nashville, TN, September 14-17, pp. 489-497.

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