Guest Editorial and Overview - IEEE Xplore

IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. OE-11, NO. 1, JANUARY 1986

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Guest Editorial and Overview

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LL OF US that have worked in the marine environment are aware of the difficulty and cost of collecting data on physical and biological processes in the ocean. One way of reducing the cost of oceanic data collectionis to use monitoringtechniques with increasedsamplingpower. In general, it is not possible to significantly increasethe sampling capability of direct measurement tools.such as current meters and nets. An alternative to making direct measurements is to employ some remote-sensing techniques. Acousticremote Special Issue, has had an sensing, the subject of this increasingly important role in oceanic data collection. In recent years, significant advances have been made in the development and implementation of a variety of underwater acoustic remotesensing techniques. Many of the advances can be directly linked to the rapidly developing fields of digital electronics and microcomputer technology. Small inexpensive microprocessor-basedsystems can now beusedto perform data processing tasks that a decade ago required a large mini or main frame computer. Furthermore, using CMOS tec.hnology the required data processing tasks can be implemented with electronics that have very low power drain and good noise immunity. Using this Iow power ekctronics, it is now possible to deploy a battery-operated acoustic remote-sensing system that will collect environmental data for extended periods of time Another major factor in the recent advances in underwater acoustic remote sensing is the advances thathave been made in the understanding of the physical and biological processes in the ocean and how these processes interact with the acoustic an remote-sensing tools. The firststepinimplementing acoustic remotesensing system istomodel the interaction between the monitored parameter and thereceivedacoustic signal. If the models are inadequate, theremote-sensing technique will have limited accuracy independent of the quality of the hardware or software used to implementthe technique. In many cases, the development of appropriate models has been an iterative process. First, an acoustic remote-sensingsystem is developed to monitor a specific ocean process. Theinitialobservations with the system are then used to refine the modelwhichthenproducesbetter results with the next version of the system and so on. Many of the acoustic remote-sensing systems in use today have been around long enough to go through a number of refinements. The papers presented in this “Special Issue on Ocean Acoustic Remote Sensing” cover a range of different topics in the general field of acoustic remote sensing. The purpose of the issue was not to cover all the important developments in remote sensing but rather to give the reader a sense of the diversity of work being done in this field. Acoustic current profiling using Doppler backscattering is the subject addressed in the first four papers. The first paper

by Woodward and Appell contains an excellent review of the development of acoustic DoppIer current profiling systems. It also briefly describes the commercial systems available and discusses some of the problems and successes that have been encountered in the application of these systems. The next two papers by Theriault consider two measures of performance for current profiling systems. In the first paper, Theriault develops an expression for the variance of the velocity estimate using theCramer-Rao lower bound, He then goes on to use his results to illustrate the trade-offs betweenaccuracyand resolution. His second paper deals with the spatial response of three- and four-beam Doppler current profiIers. As he points out, the Doppler current profiler is not a point measurement device and its spatial performance is affected by the physical parameters of the system such as beamwidth, pulsewidth, beam separation, and the averaging techniques used to reduce variabihty in the data. The paper by Hansen considers three approaches for extracting the Doppler information from the received signal. The techniques he considers are a fast Fourier transform (FFT) approach, a spectra1 moment estimator (SME) method, and an autoregressive (AR)model fitting approach. The AR analysis technique has received considerable attention in time-series analysis. The results in this paper show that it may also prove to be a powerful tool for remote current profiling. The paper by Farmer and Clifford also deals with a technique to remotelymeasure ocean flows. Their method differs considerably from the more common Doppler current profiler. They use a bistaticacousticsystem with multiple receiving transducers. In their technique, the temporal cross correlation of the received signals is used to determine the average flow at right angles to the acoustic paths. Experiments with their system have shown a good correlation with current meter measurements. The paper on “Acoustic Oceanography by Remote Sensing” by Mercer looks atthe history of the application of acoustics tostudy largescale oceanography. He points out that some of the techniques used, including acoustic tomography, have had onlylimited success. It is Mercer’s view thatthe problems encountered are not fundamental limitations and can probably be overcome in the future. Remote sensing of largescaleoceanographyusing acoustic tomography is also the subject area addressed in the paper by Georges et al. They investigate the effects of largescale oceanographic features on acoustic. propagation using Hamiltonian ray tracing and discuss the implications of their results for acoustic tomography. Eislerand Stevenson also investigate the performance of remote sensing using acoustic tomography. They consider the of trade-offs between various system parameters and the effect these parameters on performance. The paper by Stanton and Clay discusses how the echolevel statistics can be used to

0364-9059/86/0100-0001$01.OO Q 1986 IEEE

remotely extract informationabout objects in the water column andaboutthewater-bottomboundary.In particular, they develop a method for processing the echo level statistics to estimate the backscattering cross-section distribution of fish and the roughness of the bottom. Although the subjects of the last two papers are not within my defined scope of acoustic remotesensing, both papers deal with the acoustic transmission of information in the ocean. “Channel Capacity in Bits Per Joule” by Kwon and Birdsall is a theoretical investigation of the underwater communication channel. The capacity bound they derive is in terms of the information transfer per unit energy rather than the transfer per unit time. This formulationwasused to emphasizethe importance of the energy constraint that is present in many

remote acoustic system. The paper by Morgera et al. describes the design andperformance of & acoustic telemetry system designed for shallow-water applications. Over the last ten years their has been a dramatic increase in the use of acoustics for remote sensing of the ocean environment. I appreciate the effort of all the authors and reviewers that made it possible to present papers on a variety of remotesensing techniques in a single issue of the IEEE JOURNAL OF OCEANIC ENGINEERING. It should beinteresting for all of us that are interested in oceanicengineering to watch the development of this field in the next few years. JOHN E. EHRENEZXG

Guest Editor

John E. Ehrenberg was born in Spokane, WA, in 1944. He received the B.S. degree from Seattle University, Seattle, WA, in 1966, the S.M. degree from M.I.T., Cambridge, MA, in 1968, and the Ph.D. degree from the University of Washington, Seattle, WA, in 1973. After completing his graduate work, he joined the University of Washington where he presently holds the positions of Research Professor of Electrical Engineering and Principal Engineer at the Applied Physics Laboratory. His research has focused on the application of signal-processing techniques to underwater acoustic, oceanographic, and seismic instrumentation problems. His major emphasis has been in developing acoustic techniques for remotely monitoring and assessing marineorganisms. In 1983, he went on leave from the University of Washington to take the position of Chief Scientist at BioSonics, Inc., in Seattle, WA. In this role, he directs the development and applicationof acoustic techniques for remotely assessing fisheries populations.

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