An Acoustic Telemetry System for a Shallow-Water UnderIce Arctic Environment (1983)

Abstract A new underwater acoustic telemetry system has been developed for long-range medium-rate, full duplex digital data transmission in a shallow water, under-ice Arctic environment. Prior to design of the system, acoustic properties of three ice-covered transmission channels were evaluated in experiments in the Canadian Arctic, near Resolute Bay. The three channels, each ranging to 4500 m in length, were characterized by their water depth profiles: In the first, one end of the channel was 100 m deep, while the other end was 10m deep; in the second, one end was 100 m deep, while the other end was 200m deep; in the third, the water depth was constant at 100 m. Results of the Arctic experiment are presented here, along with a discussion of, the design concepts used to optimize the telemetry system for low error rates in a severe multipath environment. Acoustic telemetry can be used to advantage whenever digital data must be transmitted under water. The present system was developed under contract with the Canadian Government, Department of Fisheries and Oceans, for a remotely piloted vehicle being developed by the Bedford Institute of Oceanography for evaluation and use by the Canadian Hydrographic Service. The telemetry system will be used to transmit control signals and data, both to and from the vehicle. Introduction Reliable transmission of information through the water has been an elusive goal. Many schemes have been tried with varying success over the last few decades. Most of these systems have addressed data transfer to and from fixed stations on the seafloor or transmission of simple commands to transponders, releases, control valves, etc. These systems have operated over both vertical and horizontal paths at very low effective data rates. Systems capable of transmitting much higher data rates have also been developed. 4800 baud has been successfully used to relay data and flowscan pictures from the ocean instruments along nearly vertical paths over 5 km in length. Very narrow beam systems have also been successfully tested on deep water for horizontal paths. Both these high data rate systems avoid entirely the problem of multipath propogation. A remotely piloted, non-tethered vehicle is being developed by the Bedford Institute of Oceanography. This vehicle, called the ARCS (Autonomous Remote-Controlled Submersible), will be used to perform depth soundings in certain ice-covered regions in the Canadian Arctic. In order to accomplish this, the ARCS will swim under the surface of the ice at considerable distances from the control site. A full duplex acoustic communication system is required to transmit mission-related data and operational status from the ARCS to the control site, and commands from the control site to the ARCS. Commercially Available Equipment It was found that commercially available acoustic telemetry equipment, in general, was quite expensive and did not meet the specific requirements of the ARCS, primarily due to the Doppler shift caused by its 5 knot velocity. 1ST proposed to develop an acoustic telemetry system which would solve these problems and would also serve a side variety of acoustic communications applications. It was deemed necessary to conduct experiments to investigate the feasibility of the proposed signalling principles under realistic conditions, and to gain practical experience with under-ice acoustic propogation, in order to decrease the technical risk of the project. Performance Requirements The acoustic channel is expected to be about 200 m in maximum depth, with an ice-covered surface. Acoustic communication is required in this medium over distances r