Global positioning system (GPS) receiver for recovery and tracking of signals modulated with P-code

• US 5,535,278 A
• Filed: 05/02/1994
• Issued: 07/09/1996
• Est. Priority Date: 05/02/1994
• Status: Expired due to Term

• Current Assignee: Hughes Electronics Corporation (Rtx Corporation)
• Original Assignee: Magnavox Electronic Systems Company
• Inventors: Knight, Jerry E., Cahn, Charles R., Keegan, Richard G., Stansell, Thomas A. Jr.
• Primary Examiner(s): CANGIALOSI, SALVATORE A
• Application Number:
  US08/236,291
• Time in Patent Office: 799 Days
• Field of Search: 342/357, 375/1, 380/49
• US Class Current: 380/274
• CPC Class Codes: G01S 19/24 Acquisition or tracking or demodulation of signals transmitted by the system synchronisation aspects of direct sequence spread spectrum modulation H04B1/7073
• CPC Class Codes: G01S 19/32 Multimode operation in a single same satellite system, e.g. GPS L1/L2
• CPC Class Codes: H04B 1/707 using direct sequence modulation

Abstract

Method and apparatus for improving the speed and accuracy of processing signals from global positioning system (GPS) receivers by ensuring access to GPS carrier signals that have been modulated with an encrypted P-code sequence. In one disclosed embodiment of the invention, GPS L1 and L2 signals are correlated with a locally generated P-code signal, bandpass filtered to enhance signal-to-noise ratio performance, and then cross-correlated to obtain a signal with an L1-L2 frequency component that facilitates the resolution of carrier cycle ambiguity. In another embodiment, received GPS signals are immediately converted to digital form, then digitally correlated with in-phase and quadrature components of a locally generated P-code signal. Signals resulting from the correlation are then integrated over timing intervals corresponding to a previously determined encryption period to provide in-phase (I) and quadrature (Q) samples. In one variant of this embodiment, the I and Q samples derived from the L2 GPS signal are digitally squared to obtain an L2 carrier. In another variant, I and Q samples derived from both L1 and L2 signals are cross-correlated to obtain a digital carrier with an L 1-L2 component. Tracking errors for the L1 and L2 P-code signals are computed using a similar digital technique.

Claims

1. A method for processing received signals in a satellite based positioning system, to generate a carrier difference signal having a frequency that is equivalent to the difference between first and second carrier signals that have been modulated with an encrypted P-code signal, the method comprising the following steps performed for each of a plurality of satellites:

receiving first and second signals transmitted from a satellite, the signals containing an encrypted version of a pseudorandom code sequence known as P code;

independently generating replicas of the P-code pseudorandom code sequence without encryption;

correlating each of the received first and second signals containing the encrypted P-code sequence with the locally generated replicas of the P-code sequence, to obtain two resultant signals, each having a frequency spectrum with a peak, the amplitude of which is indicative of the timing relationship between the received encrypted P-code sequence and the locally generated P-code sequence;

bandpass filtering each of the resultant signals from the correlating step;

cross-correlating the resultant signals after bandpass filtering, to obtain a carrier signal at a frequency equivalent to the difference between the frequencies of the first and second signals, with a desirably high signal-to-noise ratio; and

controlling the step of generating the replicas of the P-code sequence, to closely align each of the P-code replicas with a corresponding P-code component in the first and second signals.

2. A method as defined in claim 1, wherein:

the step of controlling the step of independently generating the replicas of the P-code sequence includes obtaining timing information indicative of a pseudo-range pertaining to the satellite.

3. A method as defined in claim 1, wherein:

the step of bandpass filtering the result of the correlating step is performed with a bandwidth of approximately ±

f_null /2 centered in the frequency spectrum obtained from the correlating step, where ±

f_null defines the positions of primary null points in the frequency spectrum.

4. A method as defined in claim 1, wherein:

the step of bandpass filtering the result of the correlating step is performed with a bandwidth less than about 10 megahertz and more than about 25 kilohertz.

5. A method as defined in claim 1, wherein:

the step of bandpass filtering the result of the correlating step is performed with a bandwidth of approximately 500 kilohertz.

6. For use in a global positioning system (GPS) receiver, apparatus for deriving carrier frequency difference signals for enhanced accuracy, the apparatus comprising the following components for use in processing signals from each of a plurality of satellites:

a receiving antenna, for receiving first and second carrier signals from a satellite, the signals containing an encrypted version of a pseudorandom code sequence known as P code;

generators of two independently timed P-code replicas of the P-code pseudorandom code sequence;

two correlator circuits, for correlating the received encrypted P-code sequence in the first and second carrier signals with the locally generated replicas of the P-code sequence;

two bandpass filters, for filtering the signals obtained from the correlator circuits; and

a cross-correlator circuit, for cross-correlating the correlated and filtered signals, to obtain a carrier signal at a frequency equivalent to the difference between the frequencies of the first and second carrier signals, with a desirably high signal-to-noise ratio.

7. Apparatus as defined in claim 6, and further comprising:

circuitry, for each of the satellites from which signals are currently received, for generating P-code timing control signals to adjust the timing of the P-code generators for the first and second carrier signals.

8. A method for processing received signals in a global positioning system (GPS) receiver, to recover a carrier signal that has been modulated with an encrypted pseudorandom code, referred to as P code, 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 referred to as P code;

converting the received signals to digital form;

generating a digital replica of the P-code pseudorandom code sequence without encryption, the replica having in-phase and quadrature components;

digitally correlating the received encrypted P-code sequence in digital form with the in-phase (I) and quadrature (Q) components of the locally generated replica of the P-code sequence, to obtain a succession of I and Q samples of correlated encrypted and non-encrypted P-code signals;

integrating the I and Q samples over a time period previously determined to be the encryption time period, to obtain successive I and Q values; and

digitally squaring I and Q values obtained from the integrating step, to obtain squared I and Q values for subsequent processing that are independent of encryption, wherein digitally squaring of the I and Q values is performed in accordance with the expression (I+jQ)², where j is a mathematical operator equal to (-1)^1/2.

9. A method as defined in claim 8, wherein:

the method further comprises controlling the step of generating a digital replica of the P-code pseudorandom code sequence; and

the step of controlling the step of generating the digital replica of the P-code pseudorandom code sequence includes obtaining timing information indicative of a pseudo-range pertaining to a satellite for which the digital P-code replica is generated.

10. For use in a global positioning system (GPS) receiver, apparatus for deriving carrier signals for enhanced accuracy, the apparatus comprising the following components for use in processing signals from each of a plurality of satellites:

a receiving antenna, for receiving a GPS signal transmitted from a satellite, the signal containing an encrypted version of a pseudorandom code sequence known as P code;

an analog-to-digital converter, for converting all received signals to digital form;

a digital P-code generator, for generating a digital replica of the P-code pseudorandom code sequence having in-phase (I) and quadrature (Q) components;

a digital correlator circuit, for correlating the received encrypted P-code sequence with the in-phase (I) and quadrature (Q) components of the locally generated replica of the P-code sequence;

two integrate-and-dump circuits, for generating a succession of I and Q values at time intervals corresponding to a previously determined encryption period of the encrypted P-code signals; and

a digital squaring circuit, for squaring the correlated signals as represented by the succession of I and Q values, to obtain the second harmonic of the carrier signal with a desirably high signal-to-noise ratio.

11. Apparatus as defined in claim 10, and further comprising:

digital signal processing circuitry, for generating a control signal to adjust the timing of the digital P-code generator to maximize output of the carrier signal, and to obtain signals indicative of a pseudo-range for each satellite.

12. A method for processing received signals in a global positioning system (GPS), to generate a carrier difference signal having a frequency that is the difference between first and second GPS carrier signals that have been modulated with an encrypted pseudorandom code sequence, referred to as P code, the method comprising the steps of:

receiving first and second carrier signals transmitted from each of a plurality of satellites, the signals containing an encrypted version of a pseudorandom code sequence known as P code;

converting the received signals to digital form;

independently generating for the first and second carrier signals separate digital replicas of the P-code pseudorandom code sequence without encryption, having in-phase (I) and quadrature (Q) components;

digitally correlating each of the received first and second carrier signals containing the encrypted P-code sequence with the I and Q components of the respective locally generated replica of the P-code sequence, to obtain two resultant I and Q signals relating to the first carrier signal and two other resultant I and Q signals relating to the second carrier signal;

separately integrating the two pairs of resultant signals over the encryption time interval, to obtain I1 and Q1 signals corresponding to the received first carrier signal, and I2 and Q2 signals corresponding to the received second carrier signal;

digitally cross-correlating the resultant I1/Q1 and I2/Q2 signals, to obtain a carrier signal at a frequency equal to the difference between the frequencies of the first and second carrier signals, with a desirably high signal-to-noise ratio; and

controlling the step of digitally generating the replicas of the P-code sequence, to maintain synchronism with the received P-code sequence.

13. A method as defined in claim 12, wherein:

the step of controlling the step of digitally generating the replicas of the P-code sequence includes obtaining timing information indicative of a pseudo-range pertaining to a satellite from which the first and second signals are received.

14. Apparatus for processing received signals in a global positioning system (GPS), to generate a carrier difference signal having a frequency that is the difference between first and second GPS carrier signals that have been modulated with an encrypted P-code signal, the apparatus comprising:

antenna means, for receiving first and second carrier signals transmitted from each of a plurality of satellites, the signals containing an encrypted version of a pseudorandom code sequence known as P code;

means for converting the received signals to digital form;

means for independently generating two digital replicas of the P-code pseudorandom code sequence without encryption, the replicas having in-phase (I) and quadrature (Q) components;

means for digitally correlating each of the received first and second carrier signals containing the encrypted P-code sequence with the I and Q components of the respective locally generated replicas of the P-code sequence, to obtain two resultant I and Q signals relating to the first carrier signal and two other resultant I and Q signals relating to the second carrier signal;

means for separately integrating the two pairs of resultant signals over the encryption time interval, to obtain I1 and Q1 signals corresponding to the received first carrier signal, and I2 and Q2 signals corresponding to the received second carrier signal;

means for digitally cross-correlating the resultant I1/Q1 and I2/Q2 signals, to obtain a carrier signal at a frequency equivalent to the difference in frequencies of the first and second carrier signals, with a desirably high signal-to-noise ratio; and

means for controlling the step of digitally generating replicas of the P-code sequence, to maintain synchronism with the received P-code sequence.

15. A method for detecting and compensating for tracking errors in a received global positioning system (GPS) 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;

converting the received signals to digital form;

deriving a digital carrier signal having components I and Q by correlating the received signals with I and Q components of a locally generated P-code sequence;

generating a digital P-code early-late signal indicative of the relative timing of the received P-code signal, and having in-phase and quadrature components;

digitally correlating the received encrypted P-code sequence in digital form with the in-phase and quadrature components of the digital P-code early-late signal, to obtain a succession of in-phase (I) and quadrature (Q) samples from the results of the digital correlation;

integrating the I and Q samples over a time period previously determined to be the encryption time period, to obtain successive I early-late (I_EL) and Q early-late (Q_EL) values;

digitally computing the vector dot product of the P-code early-late signal, defined by I_EL and Q_EL, and the carrier signal defined by I and Q, to obtain a P-code tracking error signal; and

controlling the step of generating the P-code early-late signal, to null the P-code tracking error.

16. A method as defined in claim 15, wherein:

the step of computing the vector dot product includes computing the quantity (I·

I_EL +Q·

Q_EL).

17. A method as defined in claim 15, wherein:

the steps are performed with respect to a received first GPS carrier signal; and

the method further includes a set of identical steps performed with respect to a received second GPS carrier signal.

18. Apparatus for detecting and compensating for tracking errors in a received global positioning system (GPS) P-code signal, the apparatus comprising:

antenna means, for 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;

means for converting the received signals to digital form;

means for deriving a digital carrier signal having components I and Q by correlating the received signals with a locally generated P-code sequence;

means for generating a digital P-code early-late signal indicative of the relative timing of the received P-code signal;

means for resolving the digital P-code early-late signal into in-phase and quadrature components;

means for digitally correlating the received encrypted P-code sequence in digital form with the in-phase and quadrature components of the digital P-code early-late signal, to obtain a succession of in-phase (I) and quadrature (Q) samples from the results of the digital correlation;

means for integrating the I and Q samples over a time period previously determined to be the encryption time period, to obtain successive I early-late (I_EL) and Q early-late (Q_EL) values;

means for digitally computing the vector dot product of the P-code early-late signal, defined by I_EL and Q_EL, and the carrier signal defined by I and Q, to obtain a P-code tracking error signal; and

means for controlling the means for generating the P-code early-late signal, to null the P-code tracking error.

19. A method for detecting and compensating for tracking errors in a received global positioning system (GPS) 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 and another pseudorandom code sequence known as C/A code;

deriving a carrier signal having components I and Q by correlating the received signals with a locally generated C/A code sequence;

tracking the carrier signal using a carrier tracking loop, whereby using the C/A code for carrier tracking provides an improvement in carrier signal-to-noise ratio; and

controlling the timing of the C/A code with a P-code tracking loop.

20. A method for processing received signals in a global positioning system (GPS), to generate a carrier difference signal having a frequency that is the difference between first and second GPS carrier signals that have been modulated with an encrypted P-code signal, the method comprising the steps of:

receiving first and second carrier signals transmitted from each of a plurality of satellites, the signals containing an encrypted version of a pseudorandom code sequence known as P code;

converting the received signals to digital form;

independently generating for the first and second carrier signals separate digital replicas of the P-code pseudorandom code sequence without encryption, having in-phase (I) and quadrature (Q) components;

generating a replica of the C/A code sequence;

correlating the replica of the C/A code sequence with the received first carrier signal, to permit carrier tracking of the first carrier signal;

correlating the replicas of the P-code sequence with the received first and second carrier signals, respectively;

code tracking the first and second carrier signals to synchronize a selected code sequence contained in the carrier sequence with locally generated signals; and

digitally cross-correlating signals obtained from correlation with the replicas of the P-code sequences, to obtain a derived carrier signal at a frequency equivalent to the difference in frequencies of the first and second carrier signals, with a desirably high signal-to-noise ratio.

21. A method as defined in claim 20, and further comprising:

carrier tracking the second carrier signal by combining the derived carrier signal obtained from the cross-correlation step and the first carrier signal obtained from carrier tracking of the first carrier signal.

22. A method for processing received signals in a global positioning system (GPS), to derive carrier signal measurements by squaring one of first and second GPS carrier signals that have been modulated with an encrypted P-code signal, the method comprising the steps of:

receiving first and second carrier signals transmitted from each of a plurality of satellites, the signals containing an encrypted version of a pseudorandom code sequence known as P code;

converting the received signals to digital form;

independently generating for the first and second carrier signals separate digital replicas of the P-code pseudorandom code sequence without encryption, having in-phase (I) and quadrature (Q) components;

generating a replica of the C/A code sequence;

correlating the replica of the C/A code sequence with the received first carrier signal, to permit carrier tracking of the first carrier signal;

correlating the replicas of the P-code sequence with the received first and second carrier signals, respectively, to provide correlated first and second carrier signals;

digitally squaring the correlated second carrier signal, to provide second carrier signal phase measurements at a relatively high signal-to-noise ratio; and

digitally cross-correlating the correlated first and second carrier signals, to resolve half-cycle ambiguity in the second carrier signal phase measurements obtained by digital squaring.

23. A method for processing received signals in a global positioning system to take advantage of a benefit that arises from use of a carrier difference signal having a frequency that is equivalent to the difference between the frequencies of first and second carrier signals that have been modulated with an encrypted P-code signal, the method comprising the steps of:

locally generating two separate P-code replicas;

employing the P-code replicas to aid in code tracking of the first and second carrier signals, respectively;

locally generating a C/A code replica;

employing the C/A code replica to track the first carrier signal; and

cross-correlating the first and second carrier signals to obtain a derived carrier difference signal that has a frequency equal to the difference in the frequencies of the first and second carrier signals, and has a signal-to-noise-ratio advantage over either carrier signal used separately for resolving carrier whole cycle ambiguity.

24. A method as defined in claim 23, and further comprising the step of:

employing the derived carrier difference signal to track the second carrier signal.

25. A method for processing received signals in a global positioning system to take advantage of a benefit that arises from use of a carrier difference signal having a frequency that is equivalent to the difference between the frequencies of first and second carrier signals that have been modulated with an encrypted P-code signal, the method comprising the steps of:

locally generating two separate P-code replicas;

employing the P-code replicas to aid in code tracking of the first and second carrier signals in separate tracking loops;

squaring the second carrier signal, after correlating with the P-code replica, to eliminate the effect of a P-code encryption signal and to obtain a double-frequency signal of frequency twice that of the second carrier signal, wherein the phase of the second carrier signal is derived by dividing the phase of the double-frequency signal by two;

performing a cross-correlation process on the first and second carrier signals to obtain frequency difference signal having a frequency equal to the difference in frequencies of the first and second carrier signals with all other modulation, common to the first and second carrier signals, removed; and

deriving from the frequency difference signal a polarity signal from which half-cycle ambiguity of the phase measurement of the second carrier signal, as derived from the phase of the double-frequency signal divided by two, can be resolved.

26. A method as defined in claim 25, wherein:

the cross-correlation process is one of direct cross-correlation of the first and second carrier signals; and

the method further comprises the steps of adjusting the delay of the first carrier signal relative to the second, prior to cross-correlation of the first and second carrier signals, using timing information derived from the separate code tracking loops for the first and second carrier signals.

27. A method as defined in claim 26, wherein the step of adjusting the delay of the first carrier signal relative to the second is effected by shifting one of the first and second carrier signals through a multi-stage shift register to provide a variable delay, and controlling the amount of the delay with timing signals from the separate code tracking loops.

28. A method as defined in claim 25, wherein:

the cross-correlation process is one of code-aided cross-correlation, wherein cross-correlation of the first and second carrier signals occurs after correlating each carrier with a corresponding one of the two P-code replicas derived from separate tracking loops, removing the P-code but leaving the common P-code encryption.

29. A method for detecting and compensating for tracking errors in received global positioning system (GPS) signals, the method comprising the steps of:

receiving a signal transmitted from each of a plurality of satellites, the signal containing a pseudorandom code sequence known as P code, which is subject to encryption, and another pseudorandom code sequence known as C/A code, the timing of which is coherent with the P code;

deriving a digital carrier signal having components I and Q by correlating the received signal with a C/A code sequence supplied by a local C/A code generator;

tracking the received carrier signal using a carrier tracking loop that includes the local C/A code generator, whereby use of the C/A code for carrier tracking provides a carrier signal with an improved signal-to-noise ratio as compared with using code aided squaring for carrier tracking when the P-code is encrypted;

tracking the received P code or encrypted P-code sequence using a tracking loop that includes a local P code generator, whereby use of P code for this purpose provides better code tracking because the P code sequence is a faster changing and more precise code with respect to the C/A code; and

controlling the timing of the C/A code generator with signals derived from the P-code tracking loop.