Showing papers in "IEEE Journal of Oceanic Engineering in 1986"

Channel capacity in bits per joule

Abstract: 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.

Ship detection with high-resolution HF skywave radar

Abstract: This paper presents an overview of ship detection by high-frequency (HF) skywave backscatter over-the-horizon radar (OTHR). Ships have been detected at ranges of 2000 km or more by OTHR that uses sufficient resolution in the radar spatial and Doppler frequency domains. The HF sea-echo Doppler spectrum limits the target signal-to-clutter ratio (SCR), as a function of the ocean wave-height distribution, wind direction, radio frequency, and ship target radial velocity. Maximum sea-clutter spectrum purity, and hence larger SCR, is achieved with the use of stable single-mode ionospheric propagation. Real-time measurement and interpretation of ionospheric propagation features therefore must guide the choice of OTHR operating frequency. Experimental data recorded at the ONR/SR1 Wide Aperture Research Facility (WARF) bistatic OTHR in central California demonstrate reliable ship detection in the Northeast Pacific Ocean. WARF transmits 1-MW average effective radiated power, using a linear frequency-modulated continuous-wave (FMCW) waveform, and receives with a 2.55-km broadside array of vertical monopole element pairs. Swept bandwidths as high as 200 kHz have been used. Sufficient spectral resolution is achieved with a coherent integration time (CIT) of 12.8 s. Longer CIT, and autoregressive (AR) spectral analysis techniques such as Marple's algorithm, have been used to improve Doppler resolution.

Supervisory control system for the JASON ROV

Abstract: A prototype supervisory control system for a remotely operated vehicle (ROV) is described and several key elements demonstrated in simulation and in-water tests. This system is specifically designed to fill the needs of JASON, a new ROV under development that will perform scientific tasks on the seafloor to depths of 6000 m. JASON will operate from the ARGO towed imaging platform, which is currently operational. Supervisory control is a paradigm for combined human and computer control. Several key elements of the supervisory control system are presented. These include the closed-loop positioning system based on a high-resolution acoustic navigation system, a monitoring capability for assessing performance and detecting undesirable changes, and an interface that allows the human operator and the computer system to specify the desired vehicle trajectory jointly.

A deep ROV "DOLPHIN 3K": Design and performance analysis

Abstract: DOLPHIN 3K is a tethered remotely operated vehicle (ROV) system for ocean bottom surveys down to a depth of 3300 m. The system will be completed in fiscal year 1986. This paper describes the design of the system, and analyzes the maneuverability of the vehicle and the transmission performance of the optical fiber data communication system.

Sonar echo statistics as a remote-sensing tool: Volume and seafloor

Abstract: Sonar echoes from unresolved features tend to interfere with each other. When a sonar is moved through an area where features of interest are not resolved, echoes will fluctuate from ping to ping due to the interferences. These fluctuations are the subject of this paper. We classify both fish and the rough seafloor by analyzing the nature of echo fluctuations. Similar statistical techniques apply to all of the problems. We use the techniques to determine, from echo fluctuations, parameters that describe the physical situation. Fish: Two conditions are low density and high density of fish (density is defined as number of fish per unit volume), a) In the low-density case, fish are resolved and their individual echoes are distinguishable from each other. From single fish there are interferences between echoes from the anatomical features causing fluctuations. Echoes from resolved fish also fluctuate because of beampattern effects. After deconvolution (or "removal") of beam-pattern effects, echo amplitudes from resolved fish fit a Rician probability density function (pdf). b) In the high-density case, the fish are not resolved and echoes from individual fish overlap. These echoes interfere and cause the echoes to fluctuate. The Rayleigh pdf describes the instantaneous echo amplitude from a cloud or school of fish. We process echoes by picking the peak amplitude in a finite-time gate and obtain an extremal pdf. We examine both regions of density (a) and (b) and present methods we have developed to determine, from the nature of the fluctuations, properties such as fish size frequency distribution and possibly gross anatomical features, fish density, and to count occasional large fish swimming in a cloud of plankton or near the sea surface. Seafloor: Interference of echoes from individual protuberances such as rocks, nodules, and ripples causes fluctuations. The Rice pdf describes echo amplitudes from the seafloor. We combine Eckart acoustic-scattering theory for a downward-looking sonar and Rice statistics. As a result we describe echo fluctuations in terms of rms roughness of the seafloor and correlation area (product of x and y correlation distances) of the seafloor. We use boomer data to estimate seafloor microrelief as well as to predict echo fluctuations for a variety of other sonars.

Incoherent multibeam Doppler current profiler performance: Part I--Estimate variance

Abstract: We derive statistical lower bounds on the velocity and shear profile estimation accuracies (variances) achievable with both four-beam and three-beam Doppler current profilers. We find that performance (estimate variance) is asymptotic to a constant as SNR increases; this is evidently due to the assumed incoherent scattering properties of the medium. Curves and expressions are developed which define the tradeoffs between accuracy and resolution for two systems manufactured by AMETEK-Straza. The spatial response properties of the systems are addressed in a companion paper [3].

The extension of the Practical Salinity Scale 1978 to low salinities

Abstract: The Practical Salinity Scale (PSS) 1978 is defined only for salinities within the range 2-42. We have investigated the relationship between mass-determined salinity, electrical conductivity, and temperature for salinities between 0 and 2 with the aim of developing an extension to the Practical Salinity Scale 1978. The paper presents our data, on the basis of which the following correction is proposed to extend the validity of the equations defining the scale to the entire 0-42 range: S=\sum\min{i=0}\max{5} (a_{i}+b_{i}f(t))R_{t}^{i/2}-\frac{a_{0}}{1 + 1.5x + x^{2}}-\frac{b_{0}f(t)}{1+y^{1/2} + y + y^{3/2}} where f(t)=\frac{(t-15)}{1 + k(t-15) x=400R_{t} y=100R_{t} and the constant a_{i}, b_{i} , and k are defind by the Practical Salinity Scale 1978.

On the application of HF ocean radar to the observation of temporal and spatial changes in wind direction

Abstract: The application of a 30-MHz narrow-beam ground-wave ocean radar to the observation of wind directions is described. It is found that the \cos^{s} (\theta/2) model for wind-wave directions does not apply in a specific case of shallow water where swell waves are behaving nonlinearly. To experimentally extract unambiguous wind directions from this model requires sampling three different beam angles simultaneously. In practice some time and space stationarity is assumed. Detailed analysis in time and space reveals structure in the transition of the cold front from sea to land which, although unexpected, agrees with coastline observations where they are available. The nature of the structure is only briefly discussed. The response of the \lambda = 5 -m wind waves to the frontal change was two orders of magnitude faster than time constants for similar events previously modeled using pitch-and-roll buoy data. This discrepancy needs to be reconciled before lower frequency radars can be used without ground truth for wind-direction changes.

Remote sensing with the JINDALEE skywave radar

Abstract: The JINDALEE skywave radar is being developed primarily for defense surveillance of Australia's Exclusive Economic Zone (EEZ), but its remote-sensing potential has long been recognized. Studies which commenced in 1974 led to successful measurements of sea-state and inferred surface wind fields in 1977-78 using a prototype radar; the current radar has been observing the Eastern Indian Ocean region since 1982. The JINDALEE radar is now linked to the Australian Bureau of Meteorology regional forecasting centers by a facsimile transmission network. Wind maps surveying over 1 000 000 km^{2} of ocean can be produced automatically in real time at the radar facility and transmitted directly to forecasters. This capability, which became operational in January 1985, is supported by active research programs directed at improving the scope and accuracy of the measurements, as well as investigating a variety of meteorological and oceanographic phenomena. This paper presents an overview of the JINDALEE remote-sensing program with emphasis on the design and capabilities of the radar system.

Correcting for distorted antenna patterns in CODAR ocean surface measurements

Abstract: CODAR systems employ compact antenna elements such as electrically small loops and monopoles to extract bearing information in ocean surface observations. Past analysis methods have assumed that these element patterns are perfect, i.e., cosine and omnidirectional. Operations from metallic offshore platforms usually distort these patterns because of unavoidable objects in their near field. When such distortions are ignored, previous methods are shown to produce \sim35\deg rms bearing errors. Therefore least-squares methods are presented and demonstrated that deal with differential element pattern distortions. It is shown how the required relative patterns are easily measured by a boat circling the antenna, and these patterns are then stored as look-up tables in the least-squares inversion methods. Relative patterns (i.e., one element pattern divided by the other), rather than absolute, are all that are required for extraction of surface current, wave-height directional spectra, wind direction, and drifting transponder information with CODAR.

Multifrequency HF radar observations of currents and current shears

Abstract: Techniques have been developed for using high-frequency (HF) surface-wave radar to measure ocean currents and vertical current shears in the upper 1 or 2 m of the ocean surface. An HF radar can precisely measure the phase velocity and direction of propagation of ocean waves whose wavelength is one.half the radar wavelength. In the absence of a current, the speed of the waves is given by the still-water dispersion relation. An underlying current will modify this speed. The radar measures the actual phase velocity through a Doppler shift, and the wavelength of the ocean wave is known through the first-order Bragg scattering relation, so a difference between observed and theoretical stillwater phase velocity can be calculated. In addition, longer ocean waves are affected by currents at deeper depths than are shorter ocean waves. By measuring the phase velocity at several different wavelengths, it is possible to measure a vertical current shear in the top 1 or 2 m of the ocean surface. This is a measurement that is very difficult to make by any other means. A portable coherent pulsed-Doppler HF radar system was developed at Stanford and used in several experiments, both on land on the California coast and on board a ship during the Joint Air-Sca Interaction (JASIN) experiment. The land-based experiments demonstrated that a current could be measured by an HF radar, and that its value agreed well with that measured by in-situ drifting spar buoys. In addition, there was evidence of a vertical current shear, both from the radar measurements and from the buoy measurements. The JASIN experiment was an attempt to apply these techniques to the measurement of surface current and current shear in the open ocean. The radar system was installed on board a ship, along with a receiving antenna consisting of a steerable phased array of eight wide-band loops. The steerable antenna was quite rugged and performed as expected. It produced antenna patterns consistent with the physical aperture of the array. The wind velocity during the JASIN experiment was quite low, so wind- and wave-generated currents were quite small. Nevertheless, there is some evidence of a current shear. Its magnitude is small and near the resolution limit of the radar, but it appears to be somewhat higher than estimates based on either the wind or wave conditions alone, but less than the estimates based on the sum of the two components.

Incoherent multibeam Doppler current profiler performance: Part II--Spatial response

Abstract: We consider the spatial response and sampling properties of ship-mounted four-beam and three-beam incoherent Doppler current profilers. Spatial properties are an issue because such systems cannot make a point measurement of the current field, and because the actual current field is nonuniform. We obtain expressions for the effect of beamwidth, beam separation, and block averaging on system response. We observe that spatial aliasing may occur above some critical depth, and that pulsewidth reduction cannot improve vertical resolution beyond a limit imposed by system geometry. We also find that, for the three-beam system, the along-track component is irrevocably contaminated (to some degree) by the cross-track component as a result of its nonorthogonal beam geometry. The velocity and shear estimate variances achievable with these systems are calculated in a companion paper [1].

Frequency management support for remote sea-state sensing using the JINDALEE skywave radar

Abstract: Successful operation of a skywave (over-the-horizon, OTH) radar in a remote sea-state sensing mode is critically dependent upon the application of comprehensive frequency management techniques. In addition to the problem of selecting a frequency yielding an adequate signal-to-noise ratio in the geographical area under investigation, attention must be paid to minimization of ionospheric multimode and other phenomena capable of distorting or convoluting the sea backscatter spectrum. This paper describes the manner in which these problems have been addressed in the JINDALEE skywave radar, and relates the practical difficulties inherent in the task of frequency management in support of an OTH radar involved in sea-state sensing. Measurement techniques include backscatter and oblique-incidence sounding, HF spectral surveillance, and a low-powered frequency-agile "miniradar" capable of operating in either a conventional backscatter or alternate oblique-incidence mode. In addition to providing the main radar with real-time frequency management advice, a principal emphasis has been the acquisition of a synoptic data base suitable for off-line statistical analysis.

Surface acoustic wave devices

Abstract: This book is the proceedings of the NATO Advanced Study Institute held at Lueneburg, Germany, July 30-August 10, 1984. As such it is part of the NATO Advanced Study Institute Series, published by an international board of publishers in conjunction with the NATO Scientific Affairs Division. Specifically, the book is Volume 151 of Series C: Mathematical and Physical Sciences. There are 63 papers published in this volume. The authors represent 13 different countries. There are 17 papers from the United States, plus two shared with Australia; nine from France; seven each from England and Germany; four each from Canada and Italy (including La Spezia); three each from Norway and Australia (not counting the two shared with the United States); two each from Portugal and South Africa; and one each from Denmark, Japan, and Turkey. According to the editor, a “major effort was made to obtain a commensurate contribution of tutorial and advanced research papers.” It is his hope “that the material in this volume may be equally well suited for students getting an introduction to some of the basic problems in underwater signal processing and for the professionals who may obtain an upto-date overview of the present state of the art.” In this reviewer’s judgment, he has succeeded. The subject matter of the book is divided into three categories (throughout the book, advanced research papers appear side-by-side with tutorial papers and are not specifically identified as one or the other). Part 1, devoted to the acoustical background of signal processing, contains 14 papers. The first paper in this category is entitled “Twenty Years of Signal Processing,” and is by J. W. R. Griffiths. This is a great historical paper which explains to the uninitiated reader where this book “is coming from.“ There are papers on ambient noise, propagation, and target characteristics. Part 2, theoretical and practical aspects of signal processing, is the longest of the book, with 36 papers. There are papers on time-delay estimation, spectrum analysis, and adaptive array processing. Part 3, techniques and applications, has 13 papers. There is an interesting mixture of new technologies here: digital signal processing, optical signal processing, expert systems (part of the field of artificial intelligence), ocean tomography, spaceborne remote sensing, and passive synthetic aperture. In addition, five workshops were held and their summaries published: environmental modeling and signal processing, test tanks and measuring facilities, nonlinear filtering, signal processing, and expert systems. AI1 of these topics are exciting’ and current, and many of the authors are leaders in their fields. This reviewer finds the inclusion of expert systems material, to choose an example, to be quite interesting. “Expert Systems for Ship Noise Interpretation,” by Maksym is a good tutorial paper. The next paper, “Comparison of the Statistical and the Expert System Approach for ‘the Interpretation of Ship Noise,” by Bendig and Wittig is an application of these ideas. (Of course, as is befitting a pioneer discipline like this one, there is room for disagreement with specific points in the treatment of the subject). After eading these two papers, it is interesting to read “New Advances Toward Ocean, Acoustics and Space Integration,” by Scully-Power and Stevenson, particularly the following quotation (referring to the question of manned versus unmanned space research): “While there is no question that computers can search out the “gems from the pebbles,” there is little evidence to assure us that the computer can distinguish the ‘diamond from the zircon.’ ” This sounds like the makings of a great debate, especially in the wake of the Challenger tragedy. It is difficult to single out papers for discussion-they are all so good. One that caught this reviewer’s eye was “Passive Synthetic Aperture Sonar-An Analysis of the Beamforming Process,” by Pusone and Lloyd. Curiously, one has always associated synthetic aperture ideas with active sonar! Another paper that provoked considerable discussion was “Detection and Classification Phenomena of Biological Systems,” by Altes. To quote one sentence from the conclusion, “Our appreciation of biological sensory systems is limited by our own knowledge, but we can perceive trends and signal representations in animals that could improve our own technology.” Interesting! In conclusion, the purchase of this book is recommended for those who could not attend the study institute-and that includes this reviewer. The book is certainly expensive, but, for most of us, it is less expensive than a trip to Germany!

The second-order shallow-water hydrodynamic coupling coefficient in interpretation of HF radar sea echo

Abstract: Extraction of wave.height directional spectral information from high-frequency (HF) radar sea echo requires the use of hydrodynamic and electromagnetic second-order coupling coefficients obtained from a perturbational expansion of the nonlinear boundary conditions at the ocean surface. To present, the hydrodynamic coupling coefficient derived for deep water has been given. Since most coastal HF radar observations are made in water shallow compared with the dominant ocean wavelength, that solution has proven inadequate for those applications. This paper derives the more general expression for water of arbitrary depth, and demonstrates its validity against measured data. The hydrodynamic contribution increases in importance as waves of constant energy move into shallow water. The use of these results for interpretation of both narrow-beam and CODAR data is discussed.

Measurement of seabed topography by multibeam sonar using CFFT

Abstract: A precise ocean bottom map for ocean surveying and dredging is desired. Especially in dredging, it is essential to know the seabed topography in real time without being affected by scatterers (for example floating sand and mud) in the seawater during work. To meet these requirements, the multi-narrow-beam sonar system (MBSS) has been developed. The MBSS forms beams with the use of the complex fast Fourier transform (CFFT) algorithm. In the MBSS, arithmetic mean processing is employed to eliminate echo from scatterers and the measurement error due to the oblique incident angle is reduced by peak value detection processing. By using these processes, an ocean bottom map can be accurately obtained. It is both theoretically and experimentally shown that the distribution of echo intensity from scatterers is approximated by the Rayleigh probability density function. The arithmetic mean of four to eight successively received echoes from scatterers reduces the variance of the echo intensity distribution by 6 to 12 dB.

Space-time acoustic scintillation analysis: A new technique for probing ocean flows

Abstract: We present a new approach to the measurement of ocean flows. The technique exploits the coherence of the fine structure in the ocean under the influence of advection. Sound passing through this fine structure is modulated in space and time, so that the evolution and motion of the resulting pattern at a distant receiving plane contains information about the intervening flow field. The details of the fine-scale structure itself may also be recovered, to an extent determined by the complexity of the transmitter and receiver array. Two special cases of oceanographic interest are considered. First, a fully developed turbulent flow, such as that encountered in tidal channels, for which the scale of fine structure contributing to the scintillation field lies within the inertial subrange, and second, the internal wave field more generally applicable to the open ocean. We describe an experimental test of the concept. Sound traveling across a 0.66-km path in Cordova Channel, British Columbia, Canada, is detected by two closely spaced receivers. The flow speed is derived using three separate estimators and the results compared with current measurements obtained from a moored current meter. Agreement between the two types of measurement is excellent.

Guidance and control architecture for the EAVE vehicle

Abstract: For the past several years the Marine Systems Engineering Laboratory (MSEL) has directed its efforts towards the development of the technologies required for unmanned untethered submersible vehicles. The current focus of those efforts is to develop a system architecture that will allow the implementation of a knowledge-based guidance and control system. The goal of this effort is to implement a simple system which has addressed the basic problems and will allow for expansion as insight is gained from field testing the concepts using the Experimental Autonomous Vehicle (EAVE) system at MSEL. This paper considers those factors that have driven the development of an architecture which is being implemented in the EAVE vehicle system. Its intent is to focus on those issues that have guided the application of artificial intelligence (AI) techniques to meet the requirements of the system and its mission. The architecture being implemented is outlined and some of its features detailed.

A knowledge-based system framework for environmental perception in a subsea robotics context

Abstract: A knowledge-based system structure capable of providing the information and processing framework required by an unmanned autonomous submersible is presented. The component parts of such a robotic system are first discussed, followed by a description of the information and processing subsystems that are, therefore, required. The framework within which they are implemented uses the architecture of a blackboard system to specify a modular unit (the blackboard cell) capable of manipulating the diverse types of process and uncertain information encountered. In this respect, the cell represents a departure from previously reported blackboard implementations. Interconnection of these modular units uses a standard interface, configured as a globally accessible blackboard. As a means of evaluation, a software implementation of the architecture is applied to the sonar perception subsystem. The image processing tools at the heart of a simple knowledge base are described, and results are presented to show a small sequence of system execution, and some examples of candidate object and shadow areas derived from real sonar images. It is demonstrated that the blackboard cell allows a rich feedback structure to the flow of information and processes, allowing both model and data-driven actions to support the formation of competitive and cooperative solutions.

The role of the gravity-wave dispersion relation in HF radar measurements of the sea surface

Abstract: Recent experimental and theoretical findings raise interesting questions about the applicability of the normal gravity-wave dispersion relation at wave frequencies that exceed the spectral peak frequency. The use of the dispersion relation in analysis of HF radar Doppler sea echo is examined in this paper. Drawing on the results of perturbation theory for wave-wave nonlinear interactions, we show that this relation, so essential to echo interpretation in terms of current and wave information, can be employed with no degradation in accuracy for current measurement when the dominant wave frequency is considerably less (by as much as 10) than the radar Bragg resonance frequency. This finding is supported by comparisons of currents measured by HF radar with "surface truth;" the first-order echo must only be identifiable in order to be used accurately. Wave-height directional spectral information can be extracted from the second-order echo at a given radar frequency up to the point (in wave height) where the perturbation solution employed in the inversion process fails; then a lower radar frequency must be used. On the other hand, most conventional wave measuring instruments should not use the dispersion relation for interpretation of data well beyond the spectral peak, because they do not observe wave height as a function of both space and time independently, as does HF radar.

HF radar measurements of ocean wave parameters during NURWEC

Abstract: The Netherlands/U.K. Radar, Wavebuoy Experimental Comparison (NURWEC) was a collaborative experiment between the University of Birmingham, the Dutch Rijkswaterstaat (RWS), and the Institute of Oceanographic Sciences (IOS), and was designed to assess the availability and accuracy of ocean wave data using the University of Birmingham (BU) ground-wave radar. The experiment lasted for two months during which time over 110 h of radar data were collected and transferred to Birmingham for wave-parameter extraction. Estimates of significant wave height are found to have a standard deviation of 15 percent (by comparison with the Datawell WAVEC buoy) if they were less than 2 m, and of 20 percent if greater, although in this case there is a positive bias of 41 percent. A mean difference in first-moment wave period of 0.6 s over the range 3-10 s is found. The long-wave directional spectrum is found to be in good agreement in the limited number of cases for which comparison is appropriate, with differences of 21 percent in height, 6 percent in period, and 15\deg in direction. The limitations to these measurements are identified, and suggestions are made for improvements in radar operations and for further research.

A synthetic aperture sonar system capable of operating at high speed and in turbulent media

Abstract: Despite their potential ability to produce highly resolved images of the seabed, synthetic aperture sonars are not widely used. The primary reason for this restricted use is that most synthetic aperture systems are based on the radiation and detection of short-duration modulated pulses. Due to the low speed of sound in water, the pulse repetition frequency is low and so it has been difficult to maintain the required pulse-to-pulse phase coherence. This paper describes a new approach to synthetic aperture sonars based on continuous transmission with some form of frequency modulation. That is, a sonar that transmits and receives continuously but uses some form of frequency coding (in this case a linear frequency sweep) to determine range. Using a continuous transmission frequency modulated sonar it is possible to make a synthetic aperture sonar that can produce coherent apertures many wavelengths long. In addition to the combination of synthetic apertures and continuous transmission frequency modulation, further modifications are suggested to reduce the effect of lateral towfish movement and the effects of medium turbulence resulting in random path-length variations.

A review of methods of remote sensing of sea-surface conditions by HF radar and design considerations for narrow-beam systems

Abstract: Present achievements in HF ground-wave radar remote mapping of surface currents and wave characteristics are reviewed, and the features of the different systems that are in use are described. Particular emphasis is given to the problems remaining to ensure effective mapping of nondirectional wave parameters and wave directional spectra over a sector of sea extending up to a range of 150-200 km from a coastal or island site. As an example of current progress, measurements in the United Kingdom of first-moment wave period by single-site radar have achieved measurements to within 15 percent of buoy data over a range from 4 to 9 s. Measurements of significant wave height gave standard deviations of 15 and 20 percent for wave heights less than and greater than 2 m, respectively. In the latter case, a bias of 40 percent was found, which later work suggests is reducible to about 14 percent, using a model-fitting technique. To achieve directional wave spectrum measurement with a single radar requires some 40-dB ratio between the Bragg line return and the noise floor of the Doppler spectra. A two-site radar has been shown to yield advantages for directional spectrum measurement. The design of steerable narrow-beam radars in the 6-25-MHz band to achieve such performance is discussed. Suitable transmitted waveforms to achieve good rejection of interference and to be acceptable in the HF band are put forward. The operational experience in the United Kingdom, obtained with a particular FM interrupted continuous-wave radar system associated with a steerable array, is reported. Results of simulating the effect of the antenna radiation pattern on the observed Doppler spectra are illustrated. The principal effects are on the weaker Bragg line and the second-order continuum around it. This can result in errors in deduced surface wind direction and directional wave spectrum. Finally, some useful results on propagation monitoring of a skywave radar with the aid of a range-Doppler graphical output are reported.

Current velocity measurements using acoustic Doppler backscatter: A review

Abstract: Acoustic Doppler backscattering techniques have been exploited for measuring water velocity for nearly 40 years. Although most early applications centered on measuring ship speed, much work has been done during the last 20 years to adapt the techniques to measure oceanic velocity fields. The purpose of this paper is to summarize the work that has been done and to discuss the state of acoustic Doppler technology.

CODAR in Germany--A status report valid November 1985

Abstract: The use of CODAR by the University of Hamburg has extended to a wide variety of experimental and oceanographic activities over the last three years. These have ranged from Arctic studies from land and ships to observations of the Dead Sea, all yielding surface current data. Hardware improvements have been investigated, including IF amplifier changes and loop-antenna arrays for shipboard operation.

An evaluation of least-squares and closed-form dual-angle methods for CODAR surface-current applications

Abstract: Extraction of surface currents from first-order CODAR sea echo requires use of a model that allows signals from two bearings to contribute to the Doppler spectrum at a given frequency. This is called the dual-angle situation, and it applies over much of the coverage area. Two dual-angle techniques have appeared in the literature: a least-squares algorithm used with a crossed-loop antenna system, and a closed-form approach applied to a four-element square array. We evaluate these methods against realistic signal and noise scenarios encountered in CODAR operations, and study noustatistical biases remaining after infinite-ensemble averaging of the input voltage cross-spectral data. Based on these simulations, biases produced with the closed-form methods exceed those for the crossed-loop system analyzed with least squares by typically 150 percent.

Remote sensing of icebergs by ground-wave Doppler radar

Abstract: A software detection model has been developed to predict the returned Doppler spectrum for an iceberg target for ground-wave Doppler radars. This software model is based on proposed new estimates for the backscattered Doppler-dependent iceberg cross section for assumed iceberg models, as well as the backscattered Doppler spectrum from the ocean surface. The model includes estimates for forward and reverse transmission losses, based on classical spherical earth derivations. In addition, the transmission losses account for the effects of surface roughness through a modified surface impedance. Standard estimates for man-made and atmospheric noise have been considered in the detection model. A comparison between the results predicted by the detection model and data acquired during an experiment conducted at Byron Bay, Labrador, Canada has been effected. The hardware used for the experiment was an HF Doppler radar operating at 25.40 MHz. The transmitting antenna was a three-element Yagi array and the receiving antenna a 24-element narrow-beam linear array. By using iceberg ground truthing information the Doppler spectrum for individual icebergs was predicted using the software model. The predicted spectra were compared with the received spectra on a target signal-to-noise power ratio basis. The results of this comparison give a degree of confidence to the detection model and show that ground-wave radars are effective ice hazard remote sensors.

Simulating ocean acoustic tomography measurements with Hamiltonian ray tracing

Abstract: Tomographers map mesoscale ocean structure by inverting acoustic travel-time measurements through networks of underwater paths. To know where to deploy sensors and how to interpret their measurements, one must first understand the "forward problem," that is, how the sound channel and mesoscale features refract sound in three dimensions, and how such refraction alters the pulse-arrival sequence. We use a Hamiltonian ray-tracing program called HARPO to compute the refraction by continuous three-dimensional ocean models and to display the results in ways that add insight about refractive effects. We first simulate propagation in a simple range-independent sound channel, showing how pulse-arrival sequence depends on channel parameters and sensor placement. Next, we add linear range dependence and show that it is hard to extract range information from pulse measurements at one range. Finally, we add a simple model of a mesoscale eddy including its currents and show that deflection and splitting of the sound channel significantly alter the pulse-arrival sequence. Two diagrams that have not been widely used before are useful ways to display the arrival-time and ray-focusing perturbations caused by changes in ocean structure: they are plots of range versus launch angle and range versus travel time. Examples of azimuthal deflection, three-dimensional eigenrays, and reciprocal propagation through eddy currents are shown, and simplified methods for estimating the travel time of three-dimensional eigenrays are evaluated.

TROJAN: Remotely operated vehicle

Abstract: Following success in remotely operated vehicle (ROV) designs such as MMIM, IZE, SOLO, ORVIL, CIRRUS, PIC, and OBSERVER, Slingsby Engineering Limited (SEL) has now developed its technology to build and prove a new ROV, TROJAN. TROJAN is a dedicated subsea support vehicle, which results from extensive market research being applied to a base of 15 years in the underwater technology market. SEL has been at the forefront of the evolution of this market from early manned submersible days, followed by atmospheric diving systems to ROV's. The TROJAN ROV is conceived as a subsea workhorse offering a new dimension to subsea support operations. TROJAN benefits by incorporation of proven technology developed by SEL in recent years on SOLO, PIC, and CIRRUS projects. Using standard components, TROJAN's high reliability and efficient performance offer a versatile and cost-effective work system.