Sonar Geometry Research Articles

Focusing late-time returns from elastic targets in bistatic collection geometries

Targets encountered during synthetic aperture sonar (SAS) surveys may exhibit elastic scattering behavior and re-radiate sound after initial interrogation. These re-radiated returns are often described as “late-time” energy, as they reach the sonar after the initial geometric returns. Range-specific focusing methods can enhance acoustic features such as late-time energy, but these methods typically make a monostatic sonar approximation that may not hold in very shallow water sensing geometries. This presentation will discuss the development of a late-time reconstruction algorithm applicable to bistatic sonar geometries. Using both simulated and experimental sonar data from monostatic and bistatic collection geometries, this work will first quantify the differences between traditionally beamformed imagery and late-time-focused imagery using common image quality metrics. Next, we will quantitatively compare the impact of collection geometry on the performance of late-time focusing algorithms. Finally, the work will describe methods that use information from a multi-static collection to compress late-time energy down to a high-contrast, spatiallycondensed point. The analysis of these signal processing strategies will focus on their potential applications to automatic target recognition.

Target detection with multisector and multiswath echosounders

Discontinuities in the ensonification pattern of certain multibeam echosounders can hamper the detection of targets in the water column and on the bottom. Such echosounders operate in single or multiswath modes with several independent transmit sectors per swath. Each sector has a unique acoustic frequency band and is steered across and along track to compensate for the vessel’s pitch and yaw and to achieve a specified spatial density of soundings. This sonar geometry is optimized for bathymetry but less favorable for processing and interpretation of acoustic backscatter intensities because the sectors have different acoustic absorption profiles and are disjoint across and along track. This paper presents a method of target detection in the water column that involves frequency- and depth-dependent compensation for transmission loss in each sector, equalization of gains across all the sectors, and adaptive removal of sidelobe interferences. Echo intensities in each sector are then aggregated along track and displayed as signal-to-clutter ratio vs. altitude and distance along track. This technique allows detection of targets that are in the water column between the bottom echo trace and the ring of sidelobe interference—a region that is often deemed “blind” or too noisy for target detection.Discontinuities in the ensonification pattern of certain multibeam echosounders can hamper the detection of targets in the water column and on the bottom. Such echosounders operate in single or multiswath modes with several independent transmit sectors per swath. Each sector has a unique acoustic frequency band and is steered across and along track to compensate for the vessel’s pitch and yaw and to achieve a specified spatial density of soundings. This sonar geometry is optimized for bathymetry but less favorable for processing and interpretation of acoustic backscatter intensities because the sectors have different acoustic absorption profiles and are disjoint across and along track. This paper presents a method of target detection in the water column that involves frequency- and depth-dependent compensation for transmission loss in each sector, equalization of gains across all the sectors, and adaptive removal of sidelobe interferences. Echo intensities in each sector are then aggregated along track an...

Enhanced Kalman Filter Algorithm Using the Invariance Principle

Target tracking in multistatic active sonar systems is often limited in shallow-water environments due to the high level of bottom reverberation that produces false detections. Past research has shown that these false alarms may be mitigated when complete knowledge of the environment is available for discrimination, but these methods are not robust to environmental uncertainty. Recent work has demonstrated the existence of a waveguide invariant for active sonar geometries. Since this parameter is independent of specifics of the environment, it may be used when the environment is poorly known. In this paper, the invariance extended Kalman filter (IEKF) is proposed as a new tracking algorithm that incorporates dynamic frequency information in the state vector and uses the invariance relation to improve tracker discrimination. IEKF performance is quantified with both simulated and experimental sonar data and results show that the IEKF tracks the target better than the conventional extended Kalman filter (CEKF) in the presence of false detections.

Impact of target scattering properties on active waveguide invariance.

The waveguide invariant is a parameter which quantifies time‐frequency structure observed in broadband, time‐varying sonar geometries. This structure is only weakly dependent on the specific properties of the ocean waveguide, and thus processing schemes utilizing this invariant relationship between range and frequency have the potential to be robust in unknown or poorly known environments. Recent work has extended the concept of waveguide invariance to active sonar, and experimental demonstrations in near‐monostatic active geometries using a vertical plate scatterer have been presented (Quijano, 2008). In this study, scaled tank experiments are extended to true bistatic geometries using a variety of canonical scatterers (i.e., spheres, plates, and cylinders), in order to explore effects that are unique to the bistatic scenario. The resulting spectrograms indicate that the target scattering matrix can be the dominant influence on the time‐frequency structure. Results from normal mode and raytracing simulat...

Bistatic invariance principle for multi-static active geometries

The invariance principle describes the spatial interference patterns resulting from the coherent addition of propagating normal modes in the ocean channel. The relationship simplifies propagation structure into a scalar invariant which is approximately unity for many shallow-water environments. For passive sonar geometries (i.e., source to receiver) this valuable relationship has been extensively explored and applied to the interpretation of passive lofargrams. In active bistatic geometries, a transmitted pulse travels over two distinct propagation pathways (i.e., source-to-target and target-to-receiver). Assuming a distributed network (i.e., long-range propagation), each of these paths in isolation would produce interference patterns as dictated by the invariance principle. However, for bistatic active applications, the received pulse is a product of both propagation paths and a target-dependent scattering matrix which introduces mode coupling. For broadband waveforms, the question arises as to whether there is an observable interference pattern for a moving source. In this presentation, a derivation of the bistatic invariance principle is presented and discussed. Simulation results for the predicted time-frequency structure in a shallow-water channel are presented for specific sensor geometries and choices of target scattering matrix. Implications for application to multi-static active target tracking are presented in a companion paper.

Theoretical Accuracy of Doppler Navigation Sonars and Acoustic Doppler Current Profilers

This paper addresses the problem of Doppler shift estimation in Doppler sonar systems. The analysis focuses on the single-beam geometry formed by a circular planar array and considers both narrow-band (or so-called incoherent) and wide-band (or coherent) Doppler sonars, transmitting, respectively, one long continuous-wave pulse and a train of short continuous-wave pulses. The correlation function of the reverberation signal at the beam output is derived for volume reverberation. Directive transmission or reception and a combination of both is considered. Estimation theory is applied to derive the Cramer-Rao bound of the Doppler parameter estimate. The effect of pulse duration, sonar geometry, beamwidth, and signal-to-noise ratio are discussed. The accuracy of coherent and incoherent systems is compared for a specific case.

Target re-acquisition using acoustic features with an autonomous underwater vehicle-borne sonar

Concurrent mapping and localization (CML) is a technique for unsupervised feature-based mapping of unknown environments, and is an essential tool for autonomous robots. For land robots, CML can be applied using video, laser, or acoustic sensors, while for autonomous underwater vehicles (AUVs) the only effective transducer in most situations is sonar. In the Generic Oceanographic Array Technology Sonar (GOATS) experiment series, CML was effectively demonstrated using a single AUV. A further hurdle in the full implementation of AUV minehunting is to re-acquire and identify targets of interest. Target re-acquisition allows other vehicles to be called into a target location to further investigate with adaptive sonar geometries or alternative sensor suites designed for classification. In this work, the features in the CML-generated map are extended from only spatial coordinates to include acoustic features such as spectral response. It is demonstrated that the inclusion of acoustic features aids in the global positioning within the map, although the fine positioning is still accomplished through standard CML. In addition, areas that are sparsely populated with targets, e.g., a sandy coastline, are shown to be more readily navigable using acoustic features.

Seabed characterization from statistical analysis of high-frequency data

Results are presented from statistical analysis of experimental and simulated high-frequency seabed backscatter data. The statistical laws employed in the analysis were the Rayleigh distribution, the Rayleigh mixture, and the K-law. The significance of the distribution parameters and their estimation from the data are explained. The link between the parameters and seabed type has been theoretically examined through a detailed study of the variation of statistical laws with grazing angle. Experimental data were collected using a 150-kHz wide-band sonar at three different sites, providing different sediment types. The validity of the above statistical laws was investigated and found to be dependent on sonar geometry. The estimated parameters of the three distributions have been examined for their potential in seabed characterization. Simulations have been compared with experimental data for validation purposes. These results have then been used to investigate the theory developed beyond the scope of the recorded data.

Temporal prediction of acoustic backscatter from sediments at 10–250 kHz

A temporal model of high-frequency (10–250 kHz) seafloor acoustic backscatter has been developed which simulates the average squared echo envelope from a monostatic echo sounder. Tests comparing the model to measured backscatter echoes from pelagic and terrigineous sediments are presented, and the diminishing contribution of the sediment volume with increasing acoustic frequency is demonstrated. The temporal model was developed to assist in the identification of sediments by statistically comparing its output to the echo amplitude time series measured with bottom mapping sonars. The model incorporates the sonar geometry, electroacoustic transfer function and transmitted signal characteristics, the ocean volume parameters, the statistical geoacoustic parameters which describe the water/bottom interface and the sediment volume, and the multiscale roughness statistics of the interface for the bottom types being investigated. [Work supported by ONR.]

Temporal model of high-frequency seafloor acoustic backscatter

A monostatic temporal model of seafloor acoustic backscatter has been developed to investigate the relative contributions of interface roughness and inhomogeneities in the sediment volume for measurements made at various angles of incidence. The model takes into account sonar parameters such as acoustic wavelength, beam pattern, pulse length, and angle of incidence, and it includes a theoretical angular dependence of seafloor scattering strength that is controlled by the roughness statistics of the surface insonified, by its refraction index, and by a volume reverberation term. Simulations have been carried over angles of incidence from 0° to 60°, and at acoustic frequencies ranging from 10 to 100 kHz for various types of substrates. The theoretical angular dependence predicts that the contribution from the sediment volume becomes noticeable for angles of incidence greater than about 6°; however, in the temporal model it is shown that this contribution can be detected closer to normal incidence for small beamwidths (e.g., 2°). When applied to a multi-narrow-beam sonar geometry this temporal model shows greater potential for bottom classification than the angular dependence function of acoustic backscatter obtained by integrating the returns received in each beam. [Work supported by ONR.]

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data

Acoustic backscatter images of the seafloor obtained with sidescan sonar systems are displayed most often using a flat bottom assumption. Whenever this assumption is not valid, pixels are mapped incorrectly in the image frame, yielding distorted representations of the seafloor. Here, such distortions are corrected by using an appropriate representation of the relief, as measured by the sonar that collected the acoustic backscatter information. In addition, all spatial filtering operations required in the pixel relocation process take the sonar geometry into account. Examples of the process are provided by data collected in the Northeastern Pacific over Fieberling Guyot with the SeaMARC II bathymetric sidescan sonar system and the Sea Beam multibeam echo-sounder. The nearly complete (90%) Sea Beam bathymetry coverage of the Guyot serves as a reference to quantify the distortions found in the backscatter images and to evaluate the accuracy of the corrections performed with SeaMARC II bathymetry. As a byproduct, the processed SeaMARC II bathymetry and the Sea Beam bathymetry adapted to the SeaMARC II sonar geometry exhibit a 35m mean-square difference over the entire area surveyed.

Evaluation and verification of bottom acoustic reverberation statistics predicted by the point scattering model

The point scattering model offers a parameterization of the reverberation probability density function (pdf) in terms of the coefficient of excess (kurtosis) and a coherent component represented by a harmonic process with random phase. In this paper the potential utility of this parametrization is investigated in the context of seafloor characterization. The problem of separating out the effect of each parameter is discussed. Computer simulations are used to verify model predictions on the reverberation quadrature, envelope, and phase pdf. As part of the verification study, the scatterer density was determined from the kurtosis of the reverberation quadrature pdf. A statistical analysis of this procedure points to reduced estimate accuracy with decreasing kurtosis. Additional computer simulations show that the chosen pdf family, developed under the assumption of a Poisson scatterer distribution, is flexible enough to fit reverberation data generated by non-Poisson scatterer distributions exhibiting a degree of clustering or regularity. A computer experiment demonstrates how this parametrization can be used in conjunction with a simple sonar geometry to generate acoustic signatures for seafloor classification. In addition, real reverberation data collected by a Sea Beam sonar system in two different seafloor areas are interpreted according to the chosen parametrization.