GPS m-code receiver tracking system

• US 20080037614A1
• Filed: 08/08/2006
• Published: 02/14/2008
• Est. Priority Date: 08/08/2006
• Status: Active Grant

• Current Assignee: Aerospace Corporation
• Original Assignee: Aerospace Corporation
• Inventors: Douglas, Randal K.
• Granted Patent: US 7,899,109 B2
• Time in Patent Office: Days
• US Class Current: 375/150
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

An m-code GPS receiver receives m-code GPS communication signals having a multimodal autocorrelation, using an m-code mode identifier unambiguously determining a mode value of one of the m-code modal peaks coherently aligned to a coherent unimodal detected envelope, based on sequential probability estimation in an m-code envelope tracking filter using filter residual estimation or with a coherent m-code and c/a-code tracking filter also based on filter residual estimation, for generating m-code phase errors, for unambiguous and precise m-code code phase tracking in closed feedback loops, for preferred use in navigation systems.

Claims

1. A system for determining a code phase error of a multimodal spread spectrum signal spread by a multimodal code being a spread spectrum chipping code having a multimodal autocorrelation function having modal peaks, the system comprising,

a demodulator for carrier demodulating the multimodal spread spectrum signal into multimodal quadrature signals,

a generator for generating multimodal code replicas of the multimodal code, the multimodal code replicas being offset in time to the multimodal code by the code phase error,

a correlator for receiving multimodal quadrature signals and correlating the multimodal quadrature signals into multimodal quadrature correlations by the multimodal code replicas, and

a tracking system for receiving the multimodal quadrature correlations and for detecting an envelope coherently encompassing the modal peaks and for identifying a mode of the modal peaks for generating the code phase error.

2. The system of claim 1 wherein, the envelope is a low-grade gradient unimodal envelope, and the modal peaks are steep gradient modal peaks.

3. The system of claim 1 wherein, the envelope is a unimodal envelope defined by modal peaks defined by the multimodal autocorrelation function.

4. The system of claim 1 wherein,

the envelope is a unimodal c/a-code envelope, and

the multimodal code is an m-code code.

5. The system of claim 1 wherein,

the envelope is a low-grade gradient unimodal envelope,

the modal peaks are steep gradient modal peaks,

the code phase error adjusts in time the multimodal code replicas for coherent correlation and code phase error tracking of the multimodal quadrature signals,

the code phase error is unambiguously tracking by identifying the mode relative to the low-grade gradient unimodal envelope, and

the code phase error is accurately tracked by generating the code phase error from the identified mode having a steep gradient.

6. A system for determining the code phase error of an m-code spread spectrum signal spread by a m-code code being a spread spectrum chipping code having an m-code autocorrelation function having modal peaks, the system comprising,

a demodulator for carrier demodulating the m-code spread spectrum signal into m-code quadrature signals,

a generator for generating m-code code replicas of the m-code code, the m-code code replicas being offset in time to the multimodal code by the code phase error,

a correlator for receiving m-code quadrature signals and correlating the m-code quadrature signals into m-code quadrature correlations using the m-code code replicas, and

a tracking system for receiving the m-code quadrature correlations and for detecting an envelope coherently encompassing the modal peaks and for identifying a mode n_m of the modal peaks for generating the code phase error r_m.

7. The system of claim 6 wherein,

the envelope is a unimodal envelope.

8. The system of claim 6 wherein,

the envelope is a unimodal envelope defined by modal peaks defined by the m-code autocorrelation function.

9. The system of claim 6 wherein,

the envelope is a low-grade gradient unimodal envelope,

the modal peaks are steep gradient modal peaks,

the code phase error r_m adjusts in time the m-code code replicas for coherent correlation and code phase error tracking of the m-code quadrature signals,

the code phase error r_m is unambiguously tracking by identifying the mode n_m relative to the low-grade gradient unimodal envelope, and

the code phase error r_m is accurately tracked by generating the code phase error r_m from the identified mode n_m having a steep gradient.

10. The system of claim 6 wherein the envelope is an m-code envelope defined by the modal peaks of the m-code autocorrelation function, the tracking system comprises,

an m-code envelope detector for determining the m-code envelope from early and late versions of the m-code quadrature correlations respectively using early and later version of the m-code replicas,

an m-code envelope tracking-filter for receiving the m-code quadrature correlations and the m-code envelope and the mode n_m, the m-code envelope-tracking filter for providing an m-code error covariance p_me and an envelope covariance p_v and an m-code envelope range error r_v and the local m-code range error state estimate r_me,

a mode identifier for receiving the m-code error covariance p_me and the envelope covariance p_v and the m-code envelope range error r_v and the m-code local range error r_me, the mode identifier generating the mode n_m, and

a phase error calculator for receiving the mode value n_m and the local m-code range error r_me and for providing the code phase error r_m.

11. The system of claim 10 wherein,

the m-code envelope detector receives a first set of the quadrature components comprising Ivee, Qvee, Ivel, Qvel, Ivpe, Qvpe, Ivpl, and Qvpl quadrature components and provides m-code envelope detector outputs y_e and y_p and y₁ communicated to the m-code envelope-tracking filter, the detector outputs y_e and y_p and y₁ defining the m-code envelope.

12. The system of claim 10 wherein the m-code envelope tracking filter comprises,

a measurement residual generator for receiving the m-code quadrature correlations and the m-code envelope and the mode n_m and propagated states for providing a measurement residual ρ

_v,

a state estimator for receiving the mode n_m and the propagated states and an envelope state covariance P_v and the measurement residual ρ

_v for providing a state estimate x_v,

a state propagator for receiving the state estimate x_v and for providing the propagated states, and

a state error covariance estimator for receiving the mode n_m and the propagated states for providing the m-code error covariance p_me, the envelope covariance p_v and the envelope state covariance P_v.

13. The system of claim 10 wherein,

the m-code mode identifier generates the code phase error as r_m=r_me+n_mλ

_m/4 where λ

_m is a nominal m-code chip width.

14. The system of claim 10 wherein,

the m-code mode identifier uses a Shiryayev sequential probability ratio algorithm to unambiguously determine the mode n_m.

15. The system of claim 6 is a weak-lock system, the system further comprising,

a delta pseudorange and delta rate calculator for adjusting in a carrier replica for coherent demodulation by the demodulator of the m-code spread spectrum signal and for adjusting in time the n-code replicas for coherent correlation of the m-code quadrature signals by the correlator.

16. The system of claim 6 is an ultratight system, the system further comprising,

an integration filter for receiving the code phase error r_m and for generating an error state vector,

an inertial navigation calculator for receiving the error state vector and generating position and velocity data, and

a pseudorange and rate calculator for receiving the position and velocity data for adjusting in a carrier replica for coherent demodulation by the demodulator of the m-code spread spectrum signal and for adjusting in time the m-code replicas for coherent correlation of the m-code quadrature signals by the correlator.

17. A system for determining the code phase error of an m-code spread spectrum signal spread by a m-code code being a spread spectrum chipping code having an m-code autocorrelation function having modal peaks, system is a coherent system for further coherently receiving an arbitrary code spread spectrum signal with the multimodal spread spectrum signal, arbitrary code spread spectrum signal having an arbitrary autocorrelation function, the system comprising,

a demodulator for carrier demodulating the m-code spread spectrum signal into m-code quadrature signals and for carrier demodulating the arbitrary code spread spectrum signal into arbitrary quadrature signals,

an m-code code generator for generating m-code code replicas of the m-code code, the m-code code replicas being offset in time to the multimodal code by the code phase error,

an arbitrary code generator for generating arbitrary code replicas of the arbitrary code, the arbitrary code replicas being offset in time to the m-code by the code phase error,

a correlator for receiving m-code quadrature signals and the correlating the m-code quadrature signals into m-code quadrature correlations using the m-code code replicas, the correlator further for receiving arbitrary quadrature signals and the correlating the arbitrary code quadrature signals into arbitrary code quadrature correlations using the arbitrary code replicas and

a tracking system for receiving the m-code quadrature correlations and the arbitrary code quadrature correlation and for detecting an arbitrary envelope of the arbitrary autocorrelation function coherently encompassing the modal peaks for the m-code and for identifying a mode n_m of the modal peaks for generating the code phase error r_m.

18. The system of claim 17 wherein,

the arbitrary spread spectrum signal is a c/a-code spread spectrum signal,

the arbitrary quadrature signals are c/a-code quadrature signals,

the arbitrary autocorrelation function is a c/a-code autocorrelation function,

the arbitrary quadrature correlations are c/a-code quadrature correlations, and

the arbitrary code replicas are c/a-code replicas.

19. The system of claim 17 wherein the system is a coherent ultratight system, the system further comprising,

an integration filter for receiving the code phase error and for generating an error state vector,

an inertial navigation calculator for receiving the error state vector and generating position and velocity data, and

a pseudorange and rate calculator for receiving the position and velocity data for adjusting the carrier phase and m-code replicas and arbitrary code replicas for coherent demodulation by the demodulator and coherent correlation by the correlator.

20. The system of claim 17 wherein,

the arbitrary spread spectrum signal is a c/a-code spread spectrum signal,

the arbitrary quadrature signals are c/a-code quadrature signals,

the arbitrary autocorrelation function is a c/a-code autocorrelation function,

the arbitrary quadrature correlations are c/a-code quadrature correlations,

the arbitrary code replicas are c/a-code replicas,

the system is a coherent ultratight system, and

the system further comprises, an integration filter for receiving the code phase error and for generating an error state vector,

an inertial navigation calculator for receiving the error state vector and generating position and velocity data, and

a pseudorange and rate calculator for receiving the position and velocity data for adjusting the carrier phase and m-code replicas and arbitrary code replicas for coherent demodulation by the demodulator and coherent correlation by the correlator.