Vertical synthetic aperture sonar for ocean acoustic tomography

This paper presents the result of a first attempt to achieve a vertical synthetic aperture in the ocean for SOFAR multipath identification. The experiment was conducted during the deployment of a tomographic array in the Mediterranean Sea. Drifting the hydrophone up or down from a ship while listening to the transmitted signal created a powerful synthetic aperture at 400 Hz. Wide-band phase-coded signals, typically used in ocean tomography, were found suitable for this application. The displacement length was 100 m and the hydrophone velocity 1 m/s. The obtained resolution of 1° enabled all the rays in the tested middle range configuration to be resolved and identified. Most of them could not have been resolved with a static hydrophone. Several Doppler processing methods are presented. The narrowband approximation leading to fast algorithms is discussed. Phase time series of individual paths obtained with an array-like wave separation method show that the phase coherence of the different multipaths is nearly perfect. An angle/velocity calibration method and a first rough inversion are finally presented     

Track Parameter Estimation from Multipath Delay Information

It is desired to track the location of an underwater acoustic source with range difference measurements from a stationary passive array. Many times, the array has only one or two sensors, and the multipath and intersensor range difference measurements are insufficient to localize and track a source moving along an arbitrary path [1]. Here, we propose to track sources with one- or two-sensur stationary passive arrays by making the simplifying assumption that the source's path can be described by a small set of so-called track parameters. Range difference information can then be used to estimate the track parameter set rather than the source location as a function of time. In this paper, we choose the track parameters to specify a straight-line constant-velocity constant-depth path. Cramer-Rao bounds are presented for estimating these track parameters from the time history of multipath and intersensor range difference measurements. It is shown that this track parameter set cannot be accurately estimated from the time history of a single multipath range difference without side information (an independent velocity estimate, for instance), although multipath and intersensor range difference measurements from a two-sensor array are generally sufficient to estimate the track parameter set. Computationally efficient techniques are presented which estimate track parameters from range difference measurements taken from one- and two-sensor arrays. Monte-Carlo simulations are presented which show that these techniques have sample mean-square error approximately equal to the Cramer-Rao bound when a single multipath range difference and an independent velocity estimate are available. The sample mean-square error is shown to be in the range of two to ten times the corresponding Cramer-Rao bounds when these techniques are applied to two-sensor range difference data.     

Sensor position estimation and source ranging in a shallow water environment

Underwater acoustic transient signals are generated mechanically at known positions along a wharf. These signals are received by a wide aperture planar array of four underwater acoustic sensors, whose positions relative to the wharf are unknown. A method is described that enables the positions of the sensors to be estimated from accurate differential time-of-arrival measurements (with 0.1 /spl mu/s precision) as the signal wavefronts traverse the array. A comparison of the estimated positions with the nominal positions of the first three sensors, which form a 20-m-wide aperture horizontal line array, reveals a 2-cm displacement of the middle sensor from the line array axis. This slight bowing of the line array results in overranging (bias error of 3%) when the wavefront curvature method is used with the nominal collinear sensor positions to locate a static source of active sonar transmissions at a range of 59.2 m. The use of the spherical intersection method coupled with the estimated sensor positions of the line array provides an order of magnitude improvement in the range estimate (within 0.3% of the actual value). However, systematic ranging errors are observed when the sound propagation medium becomes nonstationary. Next, the differences in the arrival times of the direct path and boundary-reflected path signals at the middle sensor of the wide aperture line array are estimated using the differential phase residue of the analytic signal at the sensor output. These multipath delays are used to estimate the range and depth of the source. Although the average value of the multipath range estimates is within 0.5% of the actual value, the variance of the range estimates is 50 times larger when compared with the results of the spherical intersection and wavefront curvature methods. The multipath delay data are also processed to provide a reliable estimate of the temporal variation in the water depth enabling the tidal variation to be observed.     

Motion-Compensation Improvement for Widebeam, Multiple-Receiver SAS Systems

The effect that uncompensated motion errors have on synthetic aperture sonar (SAS) imagery can be severe. Time-domain beamforming SAS reconstruction is able to compensate arbitrary track errors, but the more efficient frequency-domain reconstruction algorithms, such as the range-Doppler, chirp-scaling, and wave number (aka range migration or Stolt-mapping) algorithms do not allow for simple compensation, especially for widebeam sonars. Data processed via these block algorithms is usually compensated before azimuth compression in a computationally inexpensive preprocessing step. Unfortunately, this compensation assumes a narrowbeam geometry, causing blurring in high-resolution images collected with widebeam sonars. In this paper, we demonstrate a new technique for compensation of large, but known, motion errors in data collected with widebeam geometry sonars. The technique relies on obtaining angle-of-arrival information from the multiple-receiver array configuration typical in high-resolution SAS systems. The new method of compensating for motion errors was found to significantly outperform the previous techniques in a simulation of point-reflector imagery.      

Passive systems theory with narrow-band and linear constraints: Part II -- Temporal diversity

This paper is part of a series of three papers studying passive tracking problems arising in navigation and positioning applications. The basic question here lies with the determination of the position and dynamics of a point source being tracked by an omnidirectional observer, through demodulation of the Doppler effect induced on the radiated signals by the relative motions. A simple model, fitting a finite parameter nonlinear estimation context, is developed, the receiver designed, and its mean-square error performance studied. It is shown that, besides the speed and angle estimation, simultaneous global range passive tracking is possible. The signal model precludes range acquisition from synchronous measurement of the absolute phase reference: the global range estimation is attained by processing the higher order temporal modulations (varying Doppler). Quantifying the statistical and geometric performance tradeoffs, the work presents simple expressions and graphical displays that can be used as design tools in practical passive tracking problems. A subsequent paper considers the space/ time coupling issues, generalizing the study to the context where a moving source is tracked by a directional array.     

Multiple terminal acoustic communications system design

A design is presented for a system providing highly reliable command and control acoustic communications between a mother ship and a number of small fast submersibles. The small submersibles may be employed for underwater mining, exploration, bottom mapping, or military surveillance. Modulation and coding design is presented; the techniques discussed provide multiple protection against multipath and fading, high reliability, acceptable transmitted signal total time duration, simplicity, and economy. The required decision point signal-to-noise ratio (SNR) for Rayleigh fading conditions is derived for the modulation and coding design. Particular attention is paid in the receive signal processing to the Doppler (relative velocity) and Doppler variation (relative acceleration) problems inherent in a scenario with mobile endpoints. A Figure-of-Merit (FOM) calculation is provided for typical geometrical and environmental parameters. It is shown for a realistic source level that the required SNR can be achieved at long range with considerable endpoint relative motion.     

Inversion for the compressional wave speed profile of the bottomfrom synthetic aperture experiments conducted in the Hudson Canyon area

In September 1988, a series of acoustic propagation experiments were conducted in the Hudson Canyon area. These included synthetic aperture experiments in which a source transmitting a set of four pure tones was towed toward/away from a vertical array of 24 receivers. Data obtained at 50 Hz during one of the synthetic aperture experiments are used to obtain a model for the compressional wave speed profile in the bottom using a modal inverse method. This model is further refined using 175 Hz data. The ability of the inferred model to predict the field at 50 Hz and higher frequencies is examined     

SAMI: a low-frequency prototype for mapping and imaging of theseabed by means of synthetic aperture

The objective of the Synthetic Aperture Mapping and Imaging (SAMI) project was to develop and to test at sea a wide-band synthetic aperture sonar prototype, capable of providing high-resolution seafloor images together with bathymetry maps. This system used the motion of a physically small array in order to synthesize a longer array, providing images with an across-track resolution independent of both range and transmit frequency. Such systems are clearly very relevant to the high-precision long-range (low-frequency) imaging of the sea bottom. The project has led to the construction of a prototype tested at sea on several well-known areas for comparison with existing images and maps. These areas included several types of sea bottom, depths, and geological structures. The results obtained in real time, on-board ship, have shown the relevance of the proposed wide-band techniques. The many profiles produced have provided high-resolution images and maps of various seafloors. Interpretation by geologists showed that the system was capable of providing the same or finer detail than a deep-sea short-range, high-frequency system and maintained a higher resolution over a wider swath. The sea data processed have shown that the system provided maps with a cubic meter voxel. The resolution cell is constant over the whole range (50 to 2500 m) thanks to the dynamic focusing of the synthetic aperture. Postprocessing of a part of the data stored during the experiments has been carried out in the laboratory. This work has shown that techniques such as autofocusing can give an increase in resolution (i.e., gain in contrast and resolution of about 3 dB). The results displayed in the paper show the relevance of the techniques developed to the provision of a complete high-performance imaging tool for the oceanographic community     

A new passive synthetic aperture technique for towed arrays

An algorithm that synthesizes apertures in the beam domain using FFT transformations and performs coherent processing of subaperture signals at successive time intervals is presented. Experimental tests of the algorithm show that for ocean environments with spatial coherence longer than the synthetic aperture length and for signals with temporal coherence longer than the required acquisition time, a synthetic array gain is achieved which roughly corresponds to the length of an equivalent fully populated array. In the experiments, transducer generated CW with phase stability and pseudorandom signals were used. Limitations on the spatial and temporal coherence were introduced only by the medium, the temporal coherence of the pseudorandom signal, and the shape and stability of the line array used     

Position Correction Using Echoes From a Navigation Fix for Synthetic Aperture Sonar Imaging

In synthetic aperture sonar (SAS), the platform position must be known sufficiently accurately for signals to be added coherently along the synthetic aperture. Often, the onboard navigation system is insufficiently accurate by itself, so corrections are needed. A well-known method is the displaced phase center antenna (DPCA) procedure for correcting platform position using seabed echoes. DPCA methods have the advantage of insensitivity to changing interference patterns, moving specular reflection, and changing occlusion, with aspect. However, when seabed echoes are unusable, either because they are too weak, or because they are corrupted by multipath, the seabed DCPA method may fail. Therefore, we present an alternative DPCA method using sonar echoes from a suitable navigation fix, based on an object detected after standard beamforming. In our proposed system, look angle is obtained by tracking the centroid of the rectified image of the fix object. When the standard DPCA correction equations are modified for a fixed reflector, it turns out that they provide incremental range and look-angle errors, precisely the values required when the target itself is used as the navigation fix. Moreover, the values obtained are then self-compensating for errors in estimating seabed depth or forward motion of the platform. The navigation fix is selected by bracketing in range, and beamforming overlapping subsets of the receiver array. In this paper, we present experimental results at transmitter frequencies of 25 and 100 kHz where our method enabled well-focused SAS images to be generated with little recourse to other navigation information. Hence, SAS can be carried out, even when a sophisticated inertial navigation system (INS) is not available.      

A circular passive synthetic array: an inverse problem approach

Based on the general concept of the inverse acoustic radiation problem, the temporal scanning of a stationary acoustic field along a closed contour is used to simplify the measurement approach for obtaining information on source directionality. The mathematical formulation is derived from a model of the two-dimensional acoustic field. The formulation of the inverse problem is also investigated to establish a methodology for improving the angular resolution of the array processing. The fundamental relationship between the sound sources and the circular passive synthetic array is explored, utilizing existing mathematical methods, in order to develop the processing algorithm. Other subjects of practical interest, such as directional ambiguity, effect of Doppler frequency, interference noise, and processing gain are discussed. It is concluded that the results can be used to establish guidelines for engineering design and deployment of this type of synthetic array, and to further exploit the new array signal processing technique     

Broad-band geoacoustic inversion in shallow water from waveguideimpulse response measurements on a single hydrophone: theory andexperimental results

The paper discusses an inversion method that allows the rapid determination of in situ geoacoustic properties of the ocean bottom without resorting to large acoustic receiving apertures, synthetic or real. The method is based on broad-band waterborne measurements and modeling of the waveguide impulse response between a controlled source and a single hydrophone. Results from Yellow Shark '94 experiments in Mediterranean shallow waters using single elements of a vertical array are reviewed, inversion of the bottom parameters is performed with an objective function that includes the processing gain of a model-based matched filter (MBMF) receiver relative to the conventional matched filter. The MBMF reference signals incorporate waveguide Green's functions for known geometry and water column acoustic model and hypothesized bottom geoacoustic models. The experimental inversion results demonstrated that, even for complex environmental conditions, a single transmission of a broad-band (200-800 Hz) coded signal received at a single depth and a few hundred forward modeling runs were sufficient to correctly resolve the bottom features. These included the sound speed profile, attenuation, density, and thickness of the top clay sediment layer, and sound speed and attenuation of the silty clay bottom. Exhaustive parameter search proved unequivocally the low-ambiguity and high-resolution properties of the MBMF-derived objective. The single-hydrophone results compare well with those obtained under identical conditions from matched-field processing of multitone pressure fields sampled on the vertical array. Both of these results agree with expectations from geophysical ground truth. The MBMF has been applied successfully to a field of advanced drifting acoustic buoys on the Western Sicilian shelf, demonstrating the general applicability of the inversion method presented     

Modal Inversion Analysis for Geoacoustic Properties of the New Jersey Continental Shelf in the SWAT Experiments

In this paper, inversion for bottom sediment properties at a site on the New Jersey continental shelf is studied as part of the Shallow Water Acoustic Technology (SWAT) project. A source towed at a constant water depth over a range of some tens of kilometers transmitted low-frequency continuous wave (cw) signals, which were measured on a bottom-moored vertical line array of receivers. For the along-shelf geometry, the zeroth-order asymptotic Hankel transform is then applied to the acoustic field at 50 Hz measured on the resulting synthetic aperture horizontal array created at each receiver depth. The resulting horizontal wave number spectra, which have peaks corresponding to the mode eigenvalues, are observed to have slightly different values at different receiver depths, and therefore, stochastic mode inversion is exploited to utilize all of the observed peak position information. The estimated sound-speed profile (SSP) for the upper 10 m of sediment is then compared with an inversion result obtained using midfrequency (2–16 kHz) chirp sonar pulses reflected at normal incidence from the sediment. Although obtained using totally different inversion techniques, both estimated profiles are shown to be in good agreement in the top 10 m of sediment. The acoustic field simulated using the inverted SSP also agrees well with the measured acoustic field at each receiver depth. Furthermore, simulated sound fields which use this profile as input data are shown to be effective in predicting the measurements obtained at a different frequency (125 Hz) and for a different (cross-shelf) geometry.      

Matched-field processing in a range-dependent shallow waterenvironment in the Northeast Pacific Ocean

This paper describes matched-field processing (MFP) of data collected in shallow water off the western coast of Vancouver Island in the Northeast Pacific Ocean. The data were collected from a vertical line array (VLA) as part of the PACIFIC SHELF trial carried out on the continental shelf and slope during September 1993, sensors in the 16-element VLA were evenly spaced at depths between 90 and 315 m, while the sound source was towed along radial paths or arcs. In this paper, we present results of the analysis of data from a continuous wave (CW) source which was towed downslope at a depth of 30 m in water from 150 to 375 m deep, in order to model the range-dependence of the acoustic propagation efficiently, the replica fields were calculated using the adiabatic normal mode approximation. This approximation was considered appropriate for the bottom slopes of the environment. Using sparse bathymetric data, a water sound speed profile and estimates of bottom properties, MFP correlations on individual ambiguity surfaces were found to be greater than 0.9 for the strongest signals. On account of environmental mismatch, the source position could not be determined unambiguously from most of the ambiguity surfaces even at high signal-to-noise ratios. Nevertheless, when an efficient linear tracker was applied to the ambiguity surfaces to find tracks, the source track was recovered at both low and high signal-to-noise ratios, this tracker performs the analysis at a constant depth and reports the track with the highest estimated track signal-to-noise ratio     

Synthetic aperture sonar for seabed imaging: relative merits ofnarrow-band and wide-band approached

The advantages of using wideband sonar systems in underwater acoustical imaging by means of synthetic aperture (side-looking) sonars are described and illustrated through simulation examples. The simulations are conducted for two cases of sonar platform motion: perfect trajectory and disturbed trajectory. Several schemes used for wideband synthetic aperture processing are investigated and their relative merits (resolution and complexity) in the case of both disturbed and perfect trajectories are evaluated. Quantitative image evaluation is initially achieved through the evaluation of performance as regards resolution. The problems involved in the definition of the image quality are discussed     

Depth measurement of remote sources using multipath propagation

A submerged acoustic source radiates narrowband Gaussian noise. Its signal propagates to a remote, large aperture vertical array over a multipath channel whose characteristics may or may not be fully known. The primary concern of this study is the accuracy of source depth estimates obtainable from the array output. Cramer-Rao bounds for the depth estimate are calculated. When the velocity profile is known exactly, the value of the bound is quite insensitive to the precise form of the velocity profile. A bound calculated from a constant velocity profile yields an excellent approximation for many situations likely to be encountered in practice. Introduction of an unknown parameter into the velocity profile has little effect on the Cramer-Rao bound for depth. However, a maximum likelihood estimator of depth working with an inaccurate value of the unknown parameter performs poorly. To obtain satisfactory performance, one must estimate the unknown parameters along with the source depth. Simulations demonstrate the success of this approach     

Analysis of finite-duration wide-band frequency sweep signals forocean tomography

A group of amplitude and frequency modulated signals which generate narrow synthesized pulses are described. The pulse-compression properties of these signals should approach those of maximal (M) sequence phase-modulated signals now commonly used in ocean experiments. These amplitude-tapered linear frequency-sweep (chirp) type signals should be accurately reproducible with most acoustic sources since they have controllable limited-bandwidth frequency content and differentiable phase. The Doppler response of the signals is calculated using a wideband approach, where the frequency shift from relative motion is not constant throughout the waveform. The resultant Doppler effect on the matched-filter output is a function of the signal duration. The signals are suitable for use with tunable resonant transducers, and have adequate Doppler response for use with Lagrangian ocean drifters     

基于合成孔径雷达的长江口海表流场反演

Range Doppler velocities derived from the Envisat advanced synthetic aperture radar（ASAR） wide swath images are analyzed and assessed against the numerically simulated surface current fields derived from the finite volume coastal ocean model（FVCOM） for the Changjiang Estuary. Comparisons with the FVCOM simulations show that the European Space Agency（ESA） Envisat ASAR based Doppler shift anomaly retrievals have the capability to capture quantitative information of the surface currents in the Changjiang Estuary. The uncertainty analysis of the ASAR range Doppler velocity estimates are discussed with regard to the azimuthal and range bias corrections, radar incidence angles, inaccuracy in the wind field corrections and the presence of rain cells.The corrected range Doppler velocities for the Changjiang Estuary area are highly valuable as they exhibit quantitative expressions related to the multiscale upper layer dynamics and surface current variability around the East China Sea, including the Changjiang Estuary.     

Limitations on the overlap-correlator method imposed by noise andsignal characteristics

Limitations on the performance of the overlap-correlator method of forming a passive synthetic aperture are derived. The technique uses the overlap of the array in sequential positions to estimate a series of phase correction factors that compensate for the motion of the array over time. It is of primary interest to optimize this overlap with respect to the effects of random noise. By minimizing the variance of the estimates of the set of phase correction factors, it is found that the optimal overlap is one-half the length of the physical array. Using this optimal overlap, the bounds on the usable spatial response are then determined as a function of signal-to-noise ratio and the number of hydrophones in the physical array. The ability of the overlap-correlator algorithm to synthesize a coherent aperture is investigated for the case of multiple sources in the absence of noise