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1. The recovery of information from an electrical or electromagnetic signal. (188) Note: Conventional radio waves are usually detected by heterodyning, i.e ., coherent reception/detection. In this method of reception/detection, the received signal is mixed, in some type of nonlinear device, with a signal from a local oscillator, to produce an intermediate frequency, i.e. , beat frequency, from which the modulating signal is recovered, i.e. , detected. The inherent instabilities of available optical sources have, until relatively recently, prevented practical use of coherent reception/detection in optical communication receivers. At present, coherent optical receivers, using sophisticated technology, are just beginning to emerge from the laboratory into the field. Virtually all existing optical receivers employ direct detection; that is, the received optical signal impinges directly onto a detector. Direct detection is less sensitive than coherent detection. [After FAA] 2. In tactical operations, the perception of an object of possible military interest but unconfirmed by recognition. [JP1] 3. In surveillance, the determination and transmission by a surveillance system that an event has occurred. [JP1]