Method and device for reception processing of a GPS satellite L2 signal

• US 6,317,078 B1
• Filed: 05/25/2000
• Issued: 11/13/2001
• Est. Priority Date: 09/25/1998
• Status: Expired due to Fees

• Current Assignee: Thomson-Csf Sextant
• Original Assignee: Thomson-Csf Sextant
• Inventors: Revol, Marc, Renard, Alain
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US09/554,600
• Time in Patent Office: 537 Days
• Field of Search: 342/357.05, 342/357.12, 701/213
• US Class Current: 342/357.23
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/32 Multimode operation in a single same satellite system, e.g. GPS L1/L2
• CPC Class Codes: G01S 19/40 Correcting position, velocity or attitude

Abstract

A method for position determination with the aid of GPS satellites, including reception processing of a GPS satellite signal L2 modulated by an encrypted code Y having a key not available from the GPS satellite, so as to assess a delay of the signal L2 with respect to the signal L1 and to deduce therefrom the magnitude of the ionospheric effect so as to take account thereof and improve the accuracy of the location finding. This processing includes reducing the signal L2 to a lower band and in despreading the signal L2 by means of the non-encrypted code P produced locally in reception by a local generator of a non-encrypted spreading code P synchronized with the code Y (encrypted P) transmitted, with the aid of a phase lock loop with three parallel paths: a first path demodulated by a precise version of the locally produced non-encrypted spreading code P, a second advanced path demodulated a first time by an advanced version of the locally produced non-encrypted spreading code P and a second time by the demodulated signal of the precise path, and a third delayed path demodulated a first time by a delayed version of the locally produced non-encrypted spreading code P and a second time by the demodulated signal of the precise path, synchronization being achieved by searching for an equality of power of the carrier of the signal L2 at the output of the advanced and delayed paths.

Claims

1. Method for deducing a pseudodistance to a GPS satellite by reception processing of a GPS satellite signal L2 modulated by an encrypted spreading code Y encrypted by an encryption signal W not available from the GPS satellite, comprising:

estimating a Doppler effect affecting the GPS satellite signal L2;

transposing the GPS satellite signal L2 into a lower frequency band by utilizing at least one fixed carrier signal produced locally and translated by an expected Doppler shift of the GPS satellite signal L2;

demodulating in parallel on three paths the transposed GPS satellite signal L2 with advanced, precise, and delayed versions of a non-encrypted spreading code P produced by an adjustable-phase spreading code local generator, thereby producing respective advanced, precise, and delayed demodulated signals;

filtering the respective demodulated signals with a filter matched to a phase and to a frequency of the encryption signal W;

re-demodulating the advanced and delayed demodulated signals with the precise demodulated signal so as to delete biphase modulation and to produce two doubly demodulated signals;

calculating respective powers of the two doubly demodulated signals;

locking a phase of the adjustable-phase spreading code local generator when the two doubly demodulated signals are of equal powers; and

deducing a pseudodistance to the GPS satellite from a difference in said phase of the adjustable-phase spreading code local generator when said generator is locked and a phase of the GPS reception signal L1.

2. Method according to claim 1, wherein the step of estimating of a Doppler effect affecting the GPS satellite signal L2 comprises:

extracting from the GPS satellite signal L1, modulated by a known coarse acquisition C/A spreading code, a Doppler shift affecting the carrier of the signal L1; and

applying a proportionality ratio of 120/154th to the Doppler shift affecting the carrier of the signal L1.

3. Method according to claim 1, wherein the step of transposing the signal L2 into a lower frequency band is carried out in an intermediate frequency band.

4. Method according to claim 1, wherein the step of transposing the signal L2 into a lower frequency band is carried out in a baseband with, as final step, a quadrature demodulator delivering two baseband components, one in-phase and another in quadrature.

5. Method according to claim 1, wherein the step of filtering utilizes filters limited to passbands around 500 kHz.

6. A device for reception processing of a GPS satellite signal L2 modulated by an encrypted spreading code P encrypted by an encryption signal W not available from the GPS satellite, comprising:

an estimator configured to estimate the Doppler effect affecting the GPS satellite signal L2 received from a GPS satellite;

a transposing circuit configured to transpose the GPS satellite signal L2 into a lower frequency band by utilizing a fixed carrier signal produced locally and translated by an expected Doppler shift of the signal L2;

a first demodulator configured to demodulate, in parallel on three paths, the transposed GPS satellite signal L2 with advanced A, precise P and delayed R versions of a non-encrypted spreading code P produced by an adjustable-phase spreading code local generator, thereby producing respective advanced, precise, and delayed demodulated signals;

a bandpass filter configured to filter the respective demodulated signals with a filter matched to a phase and to a frequency of the encryption signal W;

a second demodulator configured to demodulate the advanced and delayed demodulated signals with the precise demodulated signal so as to delete biphase modulation and produce two doubly demodulated signals;

a calculator configured to calculate respective powers of the two doubly demodulated signals; and

a phase locker configured to lock a phase of the adjustable-phase spreading code local generator when the two redemodulated signals are of equal powers.

7. Device according to claim 6, wherein said estimator comprises:

a circuit configured to process the GPS satellite signal L1, modulated by a known coarse acquisition C/A spreading code and to deliver a value of a Doppler shift affecting a carrier of the signal L1; and

a multiplier circuit configured to apply a proportionality ratio of 120/154th to the Doppler shift affecting the carrier of the signal L1.

8. Device according to claim 6, wherein the transposing circuit transposes the signal L2 into an intermediate frequency band.

9. Device according to claim 6, wherein the transposing circuit transposes the signal L2 into a baseband.

10. Device according to claim 6, wherein the bandpass filter is an integrate and dump filter and is synchronized by the adjustable-phase spreading code local generator.

11. Device according to claim 6, wherein the bandpass filter is limited to passbands around 500 kHz.