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​HUGIN 1000-MR on the Royal Norwegian Navy mine hunter KNM Hinnøy in april 2008. Photo: FFI
Synthetic Aperture Sonar (SAS)
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03.02.2011
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The Norwegian Defence Research Establishment (FFI) and Kongsberg Maritime have an ongoing program to develop synthetic aperture sonar (SAS) technology for the HUGIN AUV.

​This program includes development of both sonar systems and software for SAS signal processing. A result of this development is the HISAS 1030 wideband multi-aspect interferometric SAS (see picture below). This system is delivered to the Royal Norwegian Navy as part of the HUGIN 1000-MR.

 

History

The SAS-development program started in 2000, and in 2005 a high-end prototype named HISAS 1060 (or SENSOTEK) was developed. The sonar was installed on FFIs own research vehicle HUGIN I (see picture). In the period from 2005-2007 numerous sea trials were conducted, collecting valuable data for the development of SAS-processing methods. In addition, the HISAS 1030 was used as a test system for design choices for the HISAS 1030 - which today is a product in the Kongsberg Maritime.
 

Technology overview

The principle of synthetic aperture sonar (SAS) is to combine successive pings coherently along a known track, in order to increase the azimuth (along-track) resolution. Ideally, the synthetic array length increases with range, such that the along-track resolution becomes independent of frequency and range. SAS technology has potential to image the seafloor with centimetre resolution up to hundreds of meters range.
 

SAS Signal processing

The basic building blocks in the signal processing in synthetic aperture imagery are:
  • Navigation (including time-delay estimation in micronavigation)
  • Image formation algorithms (beamforming)
  • Image correction (including autofocus)
  • Interferometry for bathymetric mapping

Synthetic aperture sonar processing is very closely related to synthetic aperture radar (SAR), seismic imaging and ultrasound beamforming. One of the fundamental challenges in SAS beamforming is processing speed. Large amounts of data need to be needs to be formed into images fast.

SAS signal processing overview

 

FOCUS toolbox

FOCUS toolbox is a software package for signal processing of synthetic aperture sonar and synthetic aperture radar data. FOCUS is entirely developed at FFI, and more than 20 man-years has gone into its development. The toolbox serves two purposes: It is a high powered tool for research and development; Parts of the toolbox are integrated into the HISAS product series sold by Kongsberg Maritime. FOCUS toolbox continues to be developed by the SAS team in various FFI projects.

The flow of data from raw data at the sonar to final product is essentially divided into four sections (see figure link below):
  1. Rapid products are sidescan imagery (produced in real time), and sidescan bathymetry. These are lower resolution data products suitable for rapid environmental assessment and getting a quick overview of the mission.
  2. The navigation solution can be updated using micronavigation from the sonar.
  3. Streaming SAS imagery is high resolution imagery of all data, full swath, at a chosen resolution and fidelity.
  4. As part of classification of small objects, FOCUS toolbox provides a variety of different techniques to enhance and suppress specific features in the SAS image.
These include shadow enhancement, target enhancement, multi-aspect movies and 3D rendering of small targets.

The image link below shows a SAS image collected by HUGIN 1000-MR outside Horten, Norway at 200 m water depth. The image shows a sunken WW-II submarine at 220 m range, and two 1x1 m cubes at 275 and 320 m range. This image captures the essence of SAS: high resolution and long range at the same time.

SAS image from HISAS 1030 on HUGIN 1000-MR.
The shown sonar range is 25-325 m. The theoretical resolution is approximately 3x3 cm. 
HUGIN 1 with the SENSOTEK system during a mission in march 2005. Photo: FFI
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