 (1) and (5) are those of most utilitarian value to the interceptor. The analyses in the following subsections consider only pure FH signals. In Section 1.2.1.3, the merits of other SS signal forms in terms of detector performance are dis- cussed. (1) Wideband Energy Detector (Radiometer) By far the most elementary and easy to implement detector is the wideband energy detector or radiometer,2 shown functionally in Figure 1.1. The wide- band detector consists of a bandpass filter (BPF) of center frequency fo and bandwidth W (equal to the total FH spread-spectrum bandwidth Wss), a square-law operation followed by a T-second integrator (reset at the end of each successive T-second period), and a comparator that weighs the inte- grator output against a threshold value in order to decide if a signal is received for each T-second segment. The size of T is generally dictated by the communicator's message duration TM. When TM is short and discontin- uous, usually T is set equal to TM for highest performance. 2The generic term "radiometer" refers to a square-law measurement device covering an RF band of interest. Techniques for receivers employing such devices evolved primarily to satisfy the need to measure the extremely weak, broadband non-coherent RF energy radiated by astro- nomical objects. Interception Detectors COMPARATOR filor DECISION THRESHOLD Figure 1.1. Functional wideband energy detector (radiometer). For large TW = TMWss products (large being greater than 1,000 for the typical values of PD and PFA previously given), the wideband radiometer per- formance may be calculated using the equations (1.4) (1.5) where Q21 (.) is the inverse of the Gaussian probability integral. Recognizing the similarity between Figure 1.1 and the detection portions of the single dwell serial search PN and FH acquisition schemes discussed in Chapters 1 and 3, respectively, then (1.4) together with (1.5) is identical³ to (1.81) with the appropriate substitutions of notation, i.e., A² S,td TM, and B Wss. Figure 1.2 is a plot of PD versu