10-14 10-12 10-10 PFA=10-6 Figure 1.7d. Correction factor n for Gaussian approximation as a function of time- bandwidth product (PD = 10-4) (reprinted from [4]). 10-16 ==10-5 10-14 10-12 10-10 PFA=10-6 Figure 1.7e. Correction factor n for Gaussian approximation as a function of time- bandwidth product (PD = 10-5) (reprinted from [4]). Low Probability of Intercept Communications Po=10-6 10-16 10-14 10-12 10-10 PFA=10-8 Figure 1.7f. Correction factor n for Gaussian approximation as a function of time- bandwidth product (PD = 10-6 (reprinted from [4]). that the FBC performance may be on the order of only 1.7 dB worse than the optimum detector (1.5 dB better than the l = 1 FBC). At the modest cost of about 1.7 dB of SIN required, it is therefore seen that the reduced complexity FBC is a more practical topology for a multi- channel detector. However, even though the individual channels and their output combiner is simplified, one million channels are still required to achieve the above performance. One million less-complex channels, practi- cally speaking, still remains untenable. (4) Partial-Band Filter Bank Combiner (PB-FBC) The PB-FBC is simply an FBC with less than the optimum or maximum number of channels. Suppose that N'T < NT channels are used, each chan- nel still matched to one of the candidate FH frequencies. The channel reduction factor is defined as f = N'//NT. A new consideration now enters the detector performance. When all NT channels are implemented, a hop frequency must appear at any given time in one of the channel filters. If only N'T channels are employed, then, on a per-hop basis, a specific hop frequency will not be covered by the filter bank if it corresponds to a deleted channel. Thus, from hop to hop, it becomes a probabilistic matter as to whether or not the PB-FBC will ahve a signal within one of its filters. When a signal is present within one of the channels, Interception Detectors this condition is known as a "hit." The probability of a hit, assuming that one of the N possible hop frequen