Method and apparatus for mitigating the effects of CW interference via post correlation processing in a GPS receiver

1. A method for detecting and mitigating continuous wave (CW) interference in the correlation domain, the method comprising:

receiving a radio frequency (RF) positioning signal at a GPS receiver;

converting the RF positioning signal to a baseband signal;

correlating the baseband signal with a plurality of codes to produce punctual correlation values and offset correlation values;

coherently integrating the punctual correlation values;

coherently integrating the offset correlation values;

determining an ambient noise level;

setting a threshold value to the determined ambient noise level;

non-coherently integrating the coherently integrated punctual correlation values;

non-coherently integrating the coherently integrated offset correlation values;

detecting the CW interference based on the non-coherent integration values and the threshold; and

subtracting the detected CW interference from the coherent integration values.

2. The method of claim 1 , further including the step of estimating phase, frequency, and amplitude of the detected CW interference.

3. The method of claim 1 , further including the step of estimating a minimum signal to noise power (CNo) ratio that results in a CW detection level being established that is employed when detecting the presence of CW interference.

4. The method of claim 3 , further including the step of determining an estimated CNo as a ratio of the non-coherent integration of a 20 msec coherent integration for the punctual correlation values and the non-coherent integration of the 20 msec coherent integration for the offset correlation values.

5. The method of claim 3 , where detecting the presence of CW interference further includes identifying the presence of CW interference when the non-coherent integration of a 20 msec coherent integration for the offset correlation values multiplied by the CW detection level derived from the estimated CNo exceeds the non-coherent integration of the 20 msec coherent integration for the punctual correlation values.

6. The method of claim 3 , where detecting the presence of CW interference includes the steps of:

detecting CW interference in low signal conditions down to −160 dBm, where 100 msec of the coherent integration is available; and

determining the estimated CNo with a ratio of the non-coherent integration of the 100 msec coherent integration for the punctual correlation values and the non-coherent integration of the 100 msec coherent integration for the offset correlation values.

7. The method of claim 3 , further including the steps of:

detecting CW interference under low signal condition down to −160 dBm, where 100 msec coherent integration is available, and

detecting CW interference that is identified when the non-coherent integration of the 100 msec coherent integration for the offset correlation values is multiplied by the CW detection level derived from an estimated CNo that exceeds the non-coherent integration of the 100 msec coherent integration for the punctual correlation values.

8. The method of claim 3 , further including the step of estimating a phase, a frequency, and an amplitude of the CW interference in the correlation domain by operating on a 20 msec coherent integration of a window of offset correlation values around the punctual correlation values.

9. The method of claim 3 , further including the step of estimating when the CW interference is detected under low signal conditions, a phase, a frequency, and an amplitude of the CW interference in the correlation domain by operating on a 100 msec coherent integration values of a window of the offset correlation values around the punctual correlation values.

10. The method of claim 4 , further includes the steps of:

setting a non-coherent integration period to a multiple of 160 msec up to 5.12 sec and non-coherent integrations: and

performing non-coherent integrations by taking the square root of the sum of the square of a 20 msec coherent integration value.

11. The method of claim 7 , further including the steps of:

setting the non-coherent integration period to a multiple of 100 msec up to 5 sec; and

performing the non-coherent integration by taking the square root of the sum of the square of the 100 msec coherent integration values in a low signal condition.

12. The method of claim 3 , where estimating the minimum CNo further includes the steps of:

identifying a smallest non-coherent integration period,

establishing a minimum decision level for the selected non-coherent integration period;

comparing a measured ratio of the non-coherent integration for the punctual correlation values and that for the offset correlation values to the minimum decision level;

estimating the CNo from the simulation curve showing the relationship between the CNo and the measured ratio for the selected non-coherent integration period if the measured ratio exceeds the minimum decision level;

increasing the non-coherent integration period if the measured ratio does not exceed the minimum decision level; and

estimating the CNo from the simulation curve with a relationship between the CNo and the measured ratio for the longest non-coherent integration period if the measured ratio does not exceed the minimum decision level and the non-coherent integration period is already at the maximum integration period.

13. The method of claim 5 , where both the 20 msec coherent integration for the punctual correlation values and that for the offset correlation values are aligned with a 50 bit per second boundary.

14. The method of claim 5 , where the 20 msec coherent integration for the offset correlation values are not synchronized to any tracking channel.

15. The method of claim 5 , further including the step of maintaining a global moving window non-coherent integration of a 160 msec period as the non-coherent integration of a last eight shared 20 msec coherent integrations for the offset correlation values.

16. The method of claim 6 , further including the step of maintaining a local version of the 160 msec non-coherent integration for the offset correlation values by using a global value that best synchronizes with a track channel that is associated with a satellite vehicle.

17. The method of claim 6 , where detecting CW interference further includes the step of determining a mean of the non-coherent integration period set to any multiple of 160 msec up to 5.12 sec.

18. The method of claim 6 , further including the step of determining a mean of the non-coherent integration of a 20 msec coherent integration for a number of different adjacent offset correlation values.

19. The method of claim 8 , where detecting CW interference includes the step of setting the non coherent integration period for the CW detect to any multiple of 100 msec up to 5 sec under low signal conditions.

20. The method of claim 9 , where estimating the phase, frequency, and amplitude of the CW interference in the correlation domain for each code phase that is offset by greater than 1 chip from the punctual code phase, further includes the steps of:

estimating the phase by taking an atan −1 of the coherent integration divided by the coherent integration;

estimating a delta phase by taking the difference between an estimated phase of a current offset code phase and that of an adjacent offset code phase; and

estimating the amplitude by taking a square root of the sum of the square of the coherent integration.