P-code-aided global positioning system receiver
First Claim
1. A method of processing received signals in a global positioning system (GPS), to recover an L2 carrier signal that has been modulated with an encrypted P-code signal, the method comprising the steps of:
- receiving a signal transmitted from each of a plurality of satellites, the signal containing an encrypted version of a pseudorandom code sequence known as P code;
independently generating a replica of the P-code pseudorandom code sequence without encryption;
correlating the received encrypted P-code sequence with the locally generated replica of the P-code sequence, to obtain a frequency spectrum containing a peak indicative of the timing relationship between the received encrypted P code sequence and the locally generated P code sequence;
bandpass filtering the result of the correlation step; and
squaring the correlated and filtered signal to recover the L2 carrier signal with a favorable signal- to-noise ratio.
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Accused Products
Abstract
Method and apparatus for use in a global positioning system (GPS), to obtain the benefit of L2 carrier signals and precise-code (P-code) signals transmitted from satellites in encrypted form. Received, encrypted P-code signals are correlated with a locally generated P-code sequence and the resulting signal is bandpass filtered and squared, to separate one satellite signal from the others, to recover the carrier signal and make accurate measurement of its phase and pseudorange, even though the received P-code sequence is encrypted. Because the squaring step is performed over a narrower bandwidth than the original P-code, there is less degradation in the signal-to-noise ratio of the received signal, as compared with squaring over the entire P-code bandwidth. Therefore, performance is more reliable under weak signal conditions or when there are interference signals present, and the improvement in signal quality means that cycle ambiguity of the carrier signals can be resolved more rapidly. Moreover, the invention does not frustrate the intended purpose of P-code encryption.
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Citations
32 Claims
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1. A method of processing received signals in a global positioning system (GPS), to recover an L2 carrier signal that has been modulated with an encrypted P-code signal, the method comprising the steps of:
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receiving a signal transmitted from each of a plurality of satellites, the signal containing an encrypted version of a pseudorandom code sequence known as P code; independently generating a replica of the P-code pseudorandom code sequence without encryption; correlating the received encrypted P-code sequence with the locally generated replica of the P-code sequence, to obtain a frequency spectrum containing a peak indicative of the timing relationship between the received encrypted P code sequence and the locally generated P code sequence; bandpass filtering the result of the correlation step; and squaring the correlated and filtered signal to recover the L2 carrier signal with a favorable signal- to-noise ratio. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. For use in a global positioning system (GPS) receiver, apparatus for deriving L2 carrier signals for enhanced accuracy, the apparatus comprising:
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a receiving antenna, for receiving a GPS signal transmitted from each of a plurality of satellites, the signal containing an encrypted version of a pseudorandom code sequence known as P code; a P-code generator, for independently generating a replica of the P-code pseudorandom code sequence; a correlator circuit, for correlating the received encrypted P-code sequence with the locally generated replica of the P-code sequence; a bandpass filter, for filtering the signal obtained from the correlator circuit; and a squaring circuit, for squaring the correlated and filtered signal, to obtain the L2 carrier signal with a relatively good signal-to-noise ratio. - View Dependent Claims (11, 12, 13, 14, 15, 16)
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17. A method of processing received signals in a global positioning system (GPS) used for navigation, to derive the benefit of P-code signals that have been encrypted, the method comprising the steps of:
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receiving a signal transmitted from each of a plurality of satellites, the signal containing an encrypted version of a pseudorandom code sequence known as P code; independently generating a replica of the P-code pseudorandom code sequence; correlating the received encrypted P-code sequence with the locally generated replica of the P-code sequence; bandpass filtering the result of the correlation step; squaring the correlated and filtered signal; processing the squared signal to derive a frequency spectrum containing a distinct peak indicative of the timing relationship between the received P-code sequence and its locally generated replica; and controlling the step of generating a replica of the P-code sequence, to maximize the peak in the frequency spectrum and obtain an accurate measure of signal arrival time for use in position determination. - View Dependent Claims (18, 19, 20)
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21. For use in a global positioning system (GPS) navigation receiver, apparatus for employing P-code signals for enhanced position determination accuracy, the apparatus comprising:
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a receiving antenna, for receiving a GPS signal transmitted from one of a plurality of satellites, the signal containing an encrypted version of a pseudorandom code sequence known as P code; a P-code generator, for independently generating a replica of the P-code pseudorandom code sequence; a correlator circuit, for correlating the received encrypted P-code sequence with the locally generated replica of the P-code sequence; a bandpass filter, for filtering the signal obtained from the correlator circuit; a squaring circuit, for squaring the correlated and filtered signal; digital signal processing circuitry, for processing the squared signal to derive a frequency spectrum containing a distinct peak indicative of the timing relationship between the received P-code sequence and its locally generated replica; and a feedback circuit for controlling the P-code generator, to maximize the peak in the frequency spectrum and obtain an accurate measure of signal arrival time for use in position determination. - View Dependent Claims (22, 23, 24)
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25. A global positioning system (GPS) receiver, comprising:
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a receiving antenna, for receiving GPS signals transmitted from one of a plurality of satellites, the signals including an L1 carrier signal modulated with a pseudorandom code known as C/A code, and an L2 carrier signal modulated with a pseudorandom code known as P code, wherein the P code sequence is encrypted before transmission; an L1 receiver module for recovering the L1 carrier signal, including a C/A code generator and a correlation circuit for correlating the locally generated C/A code with the received L1 signals, to derive the L1 carrier signal and any data modulated onto it; and an L2 receiver module for recovering the L2 carrier signal in spite of encryption of the P code sequence, including a P-code generator, for independently generating a replica of the P-code pseudorandom code sequence, a correlator circuit, for correlating the received encrypted P-code sequence with the locally generated replica of the P-code sequence, a bandpass filter, for filtering the signal obtained from the correlator circuit, and a squaring circuit, for squaring the correlated and filtered signal, to obtain the L2 carrier signal with a relatively good signal-to-noise ratio. - View Dependent Claims (26, 27, 28, 29, 30, 31, 32)
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Specification