Covert Underwater Communications

To evaluate the feasibility of using the VLF acoustic channel to achieve covert point to point communication Planning Systems Incorporated (PSI) uses a broadband Finite Element Parabolic Equation (FEPE) underwater acoustic propagation model. The model generates VLF channel transfer functions used to develop Equivalent Discrete-Time (EDT) filters. The EDT filters characterize not only the channel, but also the transmitter and receiver responses. The EDTs model the intersymbol interference (ISI) that occurs in digital communications over band-limited channels. They are used to simulate ISI effects arising from the transmission of phase shift keying (PSK) and quadrature amplitude modulation (QAM) symbol sequences over the VLF channel and to design ideal compensation techniques for the interference. The Phase I goal is to develop the design data required to develop real-time adaptive VLF channel characterization algorithms in Phase II. Among the issues to be addressed are: the definition of the channel stability; the maximum number of symbols that can be transmitted and still achieve reliable communication; and the maximum communications range that can be achieved.