Sea Technology

NOV 2018

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12 ST | November 2018 www.sea-technology.com In 1969, USNMDL scientist D.H. Brown, a specialist in acoustic lens- ing and holography, published the paper "Application of the Synthetic Aperture Concept to High Resolu- tion Sonar" for the Mine Advisory Committee of the National Acade- my of Sciences, National Research Council, wherein he identified the concept's feasibility, usefulness and challenges and described the ver- nier SAS approach to increase area coverage rate. Under direction by the Naval Ship Systems Command (now NAVSEA), development at the lab commenced in 1973 and would continue through the decade. Augmented with an inertial measurement unit (IMU), one of the actively navigated SHADOW- GRAPH tow vehicles was used for the first synthetic aperture sonar field experiments. This system had a high-frequency (100 kHz) four-el- ement vernier system on top and a low-frequency (30 kHz) system sus- pended from the midsection, estab- lishing the multiband mine-hunting approach (high band for high reso- lution; low band for sediment pene- tration and buried object detection) that would continue to this day. On calm days, synthetically formed beams from the high-fre- quency system processed offline were reported to achieve nearly the- oretical resolution at 100-m ranges. Given no corrections for random motion, utility was of course limit- ed. The modest range low-frequen- cy system, with a sole transmit/ receive element and less vulnera- bility to motion, was used to create perhaps the earliest SAS images of targets from a fielded system. These tests, conducted in the Gulf of Mex- ico, illustrated numerous basic prin- ciples, as well as limitations. By 1980, an SAS testbed and rail facility was constructed and at- tached to the old Hathaway Bridge, traversing nearby St. Andrew Bay, in order to conduct detailed per- formance assessments and explore new methods and techniques. This included measurements showing that the ocean medium is sufficient- ly stable in tidal environments to form high-frequency synthetic aper- tures over the range and time inter- vals required. These early experiments with fielded systems and controlled rail tests led to more than three decades of innovation, under sponsorship including the Office of Naval Re- search (ONR), NAVSEA and the Department of Defense Strategic Environmental Research and De- velopment Program (SERDP). With the emergence of smaller, stealthier mines, more emphasis was placed on improving the resolution, as well as range, of these systems. An im- proved multichannel SAS would be fielded by the end of the 1980s. In the mid-1990s, NSWC PCD and Northrup Grumman Corp. (for- merly Westinghouse) designed and fielded a dual high- and low-fre- quency SAS payload on a 21-in.-di- ameter towfish, employing redun- dant phase center (RPC) motion estimation and real-time process- ing. In 1998, this system (integrat- ed with magnetic and electro-optic sensors) was rapidly mobilized to locate debris from Swissair Flight 111, which had tragically crashed into the sea off of Nova Scotia. The following year, it performed suc- cessfully in the mine-hunting U.S. Navy Fleet Demonstrations of GO- MEX 99 (Corpus Christi, Texas) and Kernel Blitz 99 (Camp Pendleton, California), motivating continued refinement of the technology and extension to off-board platforms. The 2000s and 2010s would see development of the first field- ed SAS on board a UUV in 2002; development of a buried object scanning SAS (BOSS); development of the ONR multiband small syn- thetic aperture minehunter (SSAM) family of systems, deployed on 12.75-in. UUVs for detection/clas- sification of exposed and buried mines; and development and inte- gration of a wide-swath, dual-fre- quency SAS and high-performance forward-looking sonar (FLS) on a 12.75-in. UUV for combined vol- ume and bottom mine hunting. Other US Initiatives Northrop Grumman would go on to develop other SAS technolo-

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