Title :
Challenges in Seafloor Imaging and Mapping With Synthetic Aperture Sonar
Author :
Hansen, Roy Edgar ; Callow, Hayden John ; Sabo, T.O. ; Synnes, Stig Asle Vaksvik
Author_Institution :
Norwegian Defence Res. Establ. (FFI), Kjeller, Norway
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.
Keywords :
bathymetry; oceanographic equipment; radar imaging; radar interferometry; seafloor phenomena; synthetic aperture sonar; underwater sound; HISAS 1030 interferometric SAS; acoustic signals; aided inertial navigation; bathymetry; centimeter resolution; coherence; grating lobes; image focusing; nonstraight tracks; ocean environment; real aperture interferometry; sea surface; seafloor imaging; seafloor mapping; shallow waters; sonar positioning; sound velocity; synthetic aperture sonar; vehicle instability; Apertures; Image resolution; Imaging; Sonar navigation; Synthetic aperture sonar; Vehicles; Interferometry; multiple reflections; navigation; nonlinear tracks; seafloor imaging; sound velocity errors; synthetic aperture radar (SAR); synthetic aperture sonar (SAS); topography errors;
Journal_Title :
Geoscience and Remote Sensing, IEEE Transactions on
DOI :
10.1109/TGRS.2011.2155071