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Uncovering the Secrets of the Deep with Synthetic Aperture Sonar

Norwegian Defence Research Establishment (FFI) is developing state-of-the-art sonar technology for imaging and mapping the seabed.

Detailed documentation of the seabed is important in many areas such as offshore oil and gas, monitoring of the marine habitat, and military applications. High resolution sonars can be valuable tools for all these purposes. Synthetic aperture sonar (SAS) is a relatively new technology that can improve image resolution and area cover-age rate compared to traditional sonars.

FFI firmly believes that SAS will be a key technology component in future naval mine countermeasures (MCM) operations. In 2000 FFI initiated a strategic collaboration with Norwegian industry, Kongsberg Maritime, on development of SAS technology for the HUGIN autonomous underwater vehicle (AUV). The Royal Norwegian Navy Mine Warfare Services, which represents the end user with very challenging tasks – searching for sea mines on the seabed, was also a key partner from the start.

International collaborationFFI participates in the joint research program on mine detection

and classification at NATO’s Centre for Maritime Research and Ex-perimentation (CMRE), the former NURC. This has given important input to FFI in critical development stages. Currently, the SAS re-search group at FFI has scientific collaboration with many groups, such as Defence Research and Development Canada (DRDC),

Synthetic aperture sonar image of the shipwreck of the Norwegian tanker Holmengraa, sunk during World War II out-side Horten, Norway. Depth from interferometry is colour coded. The green stripes over the wheelhouse are a school of fish. Data: Kongsberg Maritime.

FFI’s HUGIN HUS vehicle with the HISAS interferometric SAS extended with real-time processing and added Low frequency capability during launch from NATO’s Research Vessel Alliance in 2012. Photo: FFI

Norwegian Defence Research Establishment , P.O. Box 25, N-2027 Kjeller, Tel: +47 63 80 70 00 Office address: Instituttveien 20, N-2007 Kjeller

Defence Science and Technology Laboratory (DSTL) UK, Naval Research Laboratory USA, NAVSEA Panama City USA, Pennsylvania State University USA and University of Oslo, Norway.

The technologyThe fundamental principle of SAS is to form a long synthetic antenna by moving the sonar antenna and combining multiple pulses. This gives high resolution and long range in the sonar images unmatched by any other sonar imaging technique.

Although the principle of SAS has been known for more than 40 years, it is still an area of active research. One reason relates to the specific challenges of navigation – SAS requires very accurate navigation to work properly.

FOCUS toolboxAs part of the research activities, FFI has developed a complete software package named FOCUS, a toolbox for all signal processing related to SAS. The software package serves two purposes: It is an integrated part of the HISAS 1030 in-terferometric SAS product from Kongsberg Maritime. It also serves as the main tool for research and development in SAS at FFI.

The FOCUS toolbox contains modules for all critical components in SAS signal processing: Sidescan and sectorscan im-agery, integrated inertial navigation with sonar micro-navigation, SAS and inter- ferometry processing. The FOCUS toolbox can also produce different specialised output products such as multi-aspect SAS images,

shadow enhanced images and target enhanced images.

ApplicationsThere are numerous applications with specific demands for high resolution images and maps of the seabed, where SAS may become the dominating sensor technology. Examples are: Documenta-tion of the seabed for offshore oil and gas, underwater mining, offshore windmill farms, monitoring of the marine habitat, underwater archaeology, search for wrecks and objects, documentation of underwater waste deposits and military applications such as search for sea mines.

Contact:Roy-Edgar.Hansen@ffi.no

SAS images illustrating different applications. Left: SAS bathymetry of a German WWII plane Focke Wulf 190 A-3. Middle: SAS image of a 7.1 m long torpedo shape. Right: SAS image of a rock formation with SAS bathymetry colour coded. Data: the Royal Norwegian Navy and Kongsberg Maritime.

Combining pings in order to improve resolution.

FOCUS toolbox data flow and product overview.

Sonar data

Sonar Processing

Sidescan image Rapid result

FOCUS Toolbox overviewSynthetic aperture sonar

Navigation

High resolution

Classification

Sidescan bathy

HIFI Navigation

SAS Image

SAS Bathy

Multi-aspect

Target enhanced

Shadow enhanced

Target signature

3D Object

IntegratedNavigation

StreamingSAS imagery

Spotprocessing