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Strong multipath conditions which exist particularly in a shallow water acoustic channel make conventional synchronous transmission schemes ineffective for communication with untethered submersibles. Noncoherent detection schemes employing frequency diversity have been proposed to improve transmission reliability. These schemes generally require complex implementation. In this paper a novel demodulation scheme based on the Sliding Discrete Fourier Transform (SDFT) is described. The system operates on constant envelope signals, can tolerate large Doppler frequency shifts and multipath environment. With the availability of VLSI circuits to implement the SDFT algorithm this scheme is particularly attractive for untethered remotely controlled submersibles where power and space conservation are of prime importance.