Fast fourier transform with down sampling based navigational satellite signal tracking
View Patent ↗A method and device to track navigational satellite signals, are claimed. In this invention, a combination of down-sampling and frequency domain transformation are used to track the navigational satellite signals under dynamic environment. A Fast Fourier Transform (FFT) with long coherent integration has been employed to determine the varying frequency components with high resolution. By representing a number of correlation values with their average value, it is possible to represent a long sequence of input values by a smaller number of values and a relatively short length FFT can reveal the low frequency components that are present in the signal during tracking operation. A large reduction in the computational load may be achieved using this down-sampling method without compromising on the frequency resolution.
1. A method for tracking received signals in a global satellite navigation receiver, the method comprising:
collecting a sequence of correlation values derived from the received signals correlated with PN codes:
partitioning the collected sequence into N groups of consecutive correlation values, where N is a positive integer;
calculating from each of the N groups, a corresponding set of N values forming a downsampled sequence;
converting the down-sampled sequence from time domain to frequency domain to produce a frequency domain sequence;
identifying a maximum power value in the frequency domain sequence;
calculating a frequency error between the received signals and a locally generated replica signal based on the identified maximum power value; and
adjusting a local replica carrier frequency by an amount equal to the frequency error.
2. The method of claim 1 , wherein calculating the corresponding set of N values comprises averaging the consecutive correlation values in each of the N groups.
3. The method of claim 1 , wherein calculating the corresponding set of N values comprises taking the root mean square of the consecutive correlation values in each of the N groups.
4. The method of claim 1 , further comprising varying the value of N according to platform dynamics.
5. The method of claim 1 , wherein the N values in the down-sampled sequence are assigned to temporal midpoints of the N groups.
6. The method of claim 1 , wherein collecting the sequence of correlation values comprises selecting a total number of values to collect as a function of the receiver dynamics.
7. The method of claim 1 , wherein adjusting the local replica carrier frequency comprises reducing a rate of adjustment to avoid spontaneous burst-like changes in the local replica carrier frequency due to signal noise.
8. The method of claim 1 , further comprising fine-tuning the frequency error by interpolation.
9. The method of claim 1 , further comprising employing a Frequency Lock Loop (FLL) to further improve the tracking performance.
10. The method of claim 9 , wherein the frequency error obtained by FFT analysis is used as an input residual frequency to the Frequency Lock Loop (FLL).
11. The method of claim 1 , wherein partitioning the collected sequence comprises aligning transitions between the groups with data bit edge transitions.
12. The method of claim 1 , comprising selecting a number of frequency bins as a function of receiver dynamics.
13. The method of claim 1 , wherein converting the down-sampled sequence from time domain to frequency domain comprises using a maximum likelihood criterion based method to remove data modulation.
14. A global satellite navigation system receiver device comprising an RF front end for receiving signals from navigational satellites, a baseband section receiving IF signals from the RF front end, and a processor receiving I, Q correlation values from the baseband section, wherein the processor:
collects a sequence of the correlation values derived from the received signals correlated with PN codes;
partitions the collected sequence into N groups of consecutive correlation values, where N is a positive integer;
calculates from each of the N groups, a corresponding set of N values forming a downsampled sequence;
converts the down-sampled sequence from time domain to frequency domain to produce a frequency domain sequence;
identifies a maximum power value in the frequency domain sequence;
calculates a frequency error between the received signal and a locally generated signal based on the identified maximum power value; and
adjusts a local replica carrier frequency by an amount equal to the frequency error.
15. The device of claim 14 , wherein calculating the corresponding set of N values comprises averaging the consecutive correlation values in each of the N groups.
16. The device of claim 14 , wherein calculating the corresponding set of N values comprises taking the root mean square of the consecutive correlation values in each of the N groups.
17. The device of claim 14 , further comprising varying the value of N according to platform dynamics.
18. The device of claim 14 , wherein the N values in the down-sampled sequence are assigned to temporal midpoints of the N groups.
19. The device of claim 14 , wherein collecting the sequence of correlation values comprises selecting a total number of values to collect as a function of the receiver dynamics.
20. The device of claim 14 , wherein adjusting the local replica carrier frequency comprises reducing a rate of adjustment to avoid spontaneous burst-like changes in the local replica carrier frequency due to signal noise.
21. The device of claim 14 , further comprising fine-tuning the frequency error by interpolation.
22. The device of claim 14 , further comprising employing a Frequency Lock Loop (FLL) to further improve the tracking performance.
23. The device of claim 22 , wherein the frequency error obtained by FFT analysis is used as an input residual frequency to the Frequency Lock Loop (FLL).
24. The device of claim 14 , wherein partitioning the collected sequence comprises aligning transitions between the groups with data bit edge transitions.
25. The device of claim 14 , comprising selecting a number of frequency bins as a function of receiver dynamics.
26. The device of claim 14 , wherein converting the down-sampled sequence from time domain to frequency domain comprises using a maximum likelihood criterion based method to remove data modulation.