METHODS AND SYSTEMS FOR ACQUISITION, REACQUISITION AND TRACKING OF WEAK NAVIGATIONAL SIGNALS

• US 20080180321A1
• Filed: 03/30/2007
• Published: 07/31/2008
• Est. Priority Date: 01/30/2007
• Status: Active Grant

• Current Assignee: CSR Technology Holdings Inc. (Qualcomm, Inc.)
• Original Assignee: SiRF Technology Holdings Incorporated (Qualcomm, Inc.)
• Inventors: Wang, Chi-Shin, Jia, Zhike, Chen, Yue-Meng, Tu, EnYuan, Cheng, Jian
• Granted Patent: US 7,477,189 B2
• Time in Patent Office: Days
• US Class Current: 342/357.12
• CPC Class Codes: G01S 19/235 Calibration of receiver components
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: H04B 1/7075 with code phase acquisition

Abstract

Provided herein are systems and methods for achieving long coherent integration in a navigational receiver to improve the sensitivity of the receiver and enable the receiver to acquire, reacquire and track signals under very weak signal conditions. In an embodiment, phase compensation is computed based on estimated Doppler frequency, rate of change of the Doppler frequency with time, and second order rate of change of the Doppler frequency. The Doppler frequency may be computed from an orbital model or ephemeris. This phase compensation is used to compensate samples of the input signal for changes in the phase due to the Doppler frequency. Frequency components of the phase-compensated samples are then computed using a frequency analysis such as a Fast Fourier Transform (FFT). The maximum frequency component is taken as an error frequency and used to compensate the samples of the input signal for residual frequency error.

Claims

1. A method of operating a navigational receiver under weak signal conditions, comprising:

correlating a received signal with a local signal to obtain correlation samples;

grouping consecutive correlation samples into sub-blocks;

compensating the frequency and phase of the samples in each sub-block;

determining frequency components of the sub-blocks;

determining a frequency compensation based on the frequency components; and

performing a long coherent integration over a block of consecutive sub-blocks.

2. The method of claim 1, wherein the frequency compensation is based on the maximum frequency component.

3. The method of claim 1, further comprising using a reference signal from an external communication system.

4. The method of claim 1, wherein each sub-block is frequency and phase compensated by rotating all the samples of the sub-block by a phase of a reference sample within the sub-block.

5. The method of claim 4, wherein the phase of the reference sample is computed based on an estimation of a phase due Doppler frequency.

6. The method of claim 4, wherein the reference sample is any sample of the sub-block

7. The method of claim 4, wherein the reference sample for each sub-block is in the same relative position for all of the sub-blocks.

8. The method of claim 4, wherein a Doppler frequency, a frequency change rate and a higher order frequency change rate are used to estimate the phase of the reference sample.

9. The method of claim 4, wherein a down-sampling technique is used prior to frequency and phase rotation.

10. The method of claim 8, wherein the Doppler frequency is determined based on the prior Doppler frequency values and a time rate of change of the Doppler frequency.

11. The method of claim 10, wherein the time rate of change of the Doppler frequency is determined based on a prior variation of the Doppler frequency with time.

12. The method of claim 1, wherein a frequency domain technique is used to determine the frequency components of the phase compensated sub-blocks.

13. The method of claim 12, wherein a polynomial interpolation method is used to determine the maximum frequency component.

14. The method of claim 12, wherein the frequency resolution is variable.

15. The method of claim 10, wherein the number of frequency components computed depends upon a residual frequency range.

16. The method of claim 1, wherein the frequency components of the sub-blocks are computed by summing the respective frequency components of all the sub-blocks.

17. The method of claim 16, wherein the frequency corresponding to the frequency component having the peak sum is used as a frequency for acquisition of the signal.

18. The method of claim 16, wherein the frequency corresponding to the component having the peak sum is used as an error frequency in tracking the signal.

19. The method of claim 1, wherein a local frequency generator is corrected based on the computed phase and frequency compensation.

20. The method of claim 1, wherein satellite ephemeris or orbit model is used with the code phase to mitigate the cross-correlation.

21. The method of claim 1, wherein a sliding window technique is used in the long integration.

22. The method of claim 1, wherein the sliding ratio is variable.

23. The method of claim 21, wherein the sliding window length is adaptively changed.

24. The method of claim 1, wherein a weighting factor is used with the said long integration.

25. The method of claim 1, wherein both data aided and non-data aided methods of integration are used.

26. The method of claim 1, wherein a local frequency generator is updated periodically.

27. A navigational satellite receiver, comprising:

a radio frequency front-end for receiving satellite signals;

a baseband section for processing the received signals into correlation samples; and

a processor coupled to the baseband section, wherein the processor groups consecutive correlation samples into sub-blocks, compensates the frequency and phase of the samples in each sub-block, computes frequency components of the sub-blocks, computes a frequency compensation based on the frequency components, and performs a long coherent integration over a block of consecutive sub-blocks.

28. The receiver of claim 27, wherein the frequency compensation is based on the maximum frequency component.

29. The receiver of claim 27, wherein the processor frequency and phase compensates each sub-block by rotating all the samples of the sub-block by a phase of a reference sample within the sub-block.

30. The receiver of claim 27, wherein the processor computes the phase of the reference sample based on an estimation of a phase due Doppler frequency.

31. The receiver of claim 27, wherein processor computes a Doppler frequency, a frequency change rate, and a higher order frequency change rate, and computes the phase of the reference sample based on the computed Doppler frequency, frequency change rate and higher order frequency change rate.

32. The receiver of claim 31, wherein the processor computes the frequency change rate based on a prior variation of the Doppler frequency with time.

33. The receiver of claim 27, wherein the processor performs a frequency domain analysis on the phase compensated sub-blocks to determine the frequency components of the phase compensated sub-blocks.

34. The receiver of claim 33, wherein the processor performs a polynomial interpolation on the frequency components to determine the maximum frequency component.

35. The receiver of claim 27, wherein the processor computes the frequency components of the sub-blocks by summing the respective frequency components of all the sub-blocks.

36. The receiver of claim 35, wherein the processor determines the frequency of the frequency compensation based on the frequency component having the peak sum.