Method and system for reconstructing time of transmit from assisted or weak signal GPS observations
This invention provides methods for a GPS device to resolve timing errors as applicable to a AGPS navigation solution, and, in particular, to solutions involving GNSS positioning and timing with limited assistance such as indoors or in a heavily obscured location, where the time of transmit of satellite signals is not directly extractable from the navigation message data. In particular the invention concerns the reconstruction of time-of-transmit from the course acquisition code of weak signals or where assisted-GNSS are employed.
1. A method for determining a time of transmit of a Global Navigation Satellite System (GNSS) signal to a receiver that is not moving or is slowly moving relative to the earth, the method comprising:
(a) acquiring and tracking, in the receiver, a code phase and a carrier frequency of the GNSS signal;
(b) obtaining bit synchronization based on the code phase and the carrier frequency by performing non-coherent correlations over rounds of coherent correlations of the GNSS signal using multiple offsets at a predetermined spacing;
(c) pre-integrating the bit synchronized GNSS signal over data bit periods to form bit-aligned partial integrals;
(d) storing the bit-aligned partial integrals in a signal store;
(e) multiplying the stored bit-aligned partial integrals by data test bits; and
(f) applying a mathematical transformation on the product of the stored bit-aligned partial integrals and the data test bits to provide correlation values to be compared with at least one threshold thereby providing an ambiguity resolution candidate for time of transmit of the GNSS signal.
2. The method of claim 1 , wherein the mathematical transformation is a Fourier transform.
3. The method of claim 1 further comprising repeating steps (e) and (f) with bit-shifted sequences of the bit-aligned partial integrals to produce multiple ambiguity resolution candidates for time of transmit of the GNSS signal.
4. The method of claim 3 , wherein the mathematical transformation is a Fourier transform.
5. The method of claim 1 further comprising repeating steps (e) and (f) with bit-shifted sequences of the data test bits to produce multiple ambiguity resolution candidates for time of transmit of the GNSS signal.
6. The method of claim 5 , wherein the mathematical transformation is a Fourier transform.
7. The method of claim 1 , wherein a peak of the mathematical transformation is compared with the threshold.
8. The method of claim 7 , wherein the mathematical transformation is a Fourier transform.
9. The method of claim 3 , wherein the at least one threshold is determined from statistics of the correlations values across all of the candidates.
10. The method of claim 3 , further comprising setting the at least one threshold several standard deviations above a mean across all candidates.
11. The method of claim 3 , further comprising:
setting a secondary threshold; and
permitting no other candidate to be above the secondary threshold for a test of each candidate to be successful.