Global positioning system receiver digital processing technique

• US 5,293,170 A
• Filed: 04/06/1992
• Issued: 03/08/1994
• Est. Priority Date: 04/10/1991
• Status: Expired due to Term

• Current Assignee: MiTAC International Corporation (MiTAC Holdings Corp.)
• Original Assignee: Ashtech, Inc. (Thales SA)
• Inventors: Lorenz, Robert G., Helkey, Roger J., Abadi, Kamran K.
• Primary Examiner(s): Issing, Gregory C.
• Application Number:
  US07/864,461
• Time in Patent Office: 701 Days
• Field of Search: 342/352, 342/357, 342/356, 375/80, 375/81, 375/82, 375/96, 375/118
• US Class Current: 342/352
• 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

Abstract

A Global Positioning System (GPS) commercial receiver is provided with a digital processor that can utilize to advantage P-code modulated L1 and L2 satellite signals which have been modulated with an unknown security code. Integration of the L1 and L2 signals, after demodulation by locally generated carrier and P-code signals, is repetitively accomplished over a duration that is estimated to be the period of the modulation code. The C/A-code L1 signal, which is not modulated with the unknown security code, is also used in locking the locally generated carrier and P-code generators in phase with the received L1 and L2 satellite signals. A interpolative technique is used for adjusting the phase of the locally generated carriers and code in increments much smaller than the period clock sources. Those locked phases can then be utilized to determine position, distance, time, etc., as is done in GPS receivers not utilizing the anti-spoofed signals but with increased accuracy and resolution. A novel structure of the GPS receiver radio frequency and intermediate frequency sections, having a specific combination of demodulating frequencies, is also disclosed.

Claims

1. A system for processing L1 and L2 signals received from a satellite that include a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising:

means for generating replicas of the respective unique frequency carriers of the received L1 and L2 signals,

means for generating a replica of the known pseudo-random P-code modulated on the received L1 and L2 signals,

means receiving the L1 and L2 signals for demodulating the L1 and L2 signals with the replicas of their carriers provided by the carrier replica generating means and with the P-code replica provided by the P-code replica generating means, thereby producing demodulated L1 and L2 signals,

means receiving the demodulated L1 and L2 signals for separately integrating them over time periods equal in length to a plurality of P-code cycles, thereby producing integrated demodulated L1 and L2 signals,

means receiving the integrated demodulated L1 and L2 signals for correlating them with each other, thereby generating a correlation level, and

means communicating with said carrier replica generating means and said P-code replica generating means for adjusting the phases of the carrier replicas and of the P-code replica relative to the incoming L1 and L2 signals in a manner to substantially maximize a measure of said correlation level,

whereby useful phases of the carrier and P-code replicas are obtained.

2. The system of claim 1 wherein the integrating means includes means for integrating the demodulated L1 and L2 signals over a time period equal to approximately twenty P-code cycles.

3. The system of claim 1 wherein the integrating means includes means for integrating the demodulated L1 and L2 signals over a time period substantially equal to a period of the unknown modulation code.

4. The system of claim 1 wherein the demodulating means includes:

a first delay line having a plurality of taps therealong,

means communicating with the P-code replica generating means for applying the P-code replica to said first delay line, thereby making the P-code replica available at different relative phases at the plurality of first delay line taps, and

means receiving the P-code replica from said first delay line taps for adjusting the relative phases of the P-code replica used to separately demodulate each of the received L1 and L2 signals, whereby the generation of only one P-code replica is required in order to demodulate both of the L1 and L2 signals.

5. The system of claim 4 wherein the phase adjusting means includes means for selecting the taps of the first delay line from which the P-code is obtained for use in demodulating each of the L1 and L2 signals.

6. The system of claim 4 wherein the integrating means includes:

a second delay line having a plurality of taps,

means for generating a timing signal in phase with the generated P-code replica,

means communicating with the timing signal generating means for applying the timing signal to the second delay line, thereby making the timing signal available at different relative phases thereof at the plurality of second delay line taps, and

means receiving the timing signal from the second delay line taps for adjusting the relative phases the repetitive periods of integration of the demodulated L1 and L2 signals.

7. The system of claim 6 wherein the phase adjusting means in each of the demodulating and integrating means includes means for adjusting in synchronism the taps of both the first and second delay lines from which the P-code and integrating period timing signals are respectively obtained for each of the L1 and L2 signals.

8. A system for generating a desired binary signal shifted in phase a controlled amount from a source binary signal formed from a first periodic clock signal having a first period, comprising:

means for generating from said source binary signal a replica thereof that is delayed an integral number of said first clock signal periods,

means for generating a second periodic clock signal having a second period that is slightly different from said first period, said first and second clock signals being synchronous with each other,

means for comparing said first and second clock signals to determine periodically recurring instances of substantial coincidence therebetween and generating a pulse thereat,

means for resetting by said coincidence pulse a counter being clocked with said second clock signal,

means receiving said source and delayed signals for generating the desired binary signal in respective alternating first and second durations, said first duration beginning at the coincidence pulse and continuing until said counter reaches a predetermined count and said second duration extending from said counter reaching said predetermined count until the occurrence of another coincidence pulse, and

means for setting the predetermined count of said counter to obtain the desired signal having a desired phase relative to said source signal.

9. A system for processing L1 and L2 satellite signals that include a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising:

means for deriving from the L1 satellite signal a signal that represents an estimate of the unknown code, and

means receiving the unknown code estimate for combining the estimate signal with the L2 signal in a manner to determine the relative phase of the L2 signal.

10. A system for processing L1 and L2 signals received from at least one satellite of a global positioning system wherein each of said signals includes a unique frequency carrier with a known pseudo-random P-code modulated thereon, comprising:

means receiving the L1 and L2 signals for generating a single replica of the known P-code contained therein,

a delay line having a plurality of taps,

means for applying the single P-code replica to said delay line in a manner that the P-code replica is available at the plurality of delay line taps with different relative phases thereof, and

means for obtaining the single generated P-code replica from two different taps of the delay line, thereby to make the P-code replica available at two different relative phases.

11. A system for processing L1 and L2 signals received from a satellite that include a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising:

means for generating replicas of the known pseudo-random P-code modulated on the received L1 and L2 signals,

means receiving the L1 and L2 signals for demodulating the L1 and L2 signals with the P-code replicas provided by the P-code replica generating means, thereby producing demodulated L1 and L2 signals,

means receiving the demodulated L1 and L2 signals for separately integrating them over time periods equal in length to a plurality of P-code cycles, thereby producing integrated demodulated L1 and L2 signals,

means receiving the integrated demodulated L1 and L2 signals for combining them with each other, thereby generating at least one combined signal, and

means communicating with said P-code replica generating means for adjusting the phases of the P-code replicas relative to the incoming L1 and L2 signals until a certain characteristic of said at least one combined signal is obtained that corresponds to a condition when the P-code replicas are substantially aligned in phase with the P-code modulated on the respective received L1 and L2 signals,

whereby useful phases of the P-code replicas are obtained.

12. The system of claim 11 wherein the integrating means includes means for integrating the demodulated L1 and L2 signals over a time period equal to approximately twenty P-code cycles.

13. The system of claim 11 wherein the integrating means includes means for integrating the demodulated L1 and L2 signals over a time period substantially equal to a period of the unknown modulation code.

14. The system of claim 11 wherein said means for generating P-code replicas includes:

means for generating a single pseudo-random P-code signal, and

means receiving said single P-code signal for generating said P-code replicas with individually adjustable relative phases.

15. A method of processing L1 and L2 signals received from at least one satellite of a global positioning system wherein each of said signals includes a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising the steps of:

demodulating each of the received L1 and L2 signals with locally generated replicas of the known P-code that is contained therein,

combining a result of the demodulation step from each of the L1 and L2 signal paths with the demodulated signal of the other of the L1 and L2 signal paths, and

adjusting the phases of the locally generated P-code replicas relative to the incoming L1 and L2 signals until a given characteristic of the combined L1 and L2 signals is obtained that indicates the locally generated P-code signals are substantially aligned in phase with the P-code components of the received L1 and L2 signals, whereby the resulting locally generated P-code phases are useable to determine information of the location of the receiving position.

16. The method of claim 15 wherein the phase adjusting step includes maximizing power of the combined L1 and L2 signals as said given characteristic.

17. The method of claim 15 wherein:

the demodulating step additionally includes the step of demodulating each of the received L1 and L2 signals with locally generated replicas of their respective carrier signals, and

the phase adjusting step additionally includes the step of adjusting the phases of the locally generated carrier signal replicas until the desired characteristic of the combined L1 and L2 signals is obtained, whereby the resulting locally generated carrier replicas are useable to determine information of the location of the receiving position.

18. The method of claim 15 wherein

the demodulating step further includes the step of locally generating the P-code replicas by use of an early/late phase circuit, and

the phase adjusting step further includes minimizing power of a correlation of signals from said early/late circuit as said given characteristic.

19. The method of claim 16 wherein the demodulating step includes the step of locally generating from a single P-code generator separate phase adjusted replicas of the P-code contained in each of the received L1 and L2 signals.

20. The method of claim 15 wherein the received L1 signal additionally contains a known C/A-code modulated on its L1 carrier in phase quadrature with the P-code modulated thereon, the phase adjusting step including the initial steps of locally generating a replica of the C/A-code that is modulated on the received L1 signal and adjusting the relative phase of the locally generated C/A-code replica until it is locked in phase with that on the received L1 signal.

21. A method of processing L1 and L2 signals received from at least one satellite of a global positioning system wherein each of said signals includes a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising the steps of:

demodulating each of the received L1 and L2 signals with a locally generated replica of the known P-code that is contained therein,

repetitively and separately integrating the demodulated L1 and L2 signals over time periods equal in length to a plurality of P-code cycles,

combining a result of the integration step from each of the L1 and L2 signal paths with the integrated signal of the other of the L1 and L2 signal paths, and

adjusting the phases of the locally generated P-code replicas relative to the incoming L1 and L2 signals until a given characteristic of the combined L1 and L2 signals is obtained that indicates the locally generated P-code signals are substantially aligned in phase with the P-code components of the received L1 and L2 signals, whereby the resulting locally generated P-code phases are useable to determine information of the location of the receiving position.

22. The method of claim 21 wherein the phase adjusting step includes maximizing power of the combined L1 and L2 signals as said given characteristic.

23. The method of claim 21 wherein:

the demodulating step additionally includes the step of demodulating each of the received L1 and L2 signals with locally generated replicas of their respective carrier signals, and

the phase adjusting step additionally includes the step of adjusting the phases of the locally generated carrier signal replicas until the desired characteristic of the combined L1 and L2 signals is obtained, whereby the resulting locally generated carrier replicas are useable to determine information of the location of the receiving position.

24. The method of claim 21 wherein

the demodulating step further includes the step of locally generating the P-code replicas by use of an early/late phase circuit, and

the phase adjusting step further includes minimizing power of a correlation of signals from said early/late circuit as said given characteristic.

25. The method of claim 21 wherein the demodulating step includes the step of locally generating from a single P-code generator separate phase adjusted replicas of the P-code contained in each of the received L1 and L2 signals.

26. The method of claim 21 wherein the received L1 signal additionally contains a known C/A-code modulated on its L1 carrier in phase quadrature with the P-code modulated thereon, the phase adjusting step including the initial steps of locally generating a replica of the C/A-code that is modulated on the received L1 signal and adjusting the relative phase of the locally generated C/A-code replica until it is locked in phase with that on the received L1 signal.

27. The method of claim 21 wherein the integrating step includes performing such integration over time periods each equal to approximately twenty P-code cycles.

28. The method of claim 21 wherein the integration step includes performing such integration over time periods substantially equal to that of the unknown modulation code.

29. The method of claim 21 wherein the integration step includes performing the integration over time periods locked in phase with said P-code.

30. A method of processing L1 and L2 signals received from at least one satellite of a global positioning system wherein each of said signals includes a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising the steps of:

locally generating replicas of the known P-codes included in the L1 and L2 signals,

demodulating the received L1 signal with the locally generated L1 signal P-code replica, thereby obtaining a demodulated L1 signal,

combining together the L2 signal, the locally generated L2 signal P-code replica and the demodulated L1 signal, thereby obtaining a combined signal, and

adjusting the phase of the locally generated L2 and P-code replica relative to the incoming L2 signal until a given characteristic of the combined signal is obtained that indicates the locally generated L2 signal P-code replica is substantially aligned in phase with the P-code component of the received L2 signal, whereby the phase of the L2 signal P-code component is determined from the phase of the locally generated L2 signal P-code replica even though the unknown code is also modulated onto the L2 signal carrier.

31. The method of claim 30 wherein the combining step includes the step of repetitively integrating the demodulated L1 signal over time periods equal in length to a plurality of P-code cycles.

32. The method of claim 31 wherein the integrating step includes performing such integration over time periods each equal to approximately twenty L1 signal P-code cycles.

33. The method of claim 30 wherein the phase adjusting step includes maximizing the power of the combined signal as said given characteristic.

34. The method of claim 30 wherein

the demodulating step further includes the step of locally generating the L1 signal P-code replica by use of an early/late phase circuit, and

the phase adjusting step further includes minimizing power of a correlation of signals from said early/late circuit as said given characteristic.

35. The method of claim 30 wherein the received L1 signal additionally contains a known C/A-code modulated on its L1 carrier in phase quadrature with the P-code modulated thereon, the phase adjusting step including the initial steps of locally generating a replica of the C/A-code that is modulated on the received L1 signal and adjusting the relative phase of the locally generated C/A-code replica until it is locked in phase with that on the received L1 signal.

36. The method of claim 30 wherein the combining step includes the steps of first combining together the L2 signal and the locally generated L2 signal P-code replica, thereby obtaining a demodulated L2 signal, and then combining the demodulated L1 and demodulated L2 signals.

37. A method of processing L1 and L2 signals received from at least one satellite of a global positioning system wherein each of said signals includes a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising the steps of:

locally generating replicas of the known P-codes included in the L1 and L2 signals,

demodulating the received L2 signal with the locally generated L2 signal P-code replica, thereby obtaining a demodulated L2 signal,

combining together the L1 signal, the locally generated L1 signal P-code replica and the demodulated L2 signal, thereby obtaining a combined signal, and

adjusting the phase of the locally generated L2 signal P-code replica relative to the incoming L2 signal until a given characteristic of the combined signal is obtained that indicates the locally generated L2 signal P-code replica is substantially aligned in phase with the P-code component of the received L2 signal, whereby the phase of the L2 signal P-code component is determined from the phase of the locally generated L2 signal P-code replica even though the unknown code is also modulated onto the L2 signal carrier.

38. The method of claim 37 wherein the combining step includes the step of repetitively integrating the demodulated L2 signal over time periods equal in length to a plurality of P-code cycles.

39. The method of claim 38 wherein the integrating step includes performing such integration over time periods each equal to approximately twenty L2 signal P-code cycles.

40. The method of claim 37 wherein the phase adjusting step includes maximizing the power of the combined signal as said given characteristic.

41. The method of claim 37 wherein

the demodulating step further includes the step of locally generating the L2 signal P-code replica by use of an early/late phase circuit, and

the phase adjusting step further includes minimizing power of a correlation of signals from said early/late circuit as said given characteristic.

42. The method of claim 37 wherein the received L1 signal additionally contains a known C/A-code modulated on its L1 carrier in phase quadrature with the P-code modulated thereon, the phase adjusting step including the initial steps of locally generating a replica of the C/A-code that is modulated on the received L1 signal and adjusting the relative phase of the locally generated C/A-code replica until it is locked in phase with that on the received L1 signal.

43. The method of claim 37 wherein the combining step includes the steps of first combining together the L1 signal and the locally generated L1 signal P-code replica, thereby obtaining a demodulated L1 signal, and then combining the demodulated L1 and demodulated L2 signals.

44. A method of processing L1 and L2 signals received from at least one satellite of a global positioning system wherein each of said signals includes a unique frequency carrier with a known pseudo-random P-code and an unknown code modulated thereon, comprising the steps of:

deriving from the received L2 signal a signal that represents an estimate of the unknown code, and

combining the estimate signal with the received L1 signal in a manner to determine the relative phase of the L1 signal.