IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. XX, NO. Y, NOVEMBER 2011 1 Challenges in Seafloor Imaging and Mapping With Synthetic Aperture Sonar Roy Edgar Hansen, Member, IEEE, Hayden John Callow, Member, IEEE, Torstein Olsmo Sæbø, Member, IEEE, and Stig Asle Vaksvik Synnes Abstract—Synthetic aperture sonar (SAS) is emerging as an imaging technology that can provide centimeter resolution over hundreds of metres 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 non-straight 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 effects of insufficient mapping. Vehicle instability and non-straight tracks in combination with insufficient navigation accuracy can cause grating lobes in the SAS images, not common in single-channel SAR. 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. Index Terms—Synthetic aperture sonar, synthetic aperture radar, interferometry, seafloor imaging, navigation, sound ve- locity errors, topography errors, non-linear tracks, multiple reflections I. I NTRODUCTION S YNTHETIC aperture sonar (SAS) is less known and developed than its counterpart in radar. Although the principle of SAS is not new [1], [2], it is only during latest years that SAS systems have become commercially available. The Norwegian Defence Research Establishment (FFI) and Kongsberg Maritime have a long term collaboration to develop SAS for the HUGIN autonomous underwater vehicle (AUV) [3]. Fig. 1 shows a HUGIN 1000-MR AUV with the HISAS 1030 interferometric SAS onboard a Royal Norwegian Navy mine hunter. There are a few critical differences between SAR and SAS [4], particularly related to the environment for which the sensor is operating. This paper describes some of the specific Manuscript received September 30, 2010; revised February 10, 2011 The authors are with the Norwegian Defence Research Establishment (FFI), P.O. Box 25, N-2027 Kjeller, Norway. E-mail: Roy-Edgar.Hansen@ffi.no; Hayden-John.Callow@ffi.no; Torstein- Olsmo.Sabo@ffi.no; Stig-Asle.Synnes@ffi.no. Fig. 1. HUGIN 1000-MR AUV equipped with the HISAS 1030 interfero- metric SAS onboard the Royal Norwegian Navy mine hunter Hinnøy. challenges in SAS and how we approach them in imaging and mapping of the seafloor from autonomous underwater vehicles. The sonar has to be positioned with accuracy better than a fraction of a wavelength along the synthetic aperture. This is non-trivial under water where GPS is not available. One solution is using the sensor itself for navigation in combination with aided inertial navigation. The ocean environment, and in particular, the sound velocity, has to be accurately estimated for successful focusing of SAS images. The sound velocity can change up to 2% over a typical depth profile. SAS is nearfield imaging, and an error of 0.2% in sound velocity can cause defocus. For non-straight synthetic apertures, the bathymetry of the scene to be imaged must be known. This is a significant problem in SAS since the range of the system is relatively short compared to topography changes in heavy terrain. Because of the relatively low sound speed (which limits the maximum pulse repetition frequency), almost all ex- isting practical SAS systems today are multi-element receiver systems. Vehicle instability and non-straight tracks combined with insufficient navigation accuracy can cause periodic errors in the synthetic aperture and grating lobes in the SAS images, not common in single-channel SAR. In shallow waters, the acoustic signals will interact with the sea surface, possibly causing multipath. This will reduce the SAS image quality. In Section II we describe the sensor and the signal process- ing we have developed for the HISAS 1030 interferometric SAS. Section III describes the main differences between SAS and SAR. The specific challenges in SAS are described in Dette er en postprint-versjon / This is a postprint version. DOI til publisert versjon / DOI to published version: 10.1109/TGRS.2011.2155071