Underwater Acoustic Telemetry Research Articles

Adaptive equalization techniques for acoustic telemetry channels

A tutorial review of adaptive equalization techniques for combating intersymbol interference in high-speed digital communications over time-dispersive channels is given. Various equalizer structures and the associated adaptive algorithms, including both fractionally spaced and symbol-spaced equalizers, are presented. Also considered is the application of adaptive equalization techniques to underwater acoustic telemetry channels. >

Performance limitations in underwater acoustic telemetry

Performance limitations in digital acoustic telemetry are addressed. Increases in computational capabilities have led to a number of complex but practical solutions aimed at increasing the reliability of acoustic data links. These solutions range from ocean-basin scale data telemetry to video-image transmission at a few hundred yards' distance. The opportunity to implement highly complex tasks in real time on modest hardware is a common factor. The data rates range from 1 to 500 kb/s and are much slower than satellite channels, while acceptable system complexity is higher than virtually any other channel with comparable data throughput. The basic performance bounds are the channel phase stability, available bandwidth, and the channel impulse response fluctuation rate. Phase stability is of particular concern for long-range telemetry, channel fluctuation characteristics drive equalizer, and synchronizer design; the bandwidth limitation is a direct constraint on data rate for a given signaling method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of high-frequency multitone transmissions propagated in the marginal ice zone

This work presents estimates of frequency and time coherence functions of acoustic transmissions centered at 50 kHz over a 1-km horizontal underice path in the marginal ice zone (MIZ). The data were collected during the MIZEX ’84 experiment as part of a feasibility study for an underwater acoustic telemetry link. It is shown that the acoustic fluctuations are dominated by a 10-Hz process conjectured due to the turbulent under-ice boundary layer. Significant fluctuation energy in the 0.1-Hz band was also observed. The two processes differ in frequency coherence characteristics. An underwater telemetry system designed for this channel will be dominated by the high-frequency fluctuations, but the slower process is capable of causing disastrous data losses unless its effects are anticipated.

Characteristics of sound pulse transmission in shallow sea and correlation detection

Statistical characteristics of sound pulses and some typical marine noises in a shallow sea were analysed. With special requirements for underwater telemetric and telecontrolled devices in mind, a specific signal processing scheme (digital correlation accumulation) is put forward. Theoretical analyses on some underwater acoustic instruments showed this signal processing scheme is applicable to a shallow acoustic channel with variable spatiotemporal characteristics, multipath pulses and high noise level, and may be of general significance for underwater acoustic telemetry and telecontrol.

Algorithms for underwater position fixing

Algorithms are developed for determining the coordinates of a position in three‐dimensional space with a minimum number of measurements. They are especially useful for underwater acoustic telemetry applications, such as navigation systems for divers or submersible vehicles, in which positions are calculated with respect to a long‐baseline array of fixed hydrophones or transponders on the seabed. The theory is applicable to any array configuration, and requires very simple programming for computer implementation.

Channel capacity in bits per joule

Underwater acoustic telemetry has a total input energy constraint, since the energy is stored in the transmitter's batteries. This work is primarily rephrasing the work in channel capacity in terms of C_{J} bit/J as compared to C bit/s, to emphasize the energy efficiency and to deemphasize the speed of the telemetry. The energy channel capacity C_{J} , for any waveform channel with well-defined capacity C bit/s at signal power S watts, is defined as C/S bit/J. It is shown that for coherent binary frequency shift keying (BFSK) and waveform channels, the supremum of C_{J} over S is approached as S approaches zero. For the Gaussian channel the best coding uses narrow bands with the highest S/N.

A microprocessor-based acoustic telemetry system for tide measurement

This paper describes the ocean bottom unit and surface ship deck unit of a multi-microprocessor-based underwater acoustic telemetry system designed primarily to address the application of tidal data gathering during the course of a hydrographic survey. The problems of tidal data collection are highlighted and the objectives of the acoustic telemetry system are stated. A shallow-water environmental model is used to obtain accurate estimates of parameters such as propagation loss, band-level noise, frequency dispersion, and time dispersion. The environmental model in conjunction with in situ measurements dictates the guidelines used in the system design. A breakdown of the units into subsystems is given and each subsystem is described. The results of tests performed with the system on June 12 and 13, 1982, in the Bedford Basin, Dartmouth, Nova Scotia, are summarized. Finally, other potential applications of the system are mentioned.

A digital correlative accumulation and its application to underwater acoustic telemetry, telecontrol and remote sensing

A specific digital time correlative accumulation has been developed. It is an adaptable scheme of signal processing for underwater acoustic technology used in telemetry., telecontrol and some remote sensing. Its basic principle is here introduced and the prospects for successful use in an acoustic channel with multipath, time-space variables and high noise level are amply discussed. The performances of this scheme are analysed.

New products

AMF Electrical Products Development Division has added to its Sea‐Link oceanographic product line the Benthos underwater acoustic telemeters and telemetry systems, according to Saul H. Silver, Division Vice President and General Manager. These telemeters may be used for transmission of depth, temperature, compass heading, current speed and direction, tilt, and many other selected parameters from great ocean depths. Data encoding is accomplished by varying the time delay between emission of a precision clock pulse and a subsequent data pulse. Signals from the Model 1023 and 1024 telemeters can be received and displayed via any of the standard graphic recorders normally installed on oceanographic or survey vessels.

Applications and Limitations of Underwater Acoustic Telemetry

Underwater acoustic telemetry has the potential to increase substantially the volume and reliability of information transmitted between subsea work sites and remotely located monitor and control stations. In this paper, the applications and limitations of these techniques are discussed. The effects of the transmission media such as attenuation, dispersion, fading, and background noise are pointed out. This information is used to develop realistic design criteria for, and evaluate the performance of, a typical underwater telemetry system, which utilizes digital frequency-shift keying. Practical considerations such as complexity and reliability are then discussed. Finally, an outline of some of the problems which must be overcome if such systems are to gain widespread acceptance is given. [Partially supported by a National Science Foundation Sea Grant Program Institutional grant.]

Two-Way Hydroacoustic Communications Link for an Ocean-Bottom Seismograph

An underwater acoustic telemetry system was developed to provide a two-way communications link between an ocean-bottom seismograph and a ship overhead. Four data channels are transmitted from the instrument on IRIG Channel 12 (10.5 kc/s). Commands to the instrument are transmitted in the form of a simple pulse position code using a 14 kc/s carrier, and provide for such functions as gain change, calibration, instrument leveling, and uncaging. A mercury battery pack provides sufficient power for 15 days of continuous operation. Acoustic power output is approximately 2.5 watts at an overall efficiency of 20 percent. Instrumentation and batteries are housed in two spherical pressure vessels designed to withstand depths in excess of 2500 fathoms. The system was successfully operated in May, 1964, off Bermuda.

Air Force Eastern Test-Range Underwater-Sound Systems

The underwater-sound missile-impact location systems on the Air Force Eastern Test Range are described. A SOFAR system covers the Atlantic Oceans between Florida and Ascension Island to furnish accurate location in calibrated areas of the impact point of missiles carrying a SOFAR bomb. Three target arrays provide accurate impact location in limited areas from the impact signal. Acoustic instrumentation and techniques have been developed that permit these systems to be calibrated with high precision. The Columbia University Geophysical Field Station reported earlier [G. R. Hamilton, M. E. Drake, and C. Hartdegen, J. Acoust. Soc. Am. 35, 1879(A) (1963)] on one stndy (SOFAR velocity stability) made in connection with these systems. The geodetic accuracy of these systems is now limited by the accuracy with which the geodetic position of the calibrating ship can be measured. These systems are now the standard for missile-guidance scoring. Needed improvements are described. The results of work to develop an underwater acoustic telemetry link are described in a separate paper.