Sea Technology

JUN 2017

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Page 11 of 72 June 2017 / st 11 In addition to being used for high-resolution, wide-area surveys, SAS has more recently been used for reacquisition and identification in the form of circular synthetic aperture sonar (CSAS). Rather than scanning a target using a standard linear trajec- tory, in CSAS the sonar moves in a circular trajectory around targets of interest. The circular aperture fills in information gaps about target shapes and enables the generation of very high-resolution, low-speckle imagery for target identification. Other data products can be generated as well, such as movies that show the time history of the seafloor, shadow projec- tions and impulse response character- izations of targets. Volumetric SAS The increasing reliability and ca- pabilities of unmanned underwater vehicles and sensor payloads have en- abled further exploration into the mer- its of creative and unorthodox syn- thetic aperture trajectories. Over the past couple years, for example, a col- laboration between the Naval Surface Warfare Center Panama City Division (NSWC PCD) and the Applied Phys- ics Laboratory University of Wash- ington (APL-UW) has been applying SAS processing to vertical stacks of circular scans, sets of concentric rings and, more recently, vertical spirals. The advantages of these types of scans are that they can be used to generate volumetric 3D images. Vol- ume images, as opposed to the 3D point clouds gen- erated by interferometric SAS or side scan systems, are composed of truly three-dimensional pix- els, or "voxels." The resulting volumetric CSAS images are similar to 3D com- puted tomography (CT) and magnetic resonance imag- ery (MRI) scans used frequently in the medical community. Similar synthetic aperture imaging concepts have been evaluated in the field of radar (SAR) for over a decade, how- ever, not without difficulty: If synthetic aperture imaging is challenging along a straight path, it is exponentially more so using spirals, vertical raster scans or concentric rings. For this reason, almost all demonstrations of multipass aperture synthesis in technical radar literature make use of isotro- pic scatterers manually placed in the target field to serve as navigation references. Unfortunately, the time, expense and danger involved with placing isotropic scatterers near underwater targets of interest would render volume imaging impractical for most SAS applications. As a result, NSWC (Top and Bottom) CSAS volume image of a 155-mm Howit- zer shell (A) and photograph of the same target (B). The rope around the target was used by divers for positioning purposes. The bottom figure (C) shows the scan pattern used to generate the image. Photographs of 3D prints of a crab pot lying on the seabed with dynamic topography (left) and a sunken WWII Helldiver airplane (right) in Buzzards Bay, Massachusetts. Both 3D prints were generated using 3D CSAS volumetric data products. (Image Credit: U.S. Navy)

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