ce of background noise. The LPI advantage of an SS signal is that its power is spread over a bandwidth considerably larger than conventional transmissions, significantly increasing the noise in a receiver that is not privy to the despreading sequence. In the past, when implementable FH systems operated at low hop rates, their trans- missions could conceivably be detected by narrowband monitors capable of following the pseudorandom frequency variations (these so-called "fre- quency followers" could be used to drive repeat-back jammers which could effectively defeat the AJ capability of FH systems). Now, with hop rates of 100 KHz or more, the current generation of FH systems no longer has this vulnerability: an FH interceptor must detect multiple hops of the transmit- ted signal with a front end wide enough to accommodate the entire SS band- width. And, since FH signals can occupy much larger SS bandwidths than DS signals, they have a corresponding LPI advantage. As with all SS systems, an FH receiver must derive a synchronized replica of the received spreading sequence to perform properly. And the despread- ing operation, in which the received signal is despread by this SS reference, simultaneously spreads any accompanying interference over Wss or more, SO that most of the interfering power can be eliminated by a narrowband fil- ter matched to the data modulation bandwidth. As noted in Part 1, this AJ capability of an SS system is often measured by its processing gain (PG), a term which has been saddled with several conflicting definitions in the lit- erature. For example, in his pioneeering text on SS systems, Dixon at one point defines PG as the ratio of the SNR's at the output and input of the despreader [1, p. 8]; elsewhere (e.g., [1,p.7]), he uses the definition we pre- fer, PG = WSSIR ((3.4) in Chapter 3, Part 1), which has the advantage that Non-Coherent Frequency-Hopped Systems it does not depend on the particular choice of modulation or coding. Adding to this confusion, for FH systems, Dixon defines PG to