Method of doppler searching in a digital GPS receiver

• US 4,701,934 A
• Filed: 09/03/1985
• Issued: 10/20/1987
• Est. Priority Date: 09/03/1985
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Jasper, Steven C.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Chin, Stephen
• Application Number:
  US06/771,754
• Time in Patent Office: 777 Days
• Field of Search: 375/1, 375/115, 343/5 PN, 343/356, 343/357, 343/386, 343/387, 343/394, 343/402, 343/418, 370/23, 364/576, 364/726
• US Class Current: 375/147
• CPC Class Codes: G01S 11/10 using Doppler effect
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence

Abstract

An improved method of doppler searching in a digital GPS receiver is described. According to the principles of the present invention, an N-point fourier transform is performed on samples of a down converted GPS signal representing the entire range of Doppler-shifted GPS carrier frequencies. The magnitude of the square of each of the output samples is calculated to produce N power quantities, corresponding to N search frequencies. The power quantities for each of the N search frequencies are then summed to produce N quantities proportional to average power. The maximum of the N-power quantities indicates the presence of the corresponding GPS signal.

Claims

1. In a spread-spectrum receiver, for receiving one or more direct-sequence-coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out prior to activation of delay-tracking and carrier-recovery loops, an improved method of Doppler frequency search for each desired signal, and for each code delay search trial, comprising the steps of:

a. inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of the desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequency of said down-converted received signal;

b. calculating the N-point discrete Fourier transform of said block of N samples to produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of the samples input in step a;

c. calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies; and

d. performing steps a through c M times, summing the M power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective N search frequency, the maximum of said average power quantities exhibiting indicia of the presence and approximate Doppler frequency shift of said desired signal.

5. The method of claims 1, 2, 3 or 4, wherein the step of inputting comprises inputting 32 consecutive digitized samples.

6. The method of claim 1, 2, 3 or 4, wherein step (d) in performed 8 times

2. In a spread-spectrum receiver, for receiving one or more direct-sequence-coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out prior to activation of delay-tracking and carrier-recovery loops, an improved method of Doppler frequency search for each desired signal, and for each code delay search trial, comprising the steps of:

a. inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of the desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequencies of said down-converted received signal;

b. calculating the N-point discrete Fourier transform of said block of N samples to produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f₂ /2 to +f_s /2), f_s being the sampling frequency of the samples input in step a;

c. calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies;

d. performing steps a through c M times, summing the M power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective N search frequency;

e. selecting the maximum of the N average power quantities, and calculating the corresponding frequency; and

f. comparing the maximum average power quantity to a threshold, the fact of said threshold being exceeded signifying that the current code delay is approximately correct (local code approximately time-aligned to received code), with the frequency of maximum average power found in step e indicating the approximate Doppler frequency shift, and the fact of said threshold not being exceeded signifying that the current code delay is not approximately correct, and that another code delay should be tried.

3. In a global positioning system receiver, for receiving one or more course/access (C/A) coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out prior to activation of delay-tracking and carrier-recovery loops, an improved method of Doppler frequency search for each desired signal, and for each code delay search trial, comprising the steps of:

a. inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of the desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequencies of said down-converted received signal;

b. calculating the N-point discrete Fourier transform of said block of N samples to produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of the samples input in step a;

c. calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies; and

d. performing steps a through c M times, summing the M power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective search frequency, the maximum of said average power quantities exhibiting indicia of the presence and approximate Doppler frequency shift of said desired signal.

4. In a global positioning system receiver, for receiving one or more course/access (C/A) coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out to activation of delay-tracking and carrier-recovery loops, an improved method of Doppler frequency search for each desired signal, and for each code delay search trial, comprising the steps of:

a. inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of the desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequencies of said down-converted received signal;

b. calculating the N-point discrete Fourier transform of said block of N samples of produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of the samples input in step a;

c. calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies;

d. performing steps a through c M times, summing the M power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective search frequency;

e. selecting the maximum of the N average power quantities, and calculating the corresponding frequency; and

f. comparing the maximum average power quantity to a threshold, the face of said threshold being exceeded signifying that the current code delay is approximately correct (local code approximately time-aligned to received code), with the frequency of maximum average power found in step e indicating the approximate Doppler frequency shift, and the fact of said threshold not being exceeded signifying that the current code delay is not approximately correct, and that another code delay should be tried.

7. A spread-spectrum receiver for receiving one or more direct-sequence-coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out prior to activation of delay-tracking and carrier-recovery loops, said receiver comprising:

a. means for inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of a desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequency of said down-converted received signal;

b. means for calculating the N-point discrete Fourier transform of said block of N samples to produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of said N samples;

c. means for calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies; and

d. means for summing, M times, the power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective N search frequency, the maximum of said average power quantities exhibiting indicia of the presence and approximate Doppler frequency shift of said desired signal.

8. A spread-spectrum receiver, for receiving one or more direct-sequence-coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out prior to activation of delay-tracking and carrier-recovery loops, said receiver comprising:

a. means for inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of a desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequencies of said down-converted received signal;

b. means for calculating the N-point discrete Fourier transform of said block of N samples to produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of said N samples;

c. means for calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies;

d. means for summing, M times, the power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective N search frequency;

e. means for selecting the maximum of the N average power quantities, and calculating the corresponding frequency; and

f. means for comparing the maximum average power quantity to a threshold, the fact of said threshold being exceeded signifying that the current code delay is approximately correct (local code approximately time-aligned to received code), with the frequency of maximum average power indicating the approximate Doppler frequency shift, and the fact of said threshold not being exceeded signifying that the current code delay is not approximately correct, and that another code delay should be tried.

9. A global positioning system receiver, for receiving one or more course/access (C/A) coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out prior to activation of delay-tracking and carrier-recovery loops, said receiver comprising:

a. means for inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of a desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequencies of said down-converted received signal;

b. means for calculating the N-point discrete Fourier transform of said block of N samples to produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of said N samples;

c. means for calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies; and

d. means for summing M times, the power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective search frequency, the maximum of said average power quantities exhibiting indicia of the presence and approximate Doppler frequency shift of said desired signal.

10. A global positioning system receiver, for receiving one or more course/access (C/A) coded signals, each of which may be Doppler shifted from its nominal carrier frequency, wherein for each signal a code delay/Doppler frequency search procedure must be carried out to activation of delay-tracking and carrier-recovery loops, said receiver comprising:

a. means for inputting N consecutive digitized samples of a signal equal to a filtered product of a down-converted received signal multiplied by a locally-generated code of a desired signal, the code delay of said locally-generated code being that of the current code delay search trial, said filtering passing, at least, the band of frequencies corresponding to the entire range of possible Doppler-shifted carrier frequencies of said down-converted received signal;

b. means for calculating the N-point discrete Fourier transform of said block of N samples of produce N complex-valued transformed output samples, each corresponding to one of N equally-spaced search frequencies in the band from zero Hertz to fs (or, equivalently, in the band from -f_s /2 to +f_s /2), f_s being the sampling frequency of said N samples;

c. means for calculating the square of the magnitude of each of said N complex-valued output samples to produce N power quantities, corresponding to the N said search frequencies;

d. means for summing, M times, the power quantities produced for each of the N said search frequencies, to produce N quantities proportional to the average power of each respective search frequency;

e. means for selecting the maximum of the N average power quantities, and calculating the corresponding frequency; and

f. means for comparing the maximum average power quantity to a threshold, the face of said threshold being exceeded signifying that the current code delay is approximately correct (local code approximately time-aligned to received code), with the frequency of maximum average power found in step e indicating the approximate Doppler frequency shift, and the fact of