s once again obtained in the absence of and prior to true data transmission and, thus, the syn- chronization process is of a gated nature, being interleaved within the data sequence often enough to provide the desired degree of timing accuracy. The specific manner in which the FH timing estimate is formed may be understood by considering the simplified fine time synchronization of an FH/MFSK system such as that illustrated in Figure 3.14. The transmitted FH signal is successively advanced and delayed in time relative to its nom- inal synchronization position. The received FH signal plus jamming noise is cross-correlated with the local frequency hop generator. Alternately, the local frequency hop generator could be advanced and then delayed in time while the transmitter hop generator remains fixed in its nominal synchro- nization position. Since the relative timing offset between transmitter and receiver hop generators is all that is effectual in producing the timing ADVANCE FREQUENCY OR DELAY BAND-PASS DISCRETE oj.bj SPECTRAL BYATh GENERATOR FILTER CONVERTER FOURIER TRANSFORM ESTIMATE LOCAL FREQUENCY HOP GENERATOR ACCUMULATOR TIMING SYNCHRONIZATION ESTIMATOR Figure 3.14. Simplified time synchronization of an FH system with on-board processing. ADVANCE FREQUENCY DISCRETE BAND-PASS aj,bj SPECTRAL OR DEL AY FOURIER FILTER CONVERTER BYATh GENERATOR TRANSFORM ESTIMATE LOCAL FREQUENCY HOP ACCUMULATOR GENERATOR MODULATOR TIMING SYNCHRONIZATION DEMODULATOR ESTIMATOR Figure 3.15. Simplified time synchronization of an FH system with a passive processor. Time and Frequency Synchronization of Frequency-Hopped Receivers estimate, either implementation will conceptually produce the same per- formance with the choice governed by the particular system application. Typically, the former implementation is preferable from the standpoint of reducing receiver complexity. Assuming that the spacing between successive frequency hops is far out- side the bandwidth of the band-pass filter, then in the absence of perfect time synchronizati