Detection range of energy and feature detectors based upon communicator link budgets

Transmitted signals are vulnerable to detection with some relatively simple detectors such as radiometers, carrier detectors and chip-rate detectors. The ldquodetection rangerdquo is the greatest distance that a detector can be from the transmitter and still achieve values of probability of detection, and false alarm that result in reliable detection. In this paper, the probability of detection is given as a function of the received SNR. An expression for this SNR is expressed in terms of the link budget for both the legitimate receiver and detector. Two rules of thumb make determination of this detection range quite simple yet adequate to give the communicator a good idea of the detection threat. The first rule of thumb is that the detector probability of detection as a function of SNR has an abrupt transition between very low and very high values. This cross-over SNR is used with the two link budgets, including propagation effects, to derive a simple equation for the detection range. The second rule of thumb is that medium- to high-gain antennas have sidelobes that are approximately 0 dBi irrespective of the main peak gain. Some illustrative satcom and terrestrial examples, with and without spread spectrum, are provided that show that detection is a considerable concern. Mitigation techniques are discussed.