This paper presents a sequential block search technique for direct acquisition of long code sequences such as GPS P(Y)-code and the new M-code. Conventional methods perform time and frequency search sequentially one cell per predetection interval. As such, it is too slow under large uncertainty and becomes even more lengthy when a large processing gain is desired to work with weak signal or to operate under jamming conditions. In contrast, parallel search becomes an interesting alternative to allow for simultaneous testing of multiple code phases and frequency bins (one block at a time). Instead of searching through the uncertainty interval in a uniform manner, the present sequential block search technique allows for the design of search window such that its coverage follows a distribution similar to the initial time estimate. In this way, larger processing gains are given to more possible events. As such, it is expected to produce a higher probability of detection (PD) and smaller mean time to acquisition (MTTA). In this paper, two implementation schemes of the sequential block search are outlined. Probabilistic models are established to characterize PD and MTTA as a function of signal to noise ratio (SNR), initial errors, and search parameters.
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