P-code enhanced method for processing encrypted GPS signals without knowledge of the encryption code

• US 6,061,390 A
• Filed: 09/02/1994
• Issued: 05/09/2000
• Est. Priority Date: 09/02/1994
• Status: Expired due to Term

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Meehan, Thomas K., Thomas, Jess Brooks Jr., Young, Lawrence E.
• Primary Examiner(s): Laufer, Pinchus M.
• Assistant Examiner(s): SAYADIAN, HRAYR
• Application Number:
  US08/299,946
• Time in Patent Office: 2,076 Days
• Field of Search: 380/37, 380/255, 342/189, 342/194, 342/352, 342/357, 342/357.12, 701/213, 375/206, 375/208-210
• US Class Current: 375/150
• CPC Class Codes: G01S 19/30 code 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: H04L 27/2332 using a non-coherent carrier

Abstract

In the preferred embodiment, an encrypted GPS signal is down-converted from RF to baseband to generate two quadrature components for each RF signal (L1 and L2). Separately and independently for each RF signal and each quadrature component, the four down-converted signals are counter-rotated with a respective model phase, correlated with a respective model P code, and then successively summed and dumped over presum intervals substantially coincident with chips of the respective encryption code. Without knowledge of the encryption-code signs, the effect of encryption-code sign flips is then substantially reduced by selected combinations of the resulting presums between associated quadrature components for each RF signal, separately and independently for the L1 and L2 signals. The resulting combined presums are then summed and dumped over longer intervals and further processed to extract amplitude, phase and delay for each RF signal. Precision of the resulting phase and delay values is approximately four times better than that obtained from straight cross-correlation of L1 and L2. This improved method provides the following options: separate and independent tracking of the L1-Y and L2-Y channels; separate and independent measurement of amplitude, phase and delay L1-Y channel; and removal of the half-cycle ambiguity in L1-Y and L2-Y carrier phase.

Claims

1. A receiver for processing a received signal transmitted as an L1 or an L2 signal comprised of a single carrier modulated by a known code and by an unknown code wherein said unknown code causes adverse effects, said receiver comprising:

means for converting said L1 or L2 signal to quadrature components;

means for generating a model for said carrier;

means for generating a model for said known code;

means for counter-rotating said quadrature components of said L1 or L2 signal with said model carrier to produce counter-rotated signals;

means for correlating said model code with said counter-rotated signals to produce at least one correlation product separately for each counter-rotated signal;

means for accumulating said correlation products successively over selected time intervals;

means for combining with one another said accumulated correlation products derived from said quadrature components of said L1 or L2 signal in a manner reducing said adverse effects of said unknown code; and

,

means for generating an amplitude factor whereby said combined accumulated correlation products and amplitude factor are used to extract said signal'"'"'s amplitude, phase, and delay, in the presence of said unknown code.

2. The receiver recited in claim 1 wherein said measured signal phase is susceptible to phase ambiguity, and wherein the signal additionally contains a second signal, denoted as the L1-C/A signal, the receiver further comprising:

means for measuring the phase of said L1-C/A signal and

means responsive to said L1-C/A phase for avoiding said ambiguity in said measured phase.

3. The receiver recited in claim 1 wherein said unknown code consists of a sequence of chips and the correlation-product-accumulating means includes means for accumulating said correlation products separately for each quadrature component successively over time intervals substantially equal in length to chip widths of the unknown code.

4. The receiver recited in claim 3 wherein said time intervals of the correlation-product-accumulating means are substantially coincident with the chips of the unknown code.

5. The receiver recited in claim 1 wherein the correlating means generates for each of the two quadrature components at least four correlation products, denoted as early-I correlation product, prompt-I correlation product, late-I correlation product, and prompt-Q correlation product and wherein the accumulating means generates at least four accumulated correlation products, denoted as accumulated early-I correlation product, accumulated prompt-I correlation product, accumulated late-I correlation product, and accumulated prompt-Q correlation product.

6. The receiver recited in claim 5 wherein the accumulated-correlation-product combining means includes means for generating at least one weighted combination of said at least six accumulated I correlation products from the two said quadrature components and means for combining said at least one combined accumulated I correlation product with at least one of the said at least three accumulated I correlation products from at least one quadrature component.

7. The receiver recited in claim 5 wherein the accumulated-correlation-product-combining means includes means for adding said accumulated prompt-I correlation product from a first said quadrature component to said accumulated prompt-I correlation product from a second said quadrature component and for combining said sum of accumulated prompt-I correlation products with the early-I correlation product and with the late-I correlation product from at least one quadrature component.

8. The receiver recited in claim 5 wherein said accumulated-correlation-product-combining means includes means for generating at least one weighted combination of the said at least six accumulated I correlation products from the two said quadrature components and means for combining said at least one combined accumulated I correlation product with said at least one accumulated Q correlation product from at least one of said quadrature components.

9. The receiver recited in claim 5 wherein said accumulated-correlation-product-combining means includes means for adding the said accumulated prompt I correlation products from the two said quadrature components and means for combining said sum of accumulated prompt I correlation products with at least one said accumulated Q correlation product from at least one of said quadrature components.

10. The receiver recited in claim 5 wherein said accumulated-correlation-product-combining means comprises means for multiplying said accumulated prompt I correlation product from a first said quadrature component times at least one accumulated Q correlation product from a second said quadrature component and means for multiplying said accumulated prompt I correlation product from said second quadrature component times at least one accumulated Q correlation product from said first quadrature component.

11. The receiver recited in claim 5 wherein said accumulated-correlation-product-combining means comprises means for multiplying said accumulated prompt I correlation product from a first said quadrature component times said accumulated early-I correlation product, accumulated prompt-I correlation product and accumulated late-I correlation product from a second said quadrature component and means for multiplying said accumulated prompt I correlation product from said second quadrature component times said accumulated early-I correlation product, accumulated prompt-I correlation product and accumulated late-I correlation product from said first quadrature component.

12. The receiver recited in claim 5 wherein the amplitude factor generating means includes means for multiplying by a factor of four the product of the accumulated prompt-I correlation product from one of said quadrature components times the accumulated prompt-I correlation product from the other of said quadrature components.

13. The receiver recited in claim 1 wherein the correlating means generates for each of the two quadrature components at least three correlation products, denoted as (late-minus-early)-I correlation product, prompt-I correlation product, and prompt-Q correlation product and wherein the accumulating means generates at least three accumulated correlation products, denoted as accumulated (late-minus-early)-I correlation product, accumulated prompt-I correlation product, and accumulated prompt-Q correlation product.

14. The receiver recited in claim 13 wherein said accumulated-correlation-product combining means includes means for adding the said accumulated prompt-I correlation products from the two said quadrature components and means for combining said sum of accumulated prompt-I correlation products with at least one said accumulated Q correlation product from at least one of said quadrature components.

15. The receiver recited in claim 13 wherein the accumulated-correlation-product-combining means includes means for adding said accumulated prompt-I correlation product from a first said quadrature component to said accumulated prompt-I correlation product from a second said quadrature component and for combining said sum of accumulated prompt-I correlation products with the (late-minus-early)-I correlation product from at least one of said quadrature component.

16. The receiver recited in claim 13 wherein the accumulated-correlation-product-combining means comprises means for multiplying said accumulated prompt I correlation product from a first said quadrature component times at least one said accumulated Q correlation product from a second said quadrature component and means for multiplying said accumulated prompt I correlation product from second said quadrature component times at least one said accumulated Q correlation product from first said quadrature component.

17. The receiver recited in claim 13 wherein the accumulated-correlation-product-combining means comprises means for multiplying said accumulated prompt-I correlation product from a first said quadrature component times said accumulated (late-minus-early)-I correlation product and said accumulated prompt-I correlation product from a second said quadrature component and means for multiplying said accumulated prompt-I correlation product from said second quadrature component times said accumulated (late-minus-early)-I correlation product and said accumulated prompt-I correlation product from said first quadrature component.

18. The receiver recited in claim 1 wherein the signal additionally contains a second signal, denoted as the L1-C/A signal, the receiver further comprising:

means for tracking said L1-C/A signal; and

means responsive to said L1-C/A tracking for supplying input to said means for generating said model of said carrier or said known code or said amplitude factor.

19. A receiver for processing a received L1 signal and a received L2 signal, each having a single respective carrier modulated by a respective known code and by respective unknown codes wherein said unknown code causes adverse effects, said receiver comprising:

means for converting each said L1 and L2 signal to respective quadrature components;

means for generating a respective model for each said carrier;

means for generating a respective model for each said known code;

means for counter-rotating said respective quadrature components of said L1 and L2 signal with said respective model carrier to produce respective counter-rotated signals;

means for correlating said respective model code with said respective counter-rotated signals to produce at least one respective correlation product separately for each said respective counter-rotated signal;

means for accumulating said respective correlation products successively over selected respective time intervals;

means for combining with one another separately for the L1 and L2 signals said respective accumulated correlation products derived from said quadrature components of said L1 and L2 signals in a manner reducing said adverse effects of said unknown code; and

,

means for generating an amplitude factor whereby said respective combined accumulated correlation products and amplitude factor are used to extract said signal'"'"'s amplitude, phase, and delay, in the presence of said unknown codes.

20. The receiver recited in claim 19 wherein one of the measured phases is susceptible to phase ambiguity, the receiver further comprising:

means for cross-correlating said two signals to generate cross-correlation products; and

means responsive to said cross-correlation products for avoiding said phase ambiguity.

21. The receiver recited in claim 19 wherein said measured L1-P phase is susceptible to phase ambiguity, and wherein the L1 signal additionally contains a second signal, denoted as the L1-C/A signal, the receiver further comprising:

means for measuring the phase of said L1-C/A signal; and

means responsive to said L1-C/A phase for avoiding said phase ambiguity in said measured L1-P phase.

22. The receiver recited in claim 21 wherein said L2-P measured phase is susceptible to phase ambiguity, the receiver further comprising:

means responsive to said combined accumulated correlation products for avoiding said phase ambiguity.

23. A method for processing a received signal transmitted as an L1 or an L2 signal comprised of a single carrier modulated by a known code and by an unknown code wherein said unknown code causes adverse effects, said method comprising the following steps:

converting said L1 or L2 signal to quadrature components;

generating a model for said carrier;

generating a model for said known code;

counter-rotating said quadrature components of said L1 or L2 signal with said model carrier to produce counter-rotated signals;

correlating said model code with said counter-rotated signals to produce at least one correlation product separately for each counter-rotated signal;

accumulating said correlation products successively over selected time intervals;

combining with one another said accumulated correlation products derived from said quadrature components of said L1 or L2 signal in a manner reducing said adverse effects of said unknown code; and

,

generating an amplitude factor whereby said combined accumulated correlation products and amplitude factor are used to extract said signal'"'"'s amplitude, phase, and delay, in the presence of said unknown code.