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Interferometric synthetic aperture sonar (SAS) is a technique to image and map the seabed in very high resolution. For large area coverage rate systems with hundreds of metres swath width, the achievable performance varies significantly over the swath. The performance is a function of system, collection geometry, and seabed type. A model is suggested to optimise the collection geometry for maximising area coverage rate given certain optimisation criteria, such as observation geometry, signal‐to‐noise ratio, depth measurement accuracy, and coverage within swath. The model is fitted to measurements (or calibrated) through a simple procedure. Specifically, the effect of vehicle altitude during the interferometric SAS data collection is studied. A novel model on data collected by a HUGIN Superior autonomous underwater vehicle carrying a HISAS 1032 Dual Rx interferometric SAS is demonstrated. The authors show that optimising the collection geometry, and in this case specifically the vehicle altitude, significantly improves the overall performance. [ABSTRACT FROM AUTHOR]

We have studied the low-complexity adaptive (LCA) beamformer in active sonar imaging. LCA can be viewed as either a simplification of the minimum variance distortionless response (MVDR) beamformer, or as an adaptive extension to the delay-and-sum (DAS) beamformer. While both LCA and MVDR attempt to minimize the power of noise and interference in the image, MVDR achieves this by computing optimal array weights from the spatial statistics of the wavefield, while LCA selects the best performing weights out of a predefined set. To build confidence in the LCA method, we show that a robust MVDR implementation typically creates weight sets with shapes spanning between a rectangular and Hamming window function. We let LCA select from a set of Kaiser windows with responses in this span, and add some steered variations of each. We limit the steering to roughly half the −$3$ -dB width of the window's amplitude response. Using experimental data from the Kongsberg Maritime HISAS1030 sonar we find that LCA and MVDR produce nearly identical images of large scenes, both being superior to DAS. On point targets LCA is able to double the resolution compared to DAS, or provide half that of MVDR. This performance is achieved with a total of six windows: the rectangular window and the Kaiser window with $\beta =5$, in an unsteered version, and versions that are left and right steered to the steering limit. Slightly smoother images are produced if the window count is increased to 15, but past this we observe minimal difference. Finally, we show that LCA works just as well if Kaiser windows are substituted with trigonometric ones. All our observations and experiences point to LCA being very easy to understand and manage. It simply works, and is surprisingly insensitive to the exact type of window function, steering amount, or number of windows. It can be efficiently implemented on parallel hardware, and handles any scene without the need for parameter adjustments. [ABSTRACT FROM AUTHOR]

With the advent of advanced deep sea mining technology, seafloor mining is poised to begin on a global scale. For the success of any mining operation, it is crucial that both the operators and regulatory bodies possess detailed information of the resource and surrounding environment during all stages of the mining process. We propose that synthetic aperture sonar (SAS) is a key emerging technology that can be used by all relevant parties at only a minimal increase in cost. This technology, originally designed for military and offshore oil and gas industry applications, can be readily applied to scientific seafloor mapping. The fine resolution of this technique allows for deposit mapping of active and inactive seafloor massive sulfide deposits. By clearly distinguishing between volcanic and hydrothermal landscapes based on features finer than the resolution of conventional multibeam systems, SAS enables an entirely new level of hydrothermal deposit survey. [ABSTRACT FROM AUTHOR]

The article discusses the technology behind an interferometric synthetic aperture sonar with special reference to the high resolution synthetic aperture sonar (HISAS) 1030 system developed by Kongsberg Maritime. It outlines some of the sonar design and data processing challenges and explains how the challenge of filling the nadir gap was addressed. It suggests the adaptation of the processing chains using SAS to the quality of data that interferometric SAS systems can provide.

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. [ABSTRACT FROM AUTHOR]

The relative height of the seafloor can be estimated by using two vertically displaced receivers. In this paper, we propose techniques to improve the accuracy of the estimated height. Our results are based on the use of synthetic aperture sonar (SAS) imaging, which implies coherent addition of complex images acquired from a moving platform. The SAS processing improves the along-track (or azimuth) resolution, as well as the signal-to-noise ratio (SNR), which in turn improves the estimated height accuracy. We show that the shift of the effective center frequency induced by coherent, frequency-dependent scattering affect the time-delay estimates from complex cross correlations, and we propose a correction technique for broadband signals with uneven magnitude spectra. To reduce the effect of geometrical decorrelation and increase the coherence between the images, we beamform the sonar images onto an a priori estimate of the seafloor height before correlating. We develop a mathematical model for the imaging geometry. Finally, we demonstrate our proposed estimators by providing relative seafloor height estimates from real aperture and SAS images, obtained during the InSAS-2000 experiment at Elba Island in Italy. In particular, we demonstrate that the SAS image quality is significantly improved by inclusion of the height estimates as a priori information. [ABSTRACT FROM AUTHOR]