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Less than 1 second GPS hot-start TTF below -150dBm without A-GPS


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Less than 1 second GPS hot-start TTF below -150dBm without A-GPS

  1. 1. <1 second GPS hot-start TTF below -150dBm without A-GPS David Tester, Senior Member IEEE Air Semiconductor Cherry Orchard North, Kembrey Park Swindon, Wiltshire, SN2 8UH, UK Abstract—Minimization of TTFF has driven the architecture for III. SUMMARY OF GPS SYSTEM PERFORMANCE GPS receivers, being motivated by the historical navigation paradigm with more recent cellphone activity and applications. The GPS air interface is defined in [3]. Additional details Existing solutions minimize TTFF through use of assist on system design and operation can be found in [4] and [5]. information, eliminating any requirement to collect the live Each satellite transmits a CDMA signal at 154x the system satellite ephemeris and minimize acquisition time using frequency of 10.23MHz at 1.58GHz. The C/A signal at L1 is maximum effective correlators. Cellular solutions obtain information through A-GPS assistance. In-car solutions use spread using Gold codes [6]. Worst case cross-correlation local or remote calculated extended ephemeris. Recent receiver between the 1023 chip codes used for L1 GPS is 21.6dB. solutions deliver ‘continuous location awareness’ [1] [2]. The Unobstructed receive power is no less than -130dBm over the emergence of this new category of GPS receivers supporting satellite lifetime with a spread of 6dB due to satellite age. proactive rather than reactive operation of LBS services raises Observed power in a typical environment can be 30dB less! additional system level trade-offs, some conflicting with TTFF. Each satellite transmits a 37,500 bit navigation message Receivers optimized for TTFF can be adapted for pseudo- proactive GPS functionality. However improved performance through a 25 frame TDMA protocol. Each frame contains can be delivered through use of an architecture not explicitly 1,500 bits of data and is comprised of 5 sub-frames each optimized for TTFF. The impact of local reference frequency lasting 6 seconds containing ten 30 bit words. The entire error for these ‘dual mode’ GNSS receivers is examined. The message provides both satellite (ephemeris) and constellation different receiver architectures capable of delivering continuous (almanac) information. Ephemeris transmission requires 30s. location awareness are compared. I. INTRODUCTION IV. SUMMARY OF A-GPS NETWORK ASSISTANCE Location has emerged as core functionality for consumer A-GPS is an enhancement to GPS. Aiding information is provided to the receiver by a cellular network. Satellite data devices such as mobile phones and digital cameras. relating to orbits (ephemeris), coarse receiver location and GPS enabled mobile phones offer navigation capabilities and system time are provided. Minimum performance other location based services. GPS enabled digital cameras requirements for assisted GPS are defined for both 2G and 3G can location-stamp photographs (often called “geotagging”). cellphone networks by various standardization bodies such as This paper outlines the performance required by the emerging 3GPP, TIA and OMA [7], [8], [9], [10] and [11]. generation of proactive location-aware end-user applications. Various assistance scenarios are defined [7] and minimum Alternative architecture options to deliver proactive GPS are receiver time-to-fix (TTF) and location accuracy performance considered and the relative strengths of each are compared. is specified. Scenarios correspond to coarse-time assistance, The paper concludes with details of the architecture capable fine-time assistance, dynamic, open-sky and urban operation. of providing continuous GPS operation in this new paradigm. The open-sky coarse-time sensitivity scenario operates with II. SUMMARY OF THE PAPER eight satellites, HDOP range 1.1 to 1.6 and time assistance of ±2s. Receive power for all satellites is -130dBm and the Sections III and IV outline details of the GPS and A-GPS minimum requirement for CEP95 position error is 30m (2D systems. Section V describes the performance needed for position error to 95% probability) within a TTF of 20 seconds. various location based service (LBS) applications. Section The urban coarse-time sensitivity scenario operates with eight VII details the constraints for minimization of time-to-first- satellites, HDOP range 1.1 to 1.6 and time assistance of ±2s. fix (TTFF) and time-to-fix (TTF). Section VIII investigates Receive power for one satellite is -142dBm with the other architecture options for TTF rather than TTFF optimization. seven satellites receive power of -147dBm. The minimum Finally, section IX compares the performance of the proposed requirement for CEP95 error is 100m within a TTF of 20s. approach with alternative architectures based around A-GPS. The fine-time sensitivity scenario operates with eight satellites, HDOP range 1.1 to 1.6 and time assistance of ±10µs with receive power for all satellites at -147dBm. Minimum
  2. 2. requirement for CEP95 error is 100m with TTF of 20 seconds. LBS application only requires course location then low update The second coarse-time dynamic scenario operates with six rates are appropriate. However if the application requires satellites, HDOP range 1.4 to 2.1 and time assistance of ±2s. accurate location continuously then conventional 1Hz rates are Receive power for one satellites is -129dBm, one satellite at - suitable. Understanding that future LBS applications can 135dBm, one satellite at -141dBm and three satellites at - operate with varying update rate GPS sub-systems is critical to 147dBm. Maximum CEP95 error is 100m with 20s TTF. the delivery of proactive LBS applications [16] [17] [18]. Coarse-time multipath performance scenario operates with five satellites, HDOP range 1.8 to 2.5 and time assistance of VI. ARCHITECTURE FOR PROACTIVE LBS APPLICATIONS ±2µs. Receive power for two satellites is -130dBm with the Releasing the GPS receiver from the constraints of remaining three satellites direct receiver power of -130dBm traditional continuous-mode tracking with fixed frequency with each of these three satellites also providing a multipath update rate provides a new degree of freedom in architecture signal at -136dBm. CEP95 error is 100m with a TTF of 20s. definition and enables the development of GNSS receivers The final dynamic scenario operates with five satellites, capable of operating at very low power levels of around 5mW. HDOP range 1.8 to 2.5, time assistance of ±2s with all With total receiver power in the region of 5mW the GPS satellites at -130dBm. Requirement for CEP95 position error receiver can continuously operate in the background without is 100m with an position update rate of 2s. impacting power budget and significantly affecting the time between battery recharge cycles for mobile devices. TTF in all cases is 20s. Required position accuracy is 30m in open-sky static conditions and 100m in all other scenarios. Critically, background operation of the GPS receiver delivers three major advantages: predictable (and minimal) V. DISCUSSION OF LBS APPLICATION REQUIREMENTS TTF for immediate position update in push-to-fix operation, The killer application for GPS today is point-to-point guaranteed availability of recent location with zero second navigation. Initially used only for in-car navigation this has TTF and pseudo-operation indoors since indoor operation of become widely used with many cellphones [12]. As location GPS is difficult and low-power indoor GPS is an oxymoron. becomes available for mobile devices, further services become available [13] along with additional privacy concerns [14]. The receiver architecture described in this paper is capable of providing push-to-fix operation with TTF effectively Navigation demands a positioning solution capable of providing maximum accuracy with the maximum practical independent of GPS signal conditions. LBS operation in update rate and minimum time-to-first-fix (TTFF). Target indoor conditions is maintained as a side-effect of continuous performance is 1Hz update rate with <1m position error. background operation. Alternative approaches would fail to Early cellphone applications for GPS assumed location would detect satellite signals indoors, not deliver service quality and be required infrequently and that power consumption of the eventually report “location unavailable” to LBS applications. GPS solution would, as a result, not impact the power budget Traditional GPS receivers operate in either search mode or for the cellphone. Weak signal GPS operation and TTFF was navigation tracking mode. Minimization of TTFF has lead to a major concern. Target performance is weak signal TTFF not ever increasing numbers of effective correlators for the search exceeding 20s with position accuracy better than 100m [7]. engine, resulting in solutions offering efficient performance Emerging LBS applications require a positioning sub-system when searching huge numbers of candidate bins. A-GPS which is capable of continuous background operation. When receivers maximize the number of effective correlators. the receiver moves into or close to a geographic area of Conventional tracking operation operates the GPS radio and significance the GPS sub-system triggers the LBS application. tracking subsystems continuously. Power consumption for Continuous background operation of the GPS receiver enables radio operation approaches practical limit of 10mW [19] [20]. proactive operation of the LBS application. The user is Reduction of GPS sub-system power consumption below this automatically alerted rather than enabling the GPS to level demands the receiver is operated in discontinuous mode determine if desired services are available in that area. rather than traditional continuous mode triggering a decision Proactive LBS requires continuous operation of the GPS sub- on receiver architecture. The receiver must be periodically system which in turn requires a GPS engine with a power activated to “duty-cycle” the operation and minimize power. footprint compatible with the average power consumption of Should the receiver activate only when location is requested or the host device. For cellphone use 5mW operation is required. would a more effective solution automatically activate itself to When user location is continuously available, geographic maintain more detailed knowledge for visible GPS signals? triggers can be defined to automatically launch LBS services. VII. FREQUENCY - CODE PHASE - SV PRN SEARCH SPACE If location is not continuously available these services can only be triggered in response to the user enabling the GPS As received GPS signal strength decreases from -130dBm sub-system. Examples of proactive triggers are listed in [15]. to -160dBm the resulting coherent integration time required to Proactive LBS applications enable the immediate delivery of maintain target SNR increases with corresponding decrease in services. An example is location-aware push advertising. bandwidth of each search bin from 500Hz to below 10Hz [20]. Provision of continuous positioning requires a continuous Stability of the local reference frequency translates to error in stream of location information from the GPS sub-system to conversion of the RF signal to IF due to difference between the LBS application(s). Different LBS services will require the locally generated LO and the target mixing frequency. differing levels of position quality from the GPS sub-system. Mixing from RF to IF results in additional pre-correlation Providing a continuous stream of location updates is not the frequency error, which will depend on reference conditions. same as delivering updates as frequently as possible. If the 0.5ppm reference stability leads to frequency error of 788Hz.
  3. 3. 1,000 1,000,000 Pre-Correlation Frequency Error (Hz) 900 800 Correlation Bins for TTFF 750,000 700 600 500 500,000 400 300 250,000 200 100 0 0 0.05 0.5 -140 -145 -150 -155 -160 Reference Frequency Stability (PPM) GPS signal strength (dBm) Figure 1. Effect of Local Reference Frequency Error Figure 2. Increase in correlation bins with GPS signal strength Cold start TTFF is defined by number of code phase and The receiver has no absolute time or frequency reference frequency search bins, number of satellites, elapsed time to and must make time hypothesis which are validated against collect ephemeris and solve position. Minimum time is 30s. the GPS signal itself to ensure local receiver time predictions In contrast, the aided hot start TTFF is defined by the time to remain within suitable tolerances. This requires the receiver only search frequency and code-phase and solve for position. to periodically activate and re-lock to the GPS transmission. A-GPS hot start TTFF can be 1s in suitable signal conditions. Existence of accurate local time enables use of minimum size Reduction in GPS signal strength from -130dBm to -160dBm search windows for re-detection of the GPS satellite signals. increases the number of search bins as shown in Figure 2. The virtuous circle that results is the critical factor enabling self-assistance of the GPS receiver [22], [23] and [24]. The Provision of coarse receiver location and ephemeris allows resulting architecture needs a micro-searcher for re-detection the receiver to determine which satellites are known to be not of GPS time. Since the receiver operates by maintaining GPS visible and reduces TTFF by 30s. The resulting aided hot start time rather than accurate receiver location, only one satellite time is defined by the frequency-code phase search space, needs to be detected (in stationary conditions) for operation. number of effective correlators and time to solve for position. Location is a side-effect of fine-time receiver self-assistance. Lack of accurate time aiding and knowledge of variation in reference frequency limits the aided hot-start TTFF and as the The fine-time self-assistance approach forms the basis of a GPS signal strength decreases the increased number of search recent GPS receiver development. Tracking sensitivity of the bins coupled with increased dwell time per bin impacts TTFF. solution in self-assist mode has been measured to -150dBm and is expected to operate to at least -154dBm. Unfiltered Provision of accurate time aiding allows the code-phase CEP50 position accuracy (over 12 hours) for the receiver search space to be minimized. A-GPS offers two levels of operating without a Kalman filter is 2.8m and the self-assisted time aiding: ±10µs fine-time aiding and 2s coarse-time aiding. hot-start TTF is 2.5s over signal power -130dBm to -150dBm. With no external time aiding an autonomous GPS receiver drifts from GPS time at a rate of around 0.06ms/min Performance of the resulting self-assisting GPS receiver corresponding to loss of the 1ms epoch within 15 minutes. exceeds all sensitivity, TTF and accuracy requirements for the 3GPP standard yet needs 200 rather than 200,000+ correlators. The exponential increase in frequency-code phase search space results from the loss of accurate time and the uncertainty IX. CONCLUSIONS around local receiver reference frequency. Controlling impact Traditional GPS receivers deliver time from tracking of these two factors enables the hot-start search space to be location, alternative architectures can deliver location through minimized and delivers TTF that is (almost) independent of tracking time. Despite additional complexity tracking time the receiver GPS signal power. The architecture presented in provides the ability for a GPS receiver to provide self-assist the next section delivers push-to-fix TTF that is effectively information, enabling enhanced push-to-fix TTF compared to independent of GPS signal conditions, exceeding A-GPS alternative traditional approaches. performance, delivering TTF of 2.5s even with received signal levels of -150dBm but requiring only 200 local correlators. Earlier approaches have adapted GPS receivers originally Alternative approaches require 200,000+ correlators and intended for navigation to deliver LBS functionality [20] [25]. network aiding information to deliver comparable TTF. The approach described in this paper forms the basis for a VIII. SELF-ASSIST GPS ARCHITECTURE AND PERFORMANCE GPS receiver optimized for use with LBS applications [22]. When tracking the GPS transmission the receiver is locked The receiver has been successfully used to enable the directly to GPS time and whilst not tracking, the receiver local emerging “geotagging” application for use in digital cameras. time will drift compared to satellite time. The rate of drift Resulting performance for the self-assist GPS receiver depends on the relative performance of the receivers ability to exceeds the 3GPP sensitivity, TTF and accuracy requirements either predict reference variations or maintain the frequency for A-GPS operation without any network aiding requirement! reference with known performance. Maintaining an accurate local estimate of system GPS time enables self-assistance.
  4. 4. Figure 3. Performance of Self-Assist GPS receiver (over receiver signal power -130dBm to -150dBm) ACKNOWLEDGMENT [11] Open Mobile Alliance, “User Plane Location Protocol” OMA-TS-ULP- V2_020080627-C (candidate version 2.0) standard available from Development of any complex system-level semiconductor product is a group activity involving (and often demanding) UPL/V2_0-20080627-C/OMA-TS-ULP-V2_0-20080627-C.pdf [12] Nokia Maps press release available system, silicon and software optimizations and tradeoffs. The releases/showpressrelease?newsid=1103306 approach described in this paper forms part of a system-level [13] Google press release for Google Lattitude available from GPS semiconductor product developed by the team at Air. [14] J. Dobson and P. Fisher, “Geoslavery”, IEEE Technology and Society The product detailed in this paper results from the combined Magazine, vol. 22, pp. 47-52, 2003. contributions of all team members. [15] D. Tester, S. Graham, “Hot-Zones Trigger Method for Location-Based Applications and Services”, US patent application 11/613,280 REFERENCES [16] D. Tester, S. Graham, N. Tolson, I. Watson, “Variable Measurement Rate Method for Positioning Systems”, US patent 7,460,064 [1] Air Semiconductor airwave1 product datasheet, available online from [17] D. Tester, S. Graham, N. Tolson, I. Watson, “Variable Measurement Rate Method for Positioning Systems”, US patent application [2] CSR/SiRF SiRFstarIV product datasheet, available online from 12/326,219 [18] D. Tester, S. Graham, N. Tolson, I. Watson, “Variable Measurement [3] GPS interface control document IS-GPS-200-D, available online from Rate Method for Positioning Systems”, US patent application 12/563,288 [4] B. Parkinson, J. Spilker , Global Positioning System: Theory and [19] D. Tester and I. Watson, “10.7mW, 2.1mm2 , 0.13µm 3.24mm2 CMOS Applications (Volume I), Washington: American Institute of GPS radio, Proceedings of IEEE ISCAS 2010. Astronautics and Aeronautics, 1996. [20] V. Della Torre, M. Conta, R. Chokkalingam, G, Cusmai, P. Rossi and [5] B. Parkinson, J. Spilker , Global Positioning System: Theory and F. Svelto, “A 20mW 3.24mm2 Fully Integrated GPS Radio for Location Applications (Volume II), Washington: American Institute of Based Services”, IEEE J. Solid-State Circuits, vol. 42, pp. 602-612, Astronautics and Aeronautics, 1996. Mar. 2007. [6] R. Gold, "Optimal binary sequences for spread spectrum multiplexing", [21] F. van Diggelen, A-GPS: Assisted GPS, GNSS and SBAS, Artech IEEE Transactions on Information Theory,, vol 13, pp.619–621, 1967 House, 2009. [7] 3GPP A-GPS Terminal Conformance Specification, 3GPP TS 34.171 [22] D. Tester, N. Tolson, S. Graham, “Tracking Loop for an Always-ON available from GPS Receiver”, US patent application 60/909,311 [8] 3GPP2 Position Determination Service Standard for Dual-Mode Spread [23] D. Tester, N. Tolson, S. Graham, “Tracking Loop for an Always-ON Spectrum Systems 3GPP2 C.S0022-0 standard available from GPS Receiver”, US patent application 12/039,865 [24] D. Tester, N. Tolson, S. Graham, “Tracking Loop for an Always-ON [9] TIA/EIA Interim Standard Position Determination Service Standard for GPS Receiver”, US patent application 12/575,715 Dual-Mode Spread Spectrum Systems standard available from [25] A. Miskiewicz, et al, “GPS/Galileo System-on-Chip with UMTS/GSM Support for Location Based Services”, Proceedings of IEEE SIRF IA-EIA-IS-801-1.pdf GNSS 2009, pp. 1-4. [10] Open Mobile Alliance, “Internal Location Protocol” OMA-TS-ILP- [26] D. Tester, “Implementation Methodology for Dual-Mode GPS V2_020080627-C (candidate version 2.0) standard available from Receiver”, Proceedings of SNUG Europe 2009, available online from UPL/V2_0-20080627-C/OMA-TS-ILP-V2_0-20080627-C.pdf per.pdf