System and method for fast code phase and carrier frequency acquisition in GPS receiver
First Claim
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.
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Accused Products
Abstract
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.
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Citations
12 Claims
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1. A GPS receiver, comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6)
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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:
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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. - View Dependent Claims (8, 9, 10, 11, 12)
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Specification