Effects of multipath transmission on the measured propagation delay of an FM signal

Position location techniques based on propagation delay have been proposed previously. A narrow-band version of this technique involves the transmission of RF carrier, modulated by a single audio frequency. At a receiver, the audio phase is a measure of distance provided the propagation delay is less than one quarter cycle. The transmission medium introduces multipath distortion and the received signal consists of a set of signals, each an attenuated and delayed replica of the transmitted signal, having traversed a different path. When FM is employed, the phase of the demodulated composite is a nonlinear function of the parameters of the multipath structure of the channel. In this paper, this functional relationship is derived.