METHOD AND APPARATUS FOR DETECTING CROSS CORRELATION BASED IN LIMITED RANGE CODE PHASE OFFSET OBSERVATIONS

1 . A method for identifying a false correlation between a first pseudo random noise (PRN) code and a second PRN code, each of the first and second PRN codes being defined by a respectively different pseudo-random sequence of chips, the method comprising:

correlating the first PRN code to the second PRN code to produce a plurality of correlation values spanning N chips;

identifying a main peak correlation value in the plurality of correlation values;

calculating a threshold value based on the identified main peak;

comparing each of the plurality of correlation values within N code chips of the main peak correlation value to the threshold value, where N is an integer; and

identifying the main peak correlation value as a false correlation if one of the plurality of correlation values other than the main peak correlation value exceeds the threshold value.

2 . The method according to claim 1 , wherein calculating the threshold value comprises multiplying the peak value by a value M, where M represents a number of decibels by which the other correlation value may differ from the peak value.

3 . The method according to claim 1 , wherein the first PRN code is a first coarse acquisition (C/A) code of a received satellite signal and the second PRN code is second C/A code, different from the first C/A code, generated in a satellite receiver.

4 . The method according to claim 3 , wherein the satellite signal is a Quasi-Zenith Satellite System (QZSS) signal.

5 . The method according to claim 3 , wherein the satellite signal modulates a carrier signal and the carrier signal has a Doppler offset of at least 1 kHz.

6 . The method according to claim 3 , further including:

a) accumulating a non-coherent summation (NCS) of the first PRN code over an accumulation time period;

b) calculating a signal-to-noise ratio for the accumulated PRN code;

c) if the signal-to noise ratio for the accumulated PRN code is less than a threshold value, then increasing the accumulation time period in length and repeating steps a), b) and c) for the increased accumulation time period; and

d) if the signal to noise ratio for the accumulated PRN code is greater than the threshold value, setting N and M based on the length of the accumulation time period.

7 . A method for detecting a false correlation between a Global Navigation Satellite System (GNSS) satellite signal having a first pseudo-random noise (PRN) code and a coarse acquisition (C/A) code of a GNSS receiver, having a second PRN code different from the first PRN code, the method comprising:

demodulating the GNSS satellite signal to recover the first PRN code;

filtering the first PRN code in a plurality of matched filters, each matched having filter coefficients corresponding to the second PRN code, the coefficients of successive matched filters being offset, to generate a plurality of correlation values;

identifying a main peak in the plurality of correlation values;

calculating a threshold value based on the identified main peak;

comparing each of the plurality of correlation values to the threshold value; and

identifying the correlation peak as a false correlation if one of the plurality of correlation values exceeds the threshold value.

8 . the method according to claim 7 wherein the GNSS satellite signal is one of T GPS satellite signals identified by the GNSS receiver and the method further comprises:

for each of the T satellite signals:

determining a signal-to-noise level of each of the satellite signals relative to the satellite signal corresponding to the false correlation peak;

determining a carrier frequency offset modulo 1 kHz for each of the T satellite signals relative to the carrier signal of the satellite signal corresponding to the false correlation peak;

for each of the T satellite signals, identifying the satellite signal as an invalid satellite signal if the relative signal to noise ratio is less than a first threshold value and the carrier frequency offset modulo 1 kHz is less than a second threshold value.

9 . The method according to claim 7 , wherein the GNSS receiver is a GPS receiver and the GNSS satellite signal is a Quasi-Zenith Satellite System (QZSS) signal.

10 . The method according to claim 7 , wherein the GNSS satellite signal modulates a carrier signal and the carrier signal has a Doppler offset of at least 1 kHz.

11 . The method according to claim 7 , further including:

a) accumulating a non-coherent summation (NCS) of the first PRN code over an accumulation time period;

b) calculating a signal-to-noise ratio for the accumulated PRN code;

c) if the signal-to noise ratio for the accumulated PRN code is less than a threshold value, then increasing the accumulation time period in length and repeating steps a), b) and c) for the increased accumulation time period; and

d) if the signal to noise ratio for the accumulated PRN code is greater than the threshold value, setting N and M based on the length of the accumulation time period.

12 . The method according to claim 7 , where N is between 4 and 16.

13 . The method according to claim 7 , where M is between 0.5 and 1.0.

14 . A Global Navigation Satellite System (GNSS) receiver comprising:

a demodulator which demodulstes the GNSS satellite signal to recover a first pseudo random noise (PRN) code;

a PRN code generator which generates a second PRN code;

a matched filter which filters the first PRN code in a plurality of matched filters, each matched having filter coefficients corresponding to the second PRN code, the coefficients of successive matched filters being offset, to generate a plurality of correlation values and for storing the correlation values in a buffer memory;

a processor configured to:

identify a main peak in the plurality of correlation values;

calculate a threshold value based on the identified main peak;

compare each of the plurality of correlation values to the threshold value;

compare each of the plurality of correlation values to the threshold value; and

identify the correlation peak as a false correlation if one of the plurality of correlation values exceeds the threshold value.

15 . the GNSS receiver of claim 14 wherein the GNSS satellite signal is one of T GPS satellite signals identified by the GNSS receiver and, for each of the T satellites, the processor is further configured to:

determine a signal-to-noise level of each of the satellite signals relative to the satellite signal corresponding to the false correlation peak;

determine a carrier frequency offset modulo 1 kHz for each of the T satellite signals relative to the carrier signal of the satellite signal corresponding to the false correlation peak;

identify the satellite signal as an invalid satellite signal if the relative signal to noise ratio is less than a first threshold value and the carrier frequency offset modulo 1 kHz is less than a second threshold value.

16 . The GNSS receiver of claim 14 , wherein the GNSS receiver is a GPS receiver and the GNSS satellite signal is a Quasi-Zenith Satellite System (QZSS) signal.

17 . The GNSS receiver of claim 14 , wherein the GNSS satellite signal modulates a carrier signal and the carrier signal has a Doppler offset of at least 1 kHz.

18 . The GNSS receiver of claim 14 , wherein the processor is further configured to:

accumulate a non-coherent summation (NCS) of the first PRN code over an accumulation time period;

calculate a signal-to-noise ratio for the accumulated PRN code;

if the signal-to noise ratio for the accumulated PRN code is less than a threshold value, then increasing the accumulation time period in length and repeating the accumulate and calculate steps for the increased accumulation time period; and

if the signal to noise ratio for the accumulated PRN code is greater than the threshold value, setting N and M based on the length of the accumulation time period.

19 . The GNSS receiver of claim 14 , where N is between 4 and 16.

20 . The GNSS receiver of claim 14 , where M is between 0.5 and 1.0.