Combined parallel and sequential detection for GPS signal acquisition
View Patent ↗A two part signal acquisition process includes a parallel signal detection process and signal verification/false alarm rejection process. A massively parallel architecture of acquisition correlators search a large region of the time-frequency uncertainty during the parallel signal detection process to identify the most likely detections for each search dwell. Concurrent with the parallel signal detection process performed by the acquisition correlators, the current list of most likely detections is examined with additional search dwells in the verification/false alarm rejection process. The verification/false alarm rejection process is performed by a plurality of independent correlators or tracking channels. Under software control, the tracking channels perform repeated dwells on the most likely detections until they can be dismissed as false alarms or verified as the desired signal.
1. A signal acquisition process comprising:
a) calculating a first set of correlations between a segment of a satellite signal received over a first time period and a reference signal over a set of phase offset values and carrier frequency values;
b) for each of the first set of correlations that exceeds a predetermined threshold value, identifying the carrier frequency value and the phase offset value associated with that correlation;
c) calculating a second set of correlations between a segment of the satellite signal received over a second time period with the reference signal using the frequency values and the phase offset values identified from the first set of correlations;
d) identifying the carrier frequency value and the phase associated with each of correlations that exceed the threshold value in the second set of correlations; and
e) calculating a third set of correlations between a segment of the satellite signal received over a third time period with the reference signal using the set of carrier frequency values and the phase offset values associated with the first set of correlations and the carrier frequency values and the phase offset values identified from the second set of correlations.
2. The process of claim 1 wherein a portion of calculating the second set of correlations overlap in time with a portion of calculating the third set of correlations.
3. The process of claim 1 wherein calculating the third set of correlations is carried out after calculating the second set of correlations.
4. The process of claim 1 wherein six phase offsets and six carrier frequencies associated with the greatest correlations are identified from the first set of correlations.
5. The process of claim 4 wherein up to a predetermined number of carrier frequency values and phase offset values are identified from the second set of correlations.
6. A signal acquisition device comprising:
a first set of correlators adapted to repeatedly (a) perform over successive time intervals correlations over a predetermined range of phase offset values and carrier frequency values and (b) identify, for each time interval, a combination of phase offset values and carrier offset values corresponding to a predetermined number of largest correlations over that time interval; and
a second set of correlators adapted to repeatedly perform correlations, for each of the successive time intervals, using the identified combination of phase offset values and carrier frequency values for that time interval and a selected subset of the phase offset values and the carrier offset values used in the immediately previous time interval.
7. The signal acquisition device of claim 6 wherein the first and second set of correlators operate in parallel.
8. The signal acquisition device of claim 6 wherein the second set of correlators uses six phase offset values and six carrier frequency values for each time interval.
9. The signal acquisition device of claim 6 , further comprising an antenna for receiving GPS signals.
10. The signal acquisition device of claim 6 , further comprises a microprocessor for controlling the operations of the first and second sets of correlators.