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1. Having no source of energy. A passive filter involves no amplification and merely attenuates certain frequencies more than others. A passive beacon is a radar reflector that merely reflects radar energy, as opposed to an active beacon that transmits in response to a signal. 2. Applied to a system that does not generate an output if there is no input. 3. A positioning system that does not involve transmitting a signal from the measuring craft but only observing and measuring angles, amplitudes, phases, times, etc., in a system that exists independently of the craft being positioned.