Indoor GPS clock

• US 6,725,157 B1
• Filed: 06/10/2002
• Issued: 04/20/2004
• Est. Priority Date: 06/10/2002
• Status: Active Grant

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Yu, Yiming
• Primary Examiner(s): Nguyen, Tan Q.
• Assistant Examiner(s): Tran, Dalena
• Application Number:
  US10/167,101
• Time in Patent Office: 680 Days
• Field of Search: 701/213, 701/214, 701/215, 342/357.12, 342/357.15, 342/357.05, 342/357.06, 342/357.09, 342/357.1, 342/352, 375/357, 375/376, 375/152, 375/343, 455/456.1, 455/13.2
• US Class Current: 701/213
• CPC Class Codes: G01S 19/246 involving long acquisition integration times, extended snapshots of signals or methods specifically directed towards weak signal acquisition
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence

Abstract

An indoor GPS clock using GPS signals lower that −143 dBm for issuing disciplined frequency and time standard signals. The indoor GPS clock includes a correlation machine using long integration periods for enabling the indoor GPS clock to operate with low signal levels; a carrier-less tracking loop for tracking the low level signals without carrier offset feedback, a clock bias loop for providing clock bias feedback; and a reference oscillator using the clock bias feedback for providing disciplined frequency and time signals having greater accuracy than is available in conventional GPS positioning receivers. The indoor GPS clock also includes a holdover driver providing compensation for predicted drift in clock bias error for disciplining the reference oscillator for several hours when the GPS signal is no longer being received.

Claims

1. A method using a GPS signal for providing a frequency standard signal, comprising:

acquiring said GPS signal using coherent acquisition integration periods having multiple GPS code epochs, said acquisition integration periods including at least forty said code epochs, wherein the step of acquiring includes;

determining first and second correlation levels for first and second integration periods, respectively;

stripping GPS data bit information from said first and second correlation levels for providing first and second data stripped correlation levels, respectively;

linearly accumulating said first and second data stripped levels for providing an extended correlation acquisition level; and

using said extended correlation level for acquiring said GPS signal;

tracking said GPS signal, after said GPS signal has been acquired, using coherent tracking integration periods having multiple said GPS code epochs; and

disciplining said frequency standard signal using said coherent tracking integration periods to track GPS frequency.

2. The method of claim 1, wherein:

the step of acquiring said GPS signal includes multiplying samples representative of said GPS signal by samples of several phases of a pseudorandom (PRN) code for providing acquisition correlation data.

3. The method of claim 1, wherein:

the step of acquiring said GPS signal includes fast Fourier transforming samples representative of said GPS signal for providing frequency domain samples representative of said GPS signal;

fast Fourier transforming samples of several phases of a pseudorandom (PRN) code for providing frequency domain samples for said PRN code;

multiplying said frequency domain samples for said GPS signal by said frequency domain samples for said PRN code for providing products; and

inverse fast Fourier transforming said products for providing acquisition correlation data.

23. The GPS clock of claim 2, wherein:

the correlation machine and the signal processor combine for providing a processing gain of at least 16 dB with respect to a processing gain for a one millisecond integration period.

24. The GPS clock of claim 1, wherein:

the correlation machine and the signal processor combine for providing a processing gain of at least 16 dB with respect to a processing gain for a one millisecond integration period.

4. A method using a GPS signal for providing a frequency standard signal, comprising:

acquiring said GPS signal using coherent acquisition integration periods having multiple GPS code epochs, wherein the step of acquiring includes;

determining first and second correlation levels for first and second integration periods, respectively;

squaring said first and second correlation levels for providing first and second squared correlations, respectively;

summing said first and second squared correlations for providing an incoherent correlation sum; and

using said incoherent correlation sum for acquiring said GPS signal;

tracking said GPS signal, after said GPS signal has been acquired, using coherent tracking integration periods having multiple said GPS code epochs; and

disciplining said frequency standard signal using said coherent tracking integration periods to track GPS frequency.

5. The method of claim 4, wherein:

said GPS signal is less than or equal to about −

146 dBm.

6. The method of claim 4, wherein:

the step of acquiring said GPS signal includes using said coherent acquisition integrations periods for a processing gain of at least 16 dB with respect to a processing gain for a one millisecond integration period.

7. The method of claim 4, wherein:

said tracking integration periods are at least ten said code epochs.

8. The method of claim 4, wherein:

said tracking integration periods are about twenty said code epochs.

9. A method using a GPS signal for providing a frequency standard signal, comprising:

acquiring said GPS signal using coherent acquisition integration periods having multiple GPS code epochs;

generating local mixing samples having a frequency having no feedback correction for an actual carrier frequency of said GPS signal;

adjusting said local mixing sample frequency according to said frequency standard signal and continuing computations of estimated Doppler shifts of said actual carrier frequency;

multiplying samples of said GPS signal by said local mixing samples for forming intermediate signal samples;

said intermediate signal samples used for carrier-less tracking of said GPS signal after said GPS signal has been acquired; and

disciplining said frequency standard signal using said carrier-less tracking to track said carrier frequency of said GPS signal.

10. The method of claim 9, wherein:

said acquisition integration periods include at least ten said code epochs.

11. The method of claim 9, wherein:

said acquisition integration periods are about twenty said code epochs.

13. The method of claim 9, wherein:

the step of disciplining said frequency standard signal includes using said GPS signal for determining a local clock bias;

filtering said clock bias for providing a conditioned clock bias;

using said conditioned clock bias for controlling a frequency of said frequency standard signal to track said carrier frequency of said GPS signal.

14. The method of claim 13, wherein:

said conditioned clock bias is filtered for a time constant between one hundred fifty seconds and two hundred fifty seconds.

15. The method of claim 9, further comprising:

predicting frequency drift of said frequency standard signal; and

while not tracking said GPS signal, controlling a frequency of said frequency standard signal for compensating for said predicted frequency drift.

16. The method of claim 15, wherein:

predicting said frequency drift includes learning aging for said frequency standard signal while said GPS signal is being tracked and correcting for said aging when said GPS signal is not being tracked.

17. The method of claim 9, wherein:

the step of disciplining includes disciplining the time-of-transmission of a 1 pulse per second (PPS) signal to track GPS time.

12. The method of claim wherein:

said acquisition integration periods include at least forty said code epochs.

18. A GPS clock for providing a frequency standard signal, comprising:

a long integration correlation machine for acquiring a GPS signal using coherent acquisition integration periods having multiple GPS code epochs, said coherent acquisition integration periods including at least forty GPS code epochs, wherein the correlation machine determines first and second correlation levels for first and second integration periods, respectively; and

comprises a signal processor including a data stripper for stripping GPS data bit information from said first and second correlation levels for providing first and second data stripped correlation levels, respectively; and

a summer for linearly accumulating said first and second data stripped correlation levels for providing an extended correlation level, said extended correlation level used for acquiring said GPS signal;

a signal tracking loop including the correlation machine for tracking said GPS signal, after said GPS signal has been acquired, using coherent tracking integration periods having multiple said GPS code epochs; and

a clock bias loop using said signal tracking loop for disciplining said frequency standard signal to track GPS frequency.

19. The GPS clock of claim 18, wherein:

the correlation machine includes multipliers for multiplying samples representative of said GPS signal by samples of several phases of a pseudorandom (PRN) code for providing acquisition correlation data.

20. The GPS clock of claim 18, wherein:

the correlation machine includes a fast Fourier transform (FFT) converter for transforming samples representative of said GPS signal for providing frequency domain samples representative of said GPS signal;

an FFT converter for transforming samples of several phases of a pseudorandom (PRN) code for providing frequency domain samples for said PRN code;

multipliers for multiplying said frequency domain samples for said GPS signal by said frequency domain samples for said PRN code for providing products; and

an inverse fast Fourier transform (IFFT) converter for inverse transforming said products for providing acquisition correlation data.

21. A GPS clock for providing a frequency standard signal, comprising:

a long integration correlation machine for acquiring a GPS signal using coherent acquisition integration periods having multiple GPS code epochs, wherein the correlation machine determines first and second correlation levels for first and second integration periods, respectively; and

comprises a signal processor including a squarer for squaring said first and second correlations for providing first and second squared correlations, respectively; and

an incoherent summer for summing said first and second squared correlations for providing an incoherent correlation sum, said incoherent correlation sum used for acquiring said GPS signal;

a signal tracking loop including the correlation machine for tracking said GPS signal, after said GPS signal has been acquired, using coherent tracking integration periods having multiple said GPS code epochs; and

a clock bias loop including said signal tracking loop for disciplining said frequency standard signal to track GPS frequency.

22. The GPS clock of claim 21, wherein:

said GPS signal is less than or equal to about −

146 dBm.

25. The GPS clock of claim 21, wherein:

said tracking integration periods are at least ten said code epochs.

26. The GPS clock of claim 21, wherein:

said tracking integration periods are about twenty said code epochs.

27. A GPS clock for providing a frequency standard signal, comprising:

a long integration correlation machine using coherent acquisition integration periods having multiple GPS code epochs for acquiring a GPS signal;

a carrier-less tracking loop including a carrier numerically controlled oscillator (NCO) for generating local mixing samples having a frequency having no feedback correction for an actual carrier frequency of said GPS signal by adjusting said local mixing sample frequency according to said frequency standard signal and continuing computations of estimated Doppler shifts of said actual carrier frequency;

wherein the correlation machine multiplies samples of said GPS signal by said local mixing samples for forming intermediate signal samples, said intermediate signal samples used for tracking of said GPS signal after said GPS signal has been acquired; and

a clock bias loop using said carrier-less tracking loop for disciplining said frequency standard signal to track said actual carrier frequency of said GPS signal.

28. The GPS clock of claim 27, wherein:

said acquisition integration periods include at least ten said code epochs.

29. The GPS clock of claim 27, wherein:

said acquisition integration periods are about twenty said code epochs.

30. The GPS clock of claim 27, wherein:

said acquisition integrated periods include at least forty said code epochs.

31. The GPS clock of claim 27, wherein:

the clock bias loop includes a navigation processor using said GPS signal for determining a local clock bias, a clock bias filter for filtering said clock bias for providing a conditioned clock bias; and

a disciplined reference oscillator using said conditioned clock bias for controlling a frequency of said GPS frequency standard signal to track said carrier frequency of said GPS signal.

32. The GPS clock of claim 31, wherein:

the clock bias filter filters a representation of said clock bias with a time constant between one hundred fifty seconds and two hundred fifty seconds.

33. The GPS clock of claim 27, further comprising:

a disciplined reference oscillator for generating a time base for providing said frequency standard signal; and

a holdover driver for controlling said reference oscillator for compensating for predicted drift in said frequency standard signal when said GPS signal is not being tracked.

34. The GPS clock of claim 33, wherein:

the holdover driver learns aging of said reference oscillator while said GPS signal is being tracked and correcting for said aging when said GPS signal is not being tracked.

35. The GPS clock of claim 27, wherein:

the clock bias loop disciplines the time-of-transmission of a 1 pulse per second (PPS) signal to track GPS time.