1) can also be applied in FH/SS systems. Recall that in the DS/SS system the estimation of the PN sequence that biphase modulates the car- rier is accomplished by low-pass filtering the received signal with a filter whose cutoff frequency equals the clock rate of the LFSR followed by hard- limiting. Furthermore, the recursion relation for the PN sequence can be used to improve the accuracy of the initial state estimate (resulting in THRESHOLD COMPARISON THRESHOLD SYNC INDICATION COMPARISON ACQUISITION CONTROL ACTIVE CORRELATORS THRESHOLD COMPARISON THRESHOLD RECEIVED PASSIVE COMPARISON FH SIGNAL CORRELATOR Figure 3.12. A two-level scheme for FH acquisition. FH Acquisition Techniques recursion-aided rapid acquisition by sequential estimation) and/or to deter- mine whether the estimate is likely to be error free, thereby reducing acqui- sition time. In the FH/SS application, the above approach translates to estimating the received frequency over say n1 successive hop intervals, thus requiring a form of sequential spectral estimation. Note that whereas in DS/SS systems an estimate based on observation of the received signal over a single chip interval provides information about only one stage of the LFSR (i.e., we need n successive chip interval estimates to load the contents of an n-stage LFSR), in FH/SS systems an estimate of the frequency in a single hop interval provides information about the entire state of the LFSR. Thus, depending on the order of the model used for the received signal, one can use either the frequency estimate in each hop interval to identify only the most significant bit (MSB) of the corresponding PN codeword, in which case we would have n1 = n, or several significant bits of the LFSR (n1 n), thus further reducing the acquisition time. Based on the above discussion, Figure 3.13 illustrates an autoregressive spectral estimation acquisition technique (ASEAT) [5, 6] that is directly analogous to the RASE technique for the DS/SS system (see Figure 3.4 of Chapter 1). The word "autoregressive" ref