Challenges in Seafloor Imaging and Mapping With Synthetic Aperture Sonar

Author
Hansen, Roy Edgar
Callow, Hayden John
Sæbø, Torstein Olsmo
Synnes, Stig Asle Vaksvik
Date Issued
2011
Permalink
http://hdl.handle.net/20.500.12242/715
https://publications.ffi.no/123456789/715
DOI
10.1109/TGRS.2011.2155071
Collection
Articles
Description
Hansen, Roy Edgar; Callow, Hayden John; Sæbø, Torstein Olsmo; Synnes, Stig Asle Vaksvik. Challenges in Seafloor Imaging and Mapping With Synthetic Aperture Sonar. IEEE Transactions on Geoscience and Remote Sensing 2011 ;Volum 49.(10) s. 3677-3687
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Abstract
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.
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