The Naval Research laboratory has developed a sea-going system, currently consisting of three acoustic communications and data storage (ACDS) buoys. The buoys can be deployed in the water column, moored to the ocean floor, or towed behind a ship to emulate an autonomous underwater vehicle (AUV). The system is intended for semi-real-time acoustic communications in situ and continuous recording of the raw acoustic data in the water for postexperiment analysis. The purpose is to study environmental issues affecting high data rate point-to-point acoustic communications as well as multiple-in-multiple-out acoustic networking. Each ACDS has an acoustic projector and an array of eight hydrophones. It is designed for two frequency bands: 2–5 kHz and 15–25 kHz. The system has been deployed in several oceans. Modulation signals used include binary and quadrature phase-shifted keying (BPSK/QPSK), frequency-hopped frequency-shifted keying (FH FSK), orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) signals. The acoustic modem is based on the Acoustic Modem Software developed by WHOI and improved by the Naval Underwater Warfare Center, Newport, RI. Acoustic networking signals are used to aid vehicle navigation. [Work supported by ONR.]
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The multiplex system described was designed for use on the NAVFAC Snoopy vehicle: a small remotely manned tethered undersea vehicle developed at the Naval Undersea Center. Using a combination of time division (TDM) and frequency multiplexing, the system allows simultaneous transmission of vehicle power. status. command: and video signals over a small diameter coaxial cable. Digital vehicle and status signals are TDM encoded, frequency modulated, and then transmitted via the tether cable to the vehicle (or control station) receiver. At the receiver the signals are frequency demodulated, TDM decoded. and sent to the appropriate function. The power and video signals are simply frequency multiplexed onto the tether cable. along with the frequency modulated command and status signals.
High frequency sonar systems have been used by the Naval Research Laboratory to study nonlinear internal gravity waves and define the fine structure of ocean temperature and salinity layers that are found in coastal waters, usually within 130 meters of the surface. Of particular interest
is the fine structure of these waves, which are being investigated using high sensitivity sonar systems that provide 1 m horizontal resolution and less than 8 cm vertical resolution. This article describes the integration of commercial and custom-designed components, including a recently patented
transmitter-receiver switch. The significance of this T-R switch is that it improves the sensitivity of short-range sonar systems, enables a more refined measurement of nonlinear internal gravity waves, and could have broad industry applications.
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