Method and apparatus for performing signal correlation at multiple resolutions to mitigate multipath interference

1. A method of mitigating multipath interference in a digital signal correlator comprising:

forming values representative of magnitude or power of complex correlation results between a digital signal and a pseudorandom reference code;

integrating said values over an integration period to produce a correlation response; and

selecting an earliest peak in the correlation response to compute a time delay, wherein the selecting step comprises:

computing a slope of the correlation response on an ascending edge thereof;

providing a mean noise level of the correlation response;

computing the intersection of the slope and the mean noise level; and

using the intersection to determine the earliest peak.

2. A method of mitigating multipath interference in a digital signal correlator comprising:

forming values representative of magnitude or power of complex correlation results between a digital signal and a pseudorandom reference code;

integrating said values over an integration period to produce a correlation response; and

selecting an earliest peak in the correlation response to compute a time delay; and

determining whether the correlation response is affected by one or more multipath signals, wherein the step of determining whether the correlation response is affected by one or more multipath signals comprises:

computing a first intersection of an ascending edge of the correlation response with a mean noise level of the correlation response;

computing a second intersection of a descending edge of the correlation response with the mean noise level of the correlation response;

computing the width between the first intersection and the second intersection.

3. A method of mitigating multipath interference in a digital signal correlator comprising:

forming values representative of magnitude or power of complex correlation results between a digital signal and a pseudorandom reference code;

integrating said values over an integration period to produce a correlation response; and

selecting an earliest peak in the correlation response to compute a time delay;

sampling the digital signal to produce a set of digital signal samples having a selected sample spacing; and

correlating the set of digital signal samples with the pseudorandom reference code, wherein the correlating step comprises:

(a) dividing the pseudorandom reference code into a plurality of code segments;

(b) selecting a code segment;

(c) forming an inner product between the selected code segment and the set of digital signal samples to produce a partial correlation;

(d) repeating steps (b) and (c) to produce a plurality of partial correlations;

(e) summing the plurality of partial correlations.

4. The method of claim 3 , wherein the pseudorandom reference code is a C/A code of the global positioning system (GPS).

5. The method of claim 4 , wherein the pseudorandom reference code is generated by extending a code segment by replicating a C/A code bit to an integer number of samples.

6. The method of claim 5 , further comprising:

generating the code segments from a lookup table by selecting one of a fixed number of non-overlapping segments of the CIA code.

7. The method of claim 3 , wherein the inner product is formed by multiplying each of the digital signal samples with a respective chip of the selected code segment and summing each multiplication result with other multiplication results to produce the partial correlation.

8. The method of claim 3 , wherein the sample spacing is at least ½ of a C/A code chip.

9. A receiver of global positioning system (GPS) signals comprising:

an RF/IF converter for filtering and frequency translating a received GPS signal to form an IF signal;

an analog-to-digital converter for digitizing the IF signal;

a tuner for removing Doppler shift form the digitized signal and producing an in-phase (I) and a quadrature (Q) signal;

a decimation circuit for subsampling the I and Q signals;

a correlation processor for correlating the subsampled I and Q signals with a C/A reference code to produce complex correlation results;

a signal normalizer for forming values representative of magnitude or power of said complex correlation results;

a magnitude accumulator for accumulating said values to produce a correlation response; and

a processor for selecting an earliest peak in the correlation response to compute a time delay, wherein the processor is further configured to:

compute a slope of the correlation response on an ascending edge thereof;

provide a mean noise level of the correlation response;

compute the intersection of the slope and the mean noise level; and

use the intersection to determine the earliest peak.

10. A receiver of global positioning system (GPS) signals comprising:

an RF/IF converter for filtering and frequency translating a received GPS signal to form an IF signal;

an analog-to-digital converter for digitizing the IF signal;

a tuner for removing Doppler shift form the digitized signal and producing an in-phase (I) and a quadrature (Q) signal;

a decimation circuit for subsampling the I and Q signals;

a correlation processor for correlating the subsampled I and Q signals with a C/A reference code to produce complex correlation results;

a signal normalizer for forming values representative of magnitude or power of said complex correlation results;

a magnitude accumulator for accumulating said values to produce a correlation response; and

a processor for selecting an earliest peak in the correlation response to compute a time delay and determining whether the correlation response is affected by one or more multipath signals, wherein the processor is further configured to:

compute a first intersection of an ascending edge of the correlation response with a mean noise level of the correlation response;

compute a second intersection of a descending edge of the correlation response with the mean noise level of the correlation response;

compute the width between the first intersection and the second intersection.

11. An apparatus for mitigating multipath interference in a digital signal correlator comprising:

means for forming values representative of magnitude or power of complex correlation results between a digital signal and a pseudorandom reference code;

means for integrating said values over an integration period to produce a correlation response; and

means for selecting an earliest peak in the correlation response to compute a time delay, wherein the means for selecting comprises:

means for computing a slope of the correlation response on an ascending edge thereof;

means for providing a mean noise level of the correlation response;

means for computing the intersection of the slope and the mean noise level; and

means for using the intersection to determine the earliest peak.

12. An apparatus for mitigating multipath interference in a digital signal correlator comprising:

means for forming values representative of magnitude or power of complex correlation results between a digital signal and a pseudorandom reference code;

means for integrating said values over an integration period to produce a correlation response;

means for selecting an earliest peak in the correlation response to compute a time delay; and

means for determining whether the correlation response is affected by one or more multipath signals, wherein the means for determining whether the correlation response is affected by one or more multipath signals comprises:

means for computing a first intersection of an ascending edge of the correlation response with a mean noise level of the correlation response;

means for computing a second intersection of a descending edge of the correlation response with the mean noise level of the correlation response;

means for computing the width between the first intersection and the second intersection.

13. A method of mitigating multipath interference in a digital signal correlator, comprising:

processing a digital signal and a pseudorandom reference code to produce a first correlation response having a first resolution;

identifying an indication of multipath interference in response to the first correlation response;

processing the digital signal and the pseudorandom reference code to produce a second correlation response having a second resolution; and

selecting an earliest peak in the second correlation response to compute a time delay.

14. The method of claim 13 , wherein the selecting step comprises:

forming values representative of magnitude or power of the second correlation response; and

selecting the earliest peak in response to the values.