This document summarizes the key aspects of a self-assisting GPS receiver architecture capable of continuous location awareness. Section III outlines GPS system performance, while Section IV covers A-GPS network assistance. Section VI proposes a 5mW receiver architecture that can continuously operate in the background. Rather than optimizing for fast TTFF, it prioritizes maintaining an accurate local GPS time estimate to minimize the search space and deliver push-to-fix operation with sub-2 second TTF even at -150dBm signal levels, outperforming A-GPS. The receiver achieves this through a virtuous cycle of periodically activating to re-lock to GPS and refine its time prediction.

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
david.tester@air-semi.com
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. 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. 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. 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-
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The product detailed in this paper results from the combined Magazine, vol. 22, pp. 47-52, 2003.
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Applications and Services”, US patent application 11/613,280
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