Underwater modem-based navigation aids

The utility of low power underwater digital acoustic communications (acomms) has a practical commercial history of fewer than 15 years. While earlier techniques provided data rates of perhaps 40 bits per second (bps), current noncoherent methods support data rates of many hundreds of bps, and coherent techniques provide many thousands of bps. Major enhancements in both DSP and memory electronics have had a major impact on communications performance. Equally important, these enhancements have enabled us to use those same electronics to design and field underwater systems that perform multiple functions in addition to communications. Modems of necessity use complex waveform for a variety of purposes, including acquisition, alignment, and modulation. These waveforms are also appropriately useful in providing undersea navigation aids for autonomous platforms. The modem-supported platforms then are able to perform autonomous tasks with remote modem-equipped sensors and distributed networks. An important example is an undersea glider acting as a data truck to extract both raw and pre-processed data from deep ocean sensor systems. Examples of modem-supported sensors are described.