OSCILLATOR FREQUENCY CORRECTION IN GPS SIGNAL ACQUISITION

• US 20030201935A1
• Filed: 04/30/2002
• Published: 10/30/2003
• Est. Priority Date: 04/30/2002
• Status: Active Grant

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: King, Thomas M., Geier, George J., Kiaei, Sayfe
• Granted Patent: US 6,661,371 B2
• Time in Patent Office: Days
• US Class Current: 342/357.15
• CPC Class Codes: G01S 19/256 relating to timing, e.g. time of week, code phase, timing offset
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence

Abstract

A method (50) for correcting oscillator frequency error in GPS signal acquisition includes a first step (52) of receiving at least one broadcasted GPS signal. A next step (54) includes estimating a plurality of Doppler offsets. A next step (56) includes correlating the signal to produce a Doppler modulation for each of the plurality of Doppler estimates. A next step (58) includes calculating a magnitude separation of peak and null states of each modulation. A next step (60) includes searching the peak and null separation to finding the Doppler estimate producing the largest magnitude separation in peak and null states. The found Doppler estimate defines a frequency error estimate for the at least one broadcasted GPS signal, which will be common across all satellites.

Claims

1. A method for correcting oscillator frequency error in GPS signal acquisition, comprising the steps of:

receiving at least one broadcasted GPS signal;

estimating a plurality of Doppler offsets;

correlating the signal to produce a plurality of correlation results for each of the plurality of Doppler estimates;

calculating a magnitude separation of peak and null states of corresponding pairs of correlation results; and

searching the peak and null separation to find the Doppler estimate producing the largest magnitude separation in peak and null states, the found Doppler estimate defining a frequency error estimate for the at least one broadcasted GPS signal.

2. The method of claim 1, further comprising the step of constructing a code phase estimate from a signal envelope accumulation.

3. The method of claim 1, further comprising the step of constructing a code phase estimate from a largest and next largest signal envelope accumulations.

4. The method of claim 3, wherein constructing the code phase estimate is defined by CP_est=w₁CP_est1+w₂CP_est2, where CP_est1 is the code phase estimate corresponding to the largest accumulated signal envelope, CP_est2 is the code phase estimate corresponding to the next largest accumulated signal envelope, w₁=E₁/E_sum, and w₂=E₂/E_sum, where E₁ is the largest accumulated signal envelope, and E₂ is the next largest accumulated signal envelope, with E_sum=E₁+E₂.

5. The method of claim 1, further comprising the step of constructing a code phase estimate from at least one signal envelope accumulation using a narrower spacing of code phase for the at least one signal envelope accumulation than is used for the initial acquisition of the broadcasted GPS signal in the receiving step.

6. The method of claim 1, further comprising the step of constructing a code phase estimate from an autocorrelation function curve fit.

7. The method of claim 1, wherein the correlating step provides correlation magnitudes that follow a sin(x)/x curve, and wherein in the calculating step the separation is estimated by a magnitude difference between the maximum and minimum correlations corresponding to the peak and null states of the curve.

8. The method of claim 7, wherein the searching step includes searching across pairs of accumulated signal envelope detections of Doppler estimates (separated by 1/T) until the maximum difference is found.

9. The method of claim 7, wherein the correlating step provides curve fitting the correlation magnitudes to a sin(x)/x curve using at least two correlation magnitudes.

10. The method of claim 1, further comprising the step of applying the frequency error estimate to an oscillator frequency of a GPS device to correct for the oscillator frequency error.

11. The method of claim 1, wherein the receiving step includes receiving a plurality of broadcasted GPS signals, and the correlating step includes determining an average of the Doppler error estimates of each signal to provide a Doppler error modulation.

12. The method of claim 11, wherein the average in the correlating step is a weighted-least-squares estimation.

13. The method of claim 1, wherein the receiving step includes receiving a plurality of broadcasted GPS signals, and the correlating step includes determining an average of a Doppler error modulation.

14. The method of claim 13, wherein the confidence level in the searching step is determined from the closeness in the peak and null levels of the largest and next largest separations in correlations, and wherein if the confidence level is below a threshold further comprising a step of deciding between abandoning the Doppler estimation and continuing integration until the confidence level goes above the threshold.

15. A method for correcting oscillator frequency error in GPS signal acquisition, comprising the steps of:

receiving at least one broadcasted GPS signal;

constructing a code phase estimate from a signal envelope accumulation;

estimating a plurality of Doppler offsets and centering the plurality of Doppler estimates using the code phase estimate;

correlating the signal to produce correlation magnitudes that follow a sin(x)/x curve of a Doppler modulation for each of the plurality of Doppler estimates;

calculating a magnitude separation between the maximum and minimum correlations corresponding to the peak and null states of the curve; and

searching the separations to finding the Doppler estimate producing the largest magnitude separation, the found Doppler estimate defining a frequency error estimate for the at least one broadcasted GPS signal.

16. The method of claim 15, further comprising the step of constructing a code phase estimate from a largest and next largest signal envelope accumulations, and wherein constructing the code phase estimate is defined by CP_est=w₁CP_est1+w₂CP_est2, where CP_est1 is the code phase estimate corresponding to the largest accumulated signal envelope, CP_est2 is the code phase estimate corresponding to the next largest accumulated signal envelope, w₁=E₁/E_sum, and w₂=E₂/E_sum, where E₁ is the largest accumulated signal envelope, and E₂ is the next largest accumulated signal envelope, with E_sum=E₁+E₂.

17. The method of claim 15, wherein the searching step includes searching across pairs of accumulated signal envelope detections of Doppler estimates (separated by 1/T) until the maximum difference is found.

18. The method of claim 15, wherein the correlating step provides curve fitting the correlation magnitudes to a sin(x)/x curve with an autocorrelation function using at least two correlation magnitudes.

19. The method of claim 15, further comprising the step of applying the frequency error estimate to an oscillator frequency of a GPS device to correct for the oscillator frequency error.

20. A method for correcting oscillator frequency error in GPS signal acquisition, comprising the steps of:

receiving at least one broadcasted GPS signal;

constructing a code phase estimate from a signal envelope accumulation;

estimating a plurality of Doppler offsets and centering the plurality of Doppler estimates using the code phase estimate;

correlating the signal to produce correlation magnitudes of a Doppler error modulation for each of the plurality of Doppler estimates;

curve fitting the Doppler error modulation to a sin(Tƒ

/2)/(Tƒ

/2) curve, where T is the coherent integration interval, and ƒ

is the frequency error, using at least two correlation magnitudes; and

using the shift in Doppler value of the curve fit to estimate a Doppler error defining a frequency error for the at least one broadcasted GPS signal.