Sea Technology

NOV 2018

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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10 ST | November 2018 A ctive synthetic aperture sonar (SAS) is a powerful imaging technique that coherently combines echoes from multiple pings along the trajectory of a towed or ro- botic platform to construct a long virtual array of hydrophones. When synthetic ap- erture techniques are applied at sufficient- ly low acoustic frequencies, where sound absorption in the ocean medium is re- duced, a modest-sized side scan sonar can generate seabed imagery with a constant azimuth resolution comparable to that of higher frequency sonar systems, but with longer range. Synthetic aperture concepts originated in the radar community in the mid-20th century. It was not until the 1970s that the U.S. Naval Surface Warfare Center Panama City Division (NSWC PCD) and others in the sonar community began to show the feasibility of applying these concepts for underwater imaging. With evolutionary advances in enabling technologies, SAS systems are now being fielded on a variety of platforms and over a wide range of commercial and military appli- cations. The first open publication in side-looking SAS was the 1969 patent awarded to G.M. Walsh of the Ray- theon Co., "Acoustic Mapping Apparatus" (U.S. Patent 3484737), stimulating a series of concept studies within the international community. In this time period there was agreement that SAS had tremendous potential; how- ever, the challenges of making a practical system were daunting. These included limitations in coherent multi- ping processing caused by the perceived severity of me- dium sound speed fluctuations; the limited area coverage rate (ACR) set by the along-track sampling distance trav- eled between pings (generally described as half a projec- tor length per ping, or finer); the inability to sufficiently measure random motion of the sonar platform for coher- ent signal processing; multi-path interference; and data processing requirements that were prohibitively complex for the computing platforms of that era. Experiments conducted during the 1970s and 1980s instilled confidence that sound propagation in the ocean is sufficiently stable over the modest range and time in- tervals required to create a synthetic aperture. Employ- ment of a multichannel vernier receive array to increase the ACR would be developed by NSWC PCD and others during the 1970s. Seabed Mapping SAS The First 50 Years of Synthetic Aperture Sonar Imaging Technology By Dr. Daniel D. Sternlicht • Dr. Michael P. Hayes • Dr. Roy E. Hansen The synthetic aperture technique combines multiple pings coherently along the path of the sonar trajectory to con- struct a synthetic array of hydrophones. In the convention- al single-ping process, the along-track resolution of the area imaged is dictated by the projection of the sensor's beam pattern onto the seafloor; however, creation of a long synthetic array allows for the creation of fine-, con- stant-resolution images at long range.

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