Abstract: A communication and position determining device for use in an underwater communication and position determining system wherein the device transmits a sequence of one or more data words, wherein each data word includes a plurality of data bits transmitted in parallel as a corresponding number of concurrently transmitted signals, each of which occupies a different frequency band. The signals have waveforms selected from a pair of fade resistant waveforms that can be discriminated from one another, to represent binary data and the frequency bands are separated from one another by guard bands having widths sufficient to prevent intersymbol interference between the signals representing the data bits of a data word due, for example, to doppler and phase/frequency shifting, and successive data words are separated sequentially in time by intervals having a duration sufficient to prevent intersymbol interference between the signals of successive data words due to multipath reverberation.