METHOD AND DEVICE FOR TRACKING WEAK GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) SIGNALS

• US 20070205940A1
• Filed: 12/29/2006
• Published: 09/06/2007
• Est. Priority Date: 07/01/2005
• Status: Active Grant

• Current Assignee: CSR Technology Holdings Inc. (Qualcomm, Inc.)
• Original Assignee: SiRF Technology Holdings Incorporated (Qualcomm, Inc.)
• Inventors: Yang, Chun, Han, Shaowei
• Granted Patent: US 7,639,181 B2
• Time in Patent Office: Days
• US Class Current: 342/357.590
• CPC Class Codes: G01S 19/24 Acquisition or tracking or demodulation of signals transmitted by the system synchronisation aspects of direct sequence spread spectrum modulation H04B1/7073
• CPC Class Codes: G01S 19/246 involving long acquisition integration times, extended snapshots of signals or methods specifically directed towards weak signal acquisition
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain

Abstract

A Global Navigation Satellite System (GNSS) receiver and associated method capable of tracking weak GNSS signals from a plurality of GNSS satellites. In a preferred embodiment, code and carrier tracking loops are initially closed around the code phase, carrier frequency, and data bit edge estimates handed over from an acquisition mode. In subsequent tracking, early, prompt, and late copies of the code replica are correlated with the incoming signal. The prompt correlations are coherently integrated over an extended updating interval for data bit edge and sign estimation as well as for carrier phase and frequency error discrimination whereas the early and late correlations are used for code error discrimination. Code delay and carrier phase and frequency errors are used to update the code and carrier tracking loop filters. Together with data bits, they form observables of a GNSS signal'"'"'s time and frequency parameters for timing and position fixing.

Claims

1. In a navigational system utilizing a plurality of satellites that transmit radio-frequency signals containing time and orbit information of said satellites, wherein said radio-frequency signals are severely attenuated upon arriving at a receiver, a method of obtaining a position solution of the receiver from weak signals comprising:

receiving a plurality of said satellite signals at the receiver;

strengthening received weak satellite signals using a block-accumulating coherent integrator;

maintaining lock onto said strengthened weak satellite signals;

extracting time, frequency, and data bits from locked satellite signals; and

obtaining a position solution based on extracted time, frequency, and data bits of locked strengthened weak satellite signals.

2. The method according to claim 1, wherein strengthening the received weak satellite signals using the block-accumulating coherent integrator includes:

obtaining weak satellite signal samples;

performing short-length correlation between said weak satellite signal samples and locally signal replicas; and

performing block-accumulating coherent integration of short-length correlations, thereby strengthening received weak satellite signals and producing coherently integrated short-length correlations.

3. The method according to claim 2, wherein maintaining lock onto said weak satellite signals includes:

running code and carrier error discriminators with said coherently integrated short-length correlations; and

updating tracking loop filters with the outputs from said code and carrier error discriminators to maintain lock onto weak signals in closed tracking loops.

4. The method according to claim 2, wherein performing short-length correlation between said weak satellite signal samples and locally signal replicas includes:

preparing at least one complex signal carrier replica at a carrier frequency;

preparing at least three signal code replicas at early, prompt, and late spacings;

mixing said weak satellite signal samples with said carrier replicas, producing a down-converted signal; and

correlating said down-converted signal with said code replicas over a short length, thereby producing short-length correlations.

5. The method according to claim 4, wherein preparing at least one complex signal carrier replica includes:

obtaining a carrier frequency command from a carrier tracking loop filter;

obtaining an initial frequency offset from an acquisition mode;

adding said carrier frequency command, said initial frequency offset, and a nominal frequency to produce a total carrier frequency; and

driving a carrier numerically controlled oscillator at said total carrier frequency, thereby producing a carrier phase for said complex signal carrier replica.

6. The method according to claim 5, further comprising:

obtaining a rate aiding from an external source when available; and

adding said carrier frequency command, said initial frequency offset, said rate aiding and a nominal frequency to produce said total carrier frequency.

7. The method according to claim 5, wherein the step of obtaining the carrier frequency command from the carrier tracking loop filter further includes adjusting said frequency command a number of times within an extended update interval by a term proportional to an estimated rate of change in frequency.

8. The method according to claim 7, wherein the step of adjusting said frequency command further includes use of an external aiding for estimating rate of change in frequency when available.

9. The method according to claim 4, wherein preparing at least three signal code replicas at early, prompt, and late spacings includes:

obtaining a chipping rate command from a code tracking loop filter;

obtaining a rate scaling from a carrier tracking loop;

adding said chipping rate command, said rate scaling, and a nominal chipping rate, thereby producing a total chipping rate;

driving a code numerically controlled oscillator at said total chipping rate, thereby producing a code phase;

setting a code generator with an initial code phase from an acquisition mode; and

driving said code generator with said code phase, thereby producing at least three signal code replicas at early, prompt, and late spacings.

10. The method according to claim 3, wherein performing code and carrier error discriminators with coherently integrated short-length correlations, for separate code and carrier tracking loops, includes:

obtaining a sequence of prompt complex correlation samples over a preset extended interval, each sample coherently summed over a data bit;

obtaining a sequence of estimated data bits over the same preset extended interval;

multiplying said sequence of prompt complex correlation samples with said sequence of estimated data bits, thereby stripping data bits from prompt correlation samples;

applying a Fourier transform to data-bit stripped sequence of prompt complex correlation samples, producing a number of coherently integrated complex correlations, each at a different frequency bin; and

choosing the frequency bin at which the complex correlation reaches the maximum magnitude as the carrier frequency error and the phase of the maximum correlation as the carrier phase error, thereby implementing a carrier error discriminator.

11. The method according to claim 3, wherein performing code and carrier error discriminators with coherently integrated short-length correlations, for separate code and carrier tracking loops when frequency error is large, includes:

obtaining a sequence of early and late complex correlation samples over a preset extended interval, each sample summed over a data bit;

obtaining a sequence of estimated data bits over the same preset extended interval;

multiplying said sequence of prompt complex correlation samples with said sequence of estimated data bits, thereby stripping data bits from early and late correlations;

applying a Fourier transform to data-bit stripped sequence of early and complex correlation samples, thereby producing a number of coherently integrated complex correlations, each at a different frequency bin;

obtaining maximum early and late values of said coherently integrated complex correlations; and

calculating delay error from said maximum early and late correlations, thereby implementing a code error discriminator.

12. The method according to claim 3, wherein performing code and carrier error discriminators with coherently integrated short-length correlations, for separate code and carrier tracking loops when frequency error is small, includes:

obtaining a sequence of early and late complex correlation samples over a preset extended interval, each sample summed over a data bit;

squaring coherently summed early and late complex correlation samples, thereby stripping data bits;

performing power-combination of data bit stripped early and late correlation samples; and

calculating delay error discriminator from said maximum early and late correlations, thereby implementing a code error discriminator.

13. The method according to claim 3, wherein performing code and carrier error discriminators with coherently integrated short-length correlations, for a joint code and carrier tracking loop, includes:

obtaining a sequence of prompt complex correlation samples over a preset extended interval, each sample coherently summed over a data bit;

obtaining a sequence of early and late complex correlation samples over the same preset extended interval, each coherently summed over a data bit;

obtaining a sequence of estimated data bits over the same preset extended interval;

multiplying said sequence of early, prompt, and complex correlation samples with said sequence of estimated data bits, thereby stripping data bit from early, prompt and late correlations, respectively;

applying a Fourier transform to data-bit stripped sequence of prompt complex correlation samples, thereby producing a number of coherently integrated complex correlations, each at a different frequency bin;

choosing the frequency bin at which the complex correlation reaches the maximum magnitude as the carrier frequency error and the phase of the maximum correlation as the carrier phase error, thus implementing a carrier error discriminator;

summing data bit stripped early and late correlation samples, thereby producing summed data bit stripped early and late correlation samples; and

calculating delay error discriminator from said summed data bit stripped early and late correlation samples, thereby implementing a joint code error discriminator.

14. The method according to claim 10, wherein obtaining a sequence of prompt complex correlation samples over a preset extended interval, each coherently summed over a data bit, includes:

obtaining short-length correlations over one data bit;

obtaining an estimated data bit edge;

forming a number of sequences of short-length correlations over a data bit, each corresponding to a possible data bit edge around said estimated data bit edge;

performing coherent integration of short-length correlations over a data bit; and

outputting coherently integrated correlations of one bit long into a complex data buffer over a preset extended interval.

15. The method according to claim 14, wherein forming a number of sequences of short-length correlations over a data bit, each corresponding to a possible data bit edge around said estimated data bit edge, includes:

determining the number of sequences necessary to cover uncertainty in estimated data bit edge; and

reversing the sign of short-length correlations successively, one at a time, thereby forming said number of sequences of short-length correlations.

16. The method according to claim 14, wherein performing coherent integration of short-length correlations over a data bit, when frequency error is small, includes:

collecting short-length correlations over a data bit for a possible data bit edge; and

summing up all short-length correlations over the same data bit, thereby producing said coherent integration of short-length correlations over a data bit.

17. The method according to claim 14, wherein performing coherent integration of short-length correlations over a data bit, when frequency error is large, includes:

collecting short-length correlations over a data bit for a possible data bit edge; and

applying a Fourier transform to all short-length correlations over the same data bit, wherein the maximum value is taken as said coherent integration of short-length correlations over a data bit.

18. The method according to claim 14, wherein performing coherent integration of short-length correlations over a data bit, when in transition from an acquisition mode to a tracking mode with a large frequency error, includes:

collecting short-length correlations over a data bit for a possible data bit edge;

padding zeros in front of and behind said short-length correlations over a data bit to said preset extended length; and

applying a Fourier transform to said zero-padded short-length correlations over a data bit, wherein the Fourier transform is taken only at selected frequency bins when a discrete Fourier transform is used.

19. The method according to claim 18, wherein said step of padding zeros in front of and behind said short-length correlations over a data bit to said preset extended length is done in such a manner that the relative timing of any of short-length correlations in the original sequence is preserved over said extended length.

20. The method according to claim 18, wherein applying a Fourier transform to said zero-padded short-length correlations over a data bit, includes:

applying a Fourier transform to a first sequence of sign-reversed short-length correlations; and

constructing a Fourier transform for subsequent sequences of successively sign-reversed short-length correlations from said Fourier transform of the first sequence of sign-reversed short-length correlations.

21. The method according to claim 8, wherein obtaining a sequence of estimated data bits over the same preset extended interval, includes:

obtaining a best data bit sequence for current extended interval;

obtaining corresponding data bits of previous subframe; and

averaging said best data bit sequence for current extended interval with said corresponding data bits of previous subframe, thereby updating data bits for current subframe and producing said sequence of estimated data bits over the same preset extended interval.

22. The method according to claim 21, wherein obtaining a best data bit sequence for current extended interval, includes:

collecting sequences of coherently integrated bit-long correlations over said preset extended interval, each corresponding to a possible data bit edge;

forming partial sums for current bit-long correlation with two possible signs to test, wherein a best sign is determined for current bit that produces the maximum partial sum for this possible data bit edge; and

repeating sign determination for all bit-long correlations over said preset extended interval, thereby producing said best data bit sequence for current extended interval.

23. The method according to claim 14, wherein obtaining an estimated data bit edge in short-length correlations, includes:

obtaining partial sums of coherently integrated bit-long correlations, each corresponding to a possible data bit edge and each corrected with the best data bit sequence for said preset extended interval; and

choosing the data bit edge that produces the maximum partial sum as said estimated data bit edge in short-length correlations.

24. The method according to claim 22, wherein said step of forming partial sums for current bit-long correlation is done for each frequency bin when the coherent sum is calculated using a Fourier transform.

27. The receiver according to claim 24, wherein said signal tracking processor includes:

a baseband converter for converting intermediate frequency signal samples to baseband using a reference carrier;

at least three complex despread correlators for stripping off spectrum-spreading codes from said signal samples using early, prompt, and late reference codes and integrating it over a short-length into short-length correlation buffers;

said block-accumulating coherent integrator;

a data bit edge and sign estimator for estimating data bit edge and data bit signs from short-length correlations;

code phase and carrier phase and frequency error discriminators for estimating the differences between signal samples and locally generated code and carrier replicas;

code and carrier tracking loop filters for said code chipping and carrier frequency commands from loop filters, respectively, using said code phase and carrier phase and frequency error discriminator outputs; and

a carrier replica generator and at least three code replica generators producing said code replicas and carrier replica at said code chipping and carrier frequency commands from said loop filters, respectively.

25. A receiver including:

an antenna capable of coupling to signals transmitted from a plurality of satellites of the Global Navigation Satellite System;

a radio frequency front-end that receives said signals from said satellites via said antenna, wherein said radio frequency front-end comprises at least a bandpass filter, a power amplifier, and a frequency down-converter;

a baseband signal processor comprising signal acquisition and tracking circuitry containing a plurality of functionally identical channels for extracting navigation data bits and other time and frequency parameters from said signals using said functionally identical channels; and

a data processor for calculating a navigation solution from said extracted navigation data bits and other time and frequency parameters;

wherein the baseband signal processor dynamically assigns said signals to the functionally identical channels, and

wherein each of the functionally identical channels performs weak signal tracking using a block-accumulating coherent integrator to coherently add successive blocks of consecutive short-length correlations over a desired extended length with data bit transition and data bit sign dynamically estimated.

26. The receiver according to claim 25, wherein each of said separate functionally identical channels includes:

a signal acquisition processor; and

a signal tracking processor that determines navigation data bits embedded in said satellite signal and extracts code time and carrier frequency parameters.