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Rapidly Deployable Systems (RDS) underwater acoustic telemetry trials report

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Abstract

Maritime surveillance of Australia’s littoral waters and approaches is important to the defence and well being of its resources. Rapidly Deployable Systems (RDS) that can be safely delivered and deployed, and then interrogated and controlled from near and/or far, may provide an important force multiplier for the ADF. However, the efficiency of these systems will, amongst other things, depend on the performance of the external data telemetry link. This is particularly true of an underwater acoustic communication or telemetry link that may be used by a patrol to interrogate and extract contact data from a surveillance barrier. This paper describes work done by the Maritime Operations Division Salisbury, to design, develop, and trial a simple, low cost, high data rate underwater acoustic telemetry system in a shallow water littoral environment.

Executive Summary

Under the Rapidly Deployable Systems (RDS) task sponsored by DGMD, DSTO is investigating offboard ocean deployed sensors that can be rapidly deployed and monitored from near and/or far. Underwater Acoustic Communications (ACOMMS) or Telemetry Systems that use the water as the transmission medium could allow a submarine or other platform to approach, interrogate, and download data from an undersea rapidly deployable sensor system. By eliminating the need to use interconnecting cable links, these systems could provide future RDS with complete stealth connectivity and greater operational flexibility. The study of high data rate underwater ACOMMS systems by the Maritime Operations Division (MOD) Salisbury, has principally been focused on developing simple systems to test this concept. This report describes the recent research conducted by the MOD Salisbury to investigate the problems/feasibility of using high data rate ACOMMS for short-range telemetry in shallow littoral waters. A relatively simple, low-cost experimental digital ACOMMS system, that used digital modulation techniques and differential signalling with no adaptive equalisation, beamforming or doppler correction, was designed, developed, and tested in the shallow littoral waters of the Gulf St. Vincent in South Australia. Test results showed that by using a relatively simple, low-cost, digital ACOMMS system, a relatively error free high data rate transmission (in our case 16 kbaud or 32 kbps) could only be obtained over short ranges (up to 100 m in a 40 m deep offshore channel). However, as MOD and others have found, these results can only be obtained with the simplest of underwater channels ie. stationary type with fixed transmit and receive platforms. Should the fundamental properties of the channel change, or relative motion occur between the transmitter and receiver, then these changes can severely limit the performance of the ACOMMS system. To obtain robust ACOMMS performance the investigation and/or use of more sophisticated modulation, phase tracking, and space-time processing techniques is recommended. Although these techniques require more complex modulation/demodulation and transmitter/receiver structure they should provide improved and robust ACOMMS performance.

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