Hayden J. Callow

Challenges in Seafloor Imaging and Mapping With Synthetic Aperture Sonar

Synthetic aperture sonar (SAS) is emerging as an imaging technology that can provide centimeter resolution over hundreds-of-meter range on the seafloor. Although the principle of SAS has been known for more than 30 years, SAS systems have only recently become commercially available. The success of SAS is critically dependent on overcoming several challenges related to the ocean environment. The sonar has to be positioned with accuracy better than a fraction of a wavelength along the synthetic aperture. We use the sensor itself for navigation, in combination with aided inertial navigation. The sound velocity has to be accurately estimated for successful focusing of SAS images. We calculate a simple rule of thumb for tolerance and show the effect of incorrect sound velocity. For nonstraight synthetic apertures, the bathymetry must be estimated. We use real aperture interferometry to map the scene before SAS processing. We calculate the required bathymetry accuracy and show the effects of insufficient mapping. Vehicle instability and nonstraight tracks, in combination with insufficient navigation accuracy, can cause grating lobes in the SAS images, which is not common in single-channel synthetic aperture radars. We show example imagery with severe grating lobes. In shallow waters, the acoustic signals will interact with the sea surface, possibly causing multipath. This will reduce the SAS quality. We use coherence to map the signal to multipath and, thereby, the valid sensor range. This paper illustrates the different challenges using examples from the HISAS 1030 interferometric SAS.

Shadow Enhancement in Synthetic Aperture Sonar Using Fixed Focusing

A shadow cast by an object on the seafloor is important information for target recognition in synthetic aperture sonar (SAS) images. Synthetic aperture imaging causes a fundamental limitation to shadow clarity because the illuminator is moved during the data collection. This leads to a blend of echo and shadow, or geometrical fill-in in the shadow region. The fill-in is most dominant for widebeam synthetic aperture imaging systems. By treating the shadow as a moving target and compensating for the motion during the synthetic aperture imagery, we avoid the geometrical shadow fill-in. We show this to be equivalent to fixing the focus at the range of the shadow caster. This novel technique, referred to as fixed focus shadow enhancement (FFSE) can be used directly as an imaging method on hydrophone data or as a postprocessing technique on the complex SAS image. We demonstrate the FFSE technique on simulated data and on real data from a rail-based SAS, and on two different SAS systems operated on a HUGIN autonomous underwater vehicle.

Bathymetric Capabilities of the HISAS Interferometric Synthetic Aperture Sonar

Multibeam echo-sounders have until now been the leading technology in seabed mapping, but typically have resolutions of around half a metre. Recently, synthetic aperture sonar (SAS) technology has matured to a commercial level, delivering sidelooking imagery with a resolution of a few centimetres to a range of several hundred metres. High resolution sidelooking imagery makes interferometry an extremely interesting technique for bathymetric processing. In this paper we discuss the theoretical accuracy in relative height mapping of the new HISAS interferometric SAS and compare the results with processing of simulated data. We also demonstrate SAS interferometry on real data collected with a prototype SAS mounted on a HUGIN AUV. Different filtering techniques are applied to the data, illustrating the trade-off between resolution, robustness and smoothness.

High fidelity synthetic aperture sonar products for target analysis

Synthetic aperture sonar (SAS) can produce images with centimetre-level resolution and area coverage of better than one square kilometer per hour. This makes SAS an ideal sensor for detection and classification of small targets over large areas. Fully automated target analysis allows improved autonomy when using autonomous underwater vehicles (AUVs) and saves a tedious manual analysis in post-mission analysis. Recognition of small targets in sonar imagery is, however, a difficult task. SAS imagery preserves wavenumber information. This gives the possibility for extra products in addition to high resolution imagery. We propose a two-stage processing where regions of interest are generated from reduced resolution SAS imagery and subsequently post processed images are used to generate relevant target analysis information. In this paper, we concentrate on the types of information available and their significance rather than the choice of intermediate resolution and initial detection methods. The extra processing products discussed in this paper are target-enhanced images by autofocus, shadow-enhanced images by fixed focusing, multi-aspect images, frequency-selective information and 3D shape from interferometry. We show examples of each of the additional products using data collected by the HISAS 1030 interferometric SAS carried by the HUGIN 1000-MR vehicle.

Height estimation on wideband synthetic aperture sonar: experimental results from InSAS -2000

This paper presents an improved method of performing relative height estimation using sidescan sonar images collected from two vertically separated receiver arrays. Delay estimation on image sub-swaths by utilising the phase of the complex cross-correlation function is described. We suggest to beamform the images in ground range to reduce baseline decorrelation and footprint shift effect. The authors have also developed a new geometrical description for the height estimation. The improved height estimation technique is tested on experimental data from a wideband interferometric synthetic aperture sonar, operated in a rail experiment at Elba Island, Italy.

Comparison of SAS processing strategies for crabbing collection geometries

This paper presents a number of different processing strategies for generating synthetic aperture sonar (SAS) imagery when data collection suffers from significant sideways crab (where we define crab as the angle difference between track and heading). Whilst large amounts of sideways crab do not present fundamental limitations to the SAS method, the assumptions SAS processing chains are built upon are often violated under such conditions. We discuss typical assumptions and compare alternate image formation strategies. We also provide a short discussion of the fundamental limitations to crabbing systems. We found that the adjustments to our standard wavenumber-based imaging kernel increased crab-tolerance from approximately 7 degrees up to approximately 30.

Effect of Approximations in Fast Factorized Backprojection in Synthetic Aperture Imaging of Spot Regions

Fast Factorized Backprojection (FFBP) provides accelerated SAS image reconstruction at the cost of a tunable level of approximation. The approximation causes image artifacts and incorrect heights in interferometry. We explain the cause and effect of FFBP approximation and compare various levels of approximation on imagery and interferometry from data collected in the field with the SENSOTEK SAS. Interferometry has shown to be more sensitive to FFBP approximation error than imagery due to its phase sensitive nature. For bistatic FFBP implementations we recommend an FFBP approximation level Efact better than 1/10 of a wavelength as a reasonable tradeoff between computational load and error

Circular synthetic aperture sonar results from autonomous underwater vehicle trials

The principle of synthetic aperture sonar (SAS) is to combine successive pings coherently from different observation angles in order to increase the azimuth resolution. By collecting data along a circular track, a circular SAS (CSAS) image, or tomographic image, can be made. The image reconstruction can be done either incoherently, fully coherently, or partially coherently where each subaperture of coherent processing consists of a section of the circle. In target classification, CSAS has several benefits: the object is observed from all aspect angles giving a better perception; the resolution in the image increases. In this paper, we calculate required accuracy in navigation, bathymetry and sound velocity for successful circular SAS. Finally, we show circular SAS images of small targets from real data collected by the HUGIN autonomous underwater vehicle carrying the Kongsberg HISAS 1030. We test different beamforming strategies, and show the effect of coherent and incoherent tomographic imaging.

Detection of Short-Tethered Objects with Interferometric Synthetic Aperture Sonar

This paper shows that conventional streaming processing of synthetic aperture sonar imagery and bathymetry data can cause performance limitations in the detection of tethered objects. This is particularly the case for wide-beam systems as the shadow response is markedly degraded compared to traditional side-scan sonar imagery. Also, standard unwrapping methods for correlation phase are not suited for height estimation of tethered objects. Alternative processing methods based on interferometry and multi-aspect imagery are proposed and demonstrated on sensor data from Kongsberg Maritimes HISAS prototype processed with FFIs FOCUS toolbox.

Wideband Synthetic Aperture Sonar Backprojection With Maximization of Wave Number Domain Support

Wideband and widebeam synthetic aperture sonar (SAS) can provide information on the frequency- and aspect-dependent scattering in a scene. We suggest an approach to predict the quality of the sensor data over the available frequencies and aspect angles. We relate the typical spatial domain quality metrics to their wave number domain (WD) counterpart, and use these to map the data quality in WD. Because SAS arrays often are undersampled along-track, we pay particular attention to data degradation from aliasing. We use the proposed approach to examine how three SAS image formation algorithms based on time domain backprojection (TDBP) access data of different quality from wideband SAS systems. We illustrate the results with predictions for a generic SAS design and demonstrate the findings on two experimental systems. We observe that the maximum support of high-quality data is achieved through BP onto a high-resolution grid followed by WD filtering.