Method and Apparatus for Mitigating the Effects of Cross Correlation in a GPS Receiver

• US 20090322604A1
• Filed: 12/19/2008
• Published: 12/31/2009
• Est. Priority Date: 06/27/2008
• Status: Active Grant

• Current Assignee: CSR Technology Incorporated (Qualcomm, Inc.)
• Original Assignee: SIRF Technology Incorporated (Qualcomm, Inc.)
• Inventors: Lennen, Gary
• Granted Patent: US 8,072,376 B2
• Time in Patent Office: Days
• US Class Current: 342/357.48
• CPC Class Codes: G01S 19/21 Interference related issues ; Issues related to cross-correlation, spoofing or other methods of denial of service
• CPC Class Codes: G01S 19/246 involving long acquisition integration times, extended snapshots of signals or methods specifically directed towards weak signal acquisition

Abstract

A positioning system receiver that mitigates cross correlation of received signals from positioning system satellite vehicles by generating the strong satellite vehicle signal and subtracting it from the received signal before correlation while eliminating the need for cross correlation signature without changing the C/A code.

Claims

1. A method for acquiring or tracking weak satellite vehicle (SV) signals in the presence of strong SV signals by mitigating the cross correlation of a received signal from a strong SV and a clear/acquisition (C/A) code of a received signal from a weak SV, comprising:

estimating the strong SV signal;

subtracting the estimated strong SV signal from the actual received signal from a weak SV; and

performing correlation of the subtracted signal with the C/A code of the weak SV.

2. The method of claim 1, where the strong SV signal is reconstructed using a code phase from tracking the strong SV.

3. The method of claim 2, where the code phase of the strong SV signal is maintained with carrier-to-code aiding to track any change in the carrier frequency caused by Doppler or oscillator error.

4. The method of claim 1, where the strong SV signal is reconstructed using demodulated data bits from tracking the strong SV.

5. The method of claim 1, where the strong SV signal is reconstructed using a carrier phase and carrier frequency from tracking the strong SV.

6. The method of claim 1, where the strong SV signal is reconstructed using an estimated amplitude of the strong SV signal.

7. The method of claim 1, where the strong SV signal is reconstructed using a carrier phase and carrier frequency from tracking the strong SV and where multiple constructed strong SV signals are combined to generate a composite signal to be subtracted.

8. The method of claim 7, where estimating the amplitude of the SV satellite signal comprises:

reconstructing the strong SV signal;

subtracting the reconstructed strong SV signal from the actual received signal from a weak SV;

performing correlation of the subtracted signal with the C/A of the strong SV signal; and

updating an estimated amplitude to minimize such correlation.

9. The method of claim 8, where the strong SV signal is reconstructed using code phase, carrier phase, carrier frequency, and demodulated data bits from tracking the strong SV.

10. The method of claim 1, where a carrier-to-noise (C/No) test is used to determine whether strong SV cross correlation mitigation is necessary.

11. The method of claim 1, where the strong SV is first acquired and tracked to derive a code phase, carrier phase, carrier frequency, and demodulated data bits.

12. The method of claim 8, where a cross correlation error output from an amplitude control loop can be monitored to measure the quality of mitigation achieved.

13. A GPS position signal receiver, comprising:

an input sample memory in receipt of digital samples of positioning signals from at least two satellite vehicles (SVs);

a carrier numerical controlled oscillator that generates a carrier offset signal;

a complex mixer that combines digital samples from the input sample memory and the carrier offset signal from the carrier numerical controlled oscillator and results in a baseband signal;

a strong satellite reconstruction module that generates an estimated strong SV signal; and

a signal combiner that subtracts the estimated strong SV signal from the base band signal prior to filtering the baseband signal with a matched filter having a weak SV pseudo random number (PRN) as an input.

14. The GPS receiver of claim 13, where the strong SV reconstruction module further includes a complex mixer that multiplies a complex carrier with a strong SV PRN and then applies a strong SV amplitude signal that results in the estimated strong SV signal.

15. The GPS receiver of claim 14, where the complex carrier is generated by a sin/cos LUT module that is in receipt of a frequency offset from a second carrier numerical controlled oscillator.

16. The GPS receiver of claim 13, where the strong satellite reconstruction module further includes the strong SV reconstruction module being in receipt of phase, carrier frequency, and data information associated with the strong SV signal.

17. The GPS receiver of claim 13, further including an amplitude control loop that estimates the strong SV'"'"'s amplitude and cross correlates the output of the signal combiner.

18. The GPS receiver of claim 17, where the amplitude control loop tracks amplitude variations of the strong SV signal in a dynamic signal environment.

19. A computer-readable medium containing controller-executable instructions that when executed result in a method for acquiring or tracking weak SV signals in the presence of strong SV signals by mitigating cross correlation of a received signal from a strong SV and the C/A code of a received signal from a weak SV, the controller-executable instructions comprising instructions for:

estimating the strong SV signal;

subtracting the estimated strong SV signal from the actual received signal from a weak SV; and

performing correlation of the subtracted signal with the C/A code of the weak SV.

20. The computer-readable medium containing controller-executable instructions of claim 19, where the strong SV signal is reconstructed using a code phase from tracking the strong SV.

21. The computer-readable medium containing controller-executable instructions of claim 20, where the code phase of the strong SV signal is maintained with carrier-to-code aiding to track any change in a carrier frequency caused by Doppler or oscillator error.

22. The computer-readable medium containing controller-executable instructions of claim 19, where the strong SV signal is reconstructed using demodulated data bits from tracking the strong SV.

23. The computer-readable medium containing controller-executable instructions of claim 19, where the strong SV signal is reconstructed using a carrier phase and carrier frequency from tracking the strong SV.

24. The computer-readable medium containing controller-executable instructions of claim 19, where the strong SV signal is reconstructed using an estimated amplitude of the strong SV signal.

25. The computer-readable medium containing controller-executable instructions of claim 19, where the strong SV signal is reconstructed using a carrier phase and carrier frequency from tracking the strong satellite and where multiple constructed strong SV signals are combined to generate a composite signal to be subtracted.

26. The computer-readable media containing controller-executable instructions of claim 25, where estimating an amplitude of the strong SV signal comprises:

reconstructing the strong SV signal;

subtracting the reconstructed strong SV signal from the actual received signal from a weak SV;

performing correlation of the subtracted signal with the C/A of the strong SV; and

updating the estimated amplitude to minimize such correlation.

27. The computer-readable medium containing controller-executable instructions of claim 26, where the strong SV signal is reconstructed using code phase, carrier phase, carrier frequency, demodulated data bits from tracking the strong SV.

28. The computer-readable medium containing controller-executable instructions of claim 26, where the cross correlation error output from the amplitude control loop can be monitored to measure quality of mitigation achieved.

29. The computer-readable media containing controller-executable instructions of claim 19, where a software C/No test is used to determine whether strong SV cross correlation mitigation is necessary.

30. The computer-readable medium containing controller-executable instructions of claim 19, where the strong SV is first acquired and tracked to derive code phase, carrier phase, carrier frequency, and demodulated data bits.