System and method for fast code phase and carrier frequency acquisition in GPS receiver
View Patent ↗A GPS receiver acquires carrier frequency and Gold code phase using short segments of a received GPS signal. In one embodiment, a 1-ms segment of the GPS signal is transformed to the frequency domain. This is multiplied by a frequency representation of the Gold code. The resulting product is converted to the time domain, and a peak is detected. The location of the peak corresponds to the code phase. If no peak is located, the carrier frequency is changed. Full- and half-bin steps in carrier frequency are considered. Processing gain is achieved by using longer segments of the input signal, for example 4 or 16 ms and integrating 1-ms segments. Considerations are provided for compensating for the effects of a transition, should it occur in the short segment of the GPS signal being processed. Integrations can be performed using non-coherent and coherent techniques. Adjustments are made for non-integral millisecond segment lengths.
1. A GPS receiver, comprising:
a receiver for receiving a composite GPS signal comprising a received spreading code having a received spreading code frequency;
a digitizer for digitizing the received composite GPS signal at a sample rate;
a spreading code generator to generate a spreading code;
a convolver to convolve received composite GPS signal with the generated spreading code to generate a convolved output, said convolved output being used to identify a code phase of the received composite GPS signal;
means for estimating a clock error due to the sample rate not being equal to an integral multiple of the received spreading code frequency;
a metric derived from the convolved output and the estimate clock error; and
means for maximizing the metric to determine a total time offset and a frequency offset.
2. The GPS receiver recited in claim 1 , wherein the convolver performs a linear convolution.
3. The GPS receiver recited in claim 1 , wherein the convolver performs a plurality of circular convolutions.
4. The GPS receiver recited in claim 1 , further comprising:
means for estimating a data sequence modulating the spreading code;
means for generating an ideal convolver output corresponding to a convolution of an ideal composite GPS signal with the generated spreading code;
a multiplier for multiplying the convolver output with a complex conjugate of the estimated data sequence and a complex conjugate of the ideal convolver output; and
wherein the metric comprises an integration of application of the multiplier to consecutive segments of the collected GPS signal.
5. The GPS receiver recited in claim 1 , wherein the means for maximizing the metric searches across different pre-determined time offsets to maximize the metric.
6. The GPS receiver recited in claim 1 , wherein the convolver performs the convolution in the frequency domain.
7. A method for deriving a code phase and carrier frequency offset for use in a global positioning system (GPS) receiver, comprising the steps of:
receiving a composite GPS signal comprising a received spreading code having a received spreading code frequency;
sampling the received composite GPS signal at a sample rate;
generating a spreading code sequence;
convolving the received GPS signal with the generated spreading code sequence;
estimating a clock error due to the sample rate not being equal to an integral multiple of the received spreading code frequency;
deriving a metric from the convolved output and the estimate clock error; and
maximizing the metric to determine a total time offset and a frequency offset, wherein said total time offset identifies a code phase of the received composite GPS signal.
8. The method recited in claim 7 , wherein the convolving step comprises the step of performing a linear convolution.
9. The method recited in claim 7 , wherein the convolving step comprises the step of performing a plurality of circular convolutions.
10. The method recited in claim 7 , further comprising the steps of:
estimating a data sequence modulating the spreading code;
generating an ideal convolver output corresponding to a convolution of an ideal composite GPS signal with the generated spreading code;
multiplying the convolver output with a complex conjugate of the estimated data sequence and a complex conjugate of the ideal convolver output; and
integrating an output of the multiplier over consecutive segments of the collected GPS signal.
11. The method recited in claim 7 , wherein the maximizing step comprises the step of searching across different pre-determined time offsets to maximize the metric.
12. The method recited in claim 7 , wherein the convolving step further comprises the steps of:
converting the received GPS signal to a frequency domain;
converting the generated spreading code sequence to the frequency domain;
multiplying the converted received GPS signal and spreading code sequences to generate a product; and
converting the product to a time domain.