Multi-pass frequency hop-acquisition correlator
A multi-pass frequency hop timing acquisition correlator that produces a more accurate time estimate from the fast acquisition frequency hop signal is disclosed. The time estimate produced by the multi-pass acquisition correlator is more accurate than the ¼ hop estimate produced by single-pass fast acquisition correlators. The multi-pass frequency hop timing acquisition correlator uses a time estimate generated by a previous pass through the multi-pass frequency hop timing acquisition correlator as a starting point for computing a new, more accurate, time estimate. Thus, the second pass through the multi-pass frequency hop timing acquisition correlator produces a time estimate that is relatively more accurate than the time estimate produced by the first pass. The more accurate time estimate produced by the multi-pass acquisition correlator reduces the hardware and time needed by the direct acquisition process to acquire the M-code. In one embodiment, the multiple-pass frequency-hop correlator on the second or third pass provides microsecond-level timing from a GPS M-code fast acquisition frequency hop signal in a jamming environment.
1. A fast acquisition system comprising:
a fast acquisition processor configured to produce a time estimate for acquiring a coded signal having an associated frequency hop acquisition signal, said fast acquisition processor comprising a plurality of fast acquisition correlators; and
control logic configured to provide a first time estimate and a first time delta value to said plurality of a fast acquisition correlators for a first pass through said fast acquisition processor, said control logic further configured to provide a second time estimate produced by said first pass through said fast acquisition processor, and a second time delta value to said plurality of fast acquisition correlators, for a second pass through said fast acquisition processor, said second pass through said fast acquisition processor producing a third time estimate.
2. The fast acquisition system of claim 1 , said fast acquisition processor configured to produce said second time estimate according to a magnitude of the data produced by an n'th correlator in said plurality of correlators.
3. The fast acquisition system of claim 1 , said fast acquisition processor configured to produce said second time estimate according to a magnitude of the data produced by an n'th correlator in said plurality of correlators and according to two correlators adjacent in time to said n'th correlator.
4. The fast acquisition system of claim 1 , said fast acquisition processor configured to produce said third time estimate according to a magnitude of the data produced by an n'th correlator in said plurality of correlators.
5. The fast acquisition system of claim 1 , said fast acquisition processor configured to produce said third time estimate according to a magnitude of the data produced by an n'th correlator in said plurality of correlators, and according to two correlators removed in time from said n'th correlator.
6. The fast acquisition system of claim 1 , wherein said second time delta value is smaller than said first time delta value.
7. In a fast acquisition processor, comprising a plurality of fast acquisition correlators, a method for fast acquisition comprising:
providing a first time estimate and a first time delta value to a said fast acquisition correlators;
executing a first pass through said fast acquisition processor to produce an output comprising a second time estimate;
providing said second time estimate and a second time delta value to said fast acquisition correlators; and
executing a second pass through said fast acquisition processor to produce an output comprising a third time estimate.
8. The method of claim 7 , wherein said second time estimate is located by locating a correlator H N , having a largest magnitude in said plurality of correlators spaced according to said first time delta value.
9. The method of claim 7 , wherein said second time estimate is located by locating correlator H N , H N−1 and H N+1 , in said plurality of correlators spaced according to said first time delta, said correlator H N having a largest output magnitude of said correlators in said plurality of correlators.
10. The method of claim 9 , wherein said correlator H N-1 and H N+1 , are adjacent in time to said correlator H N .
11. The method of claim 7 , wherein said third time estimate is located by locating correlator H N , H N−5 and H N+6 , in said plurality of correlators spaced according to said second time delta, said correlator H N having a largest output magnitude of said correlators in said plurality of correlators.
12. The method of claim 7 , wherein said second time delta value is smaller than said first time delta value.
13. A fast acquisition system comprising:
means for producing a time estimate for acquiring a coded signal having an associated frequency hop acquisition signal; and
means for providing a first time estimate and a first time delta value to said means for producing as inputs for a first acquisition pass, said means for providing further configured to provide a second time estimate produced by said first pass and a second time delta value to said means for producing as inputs for a second fast acquisition pass, said second acquisition pass producing a third time estimate.
14. A fast acquisition system comprising:
a fast acquisition processor configured to produce a time estimate for acquiring a coded signal having an associated frequency hop signal; and
control logic configured to provide a first time estimate and a first time delta value to said fast acquisition processor as inputs for a first acquisition pass, providing a second time estimate produced by said first pass and a second time delta value to said fast acquisition processor as inputs for a second acquisition pass, said second acquisition pass producing a third time estimate.