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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Page 11 of 52

www.sea-technology.com November 2018 | ST 11 In the 1980s and 1990s the evo- lution of inertial navigation systems (INS) toward smaller, less expensive packages with speed esti- mates aided by Doppler ve- locity logs (DVL), combined with techniques to employ the SAS vernier for precise estimates of lateral sonar motion, led to development of the sub-wavelength-scale motion estimation tech- niques required for SAS im- age formation. This was followed by techniques correcting re- sidual phase errors along the synthetic aperture due to unaccounted-for plat- form motion or medium sound speed fluctuations, developed in the form of data-driven autofocus algo- rithms. From the 1990s through the 2000s, the burdensome computational requirements of SAS beamforming were addressed by innovations in SAS algorithms, such as Fourier-based image reconstruc- tion techniques, and large-scale parallel computing ar- chitectures for fielded systems—until DSP/FPGA boards and, finally, single-board computers were able to man- age the processing loads in the 2000s and 2010s, respec- tively. Sea surface multi-path interference continues to be a nuisance; however, mitigation has come in the form of range-dependent beam shading and adaptive beamform- ing made possible by vertical apodization of the receive array. SAS Development by NSWC PCD and Partners The tradition of pioneering new sensing technologies for maritime reconnaissance dates back to the founding of NSWC PCD at the end of WWII, known also through- out the years as the USN Mine Defense Laboratory (USN- MDL) and the Coastal Systems Station (CSS). An example is the 1950s invention of the seabed imaging side scan sonar. This technology transitioned to the Westinghouse C-MK-1 SHADOWGRAPH mine-hunting sonar fielded on minesweepers in the early 1960s, which in turn led to development of the U.S. Navy's AN/AQS-14 (predeces- sor to the AN/AQS-24) and French DUBM-series mine detection sonars. (Top and middle) The image products routinely presented to the SAS user have advanced considerably in 40 years. Note the vintage photograph (1976) of the SHADOWGRAPH SAS and corresponding LF imagery of bur- ied targets compared to recent exam- ples of single-pass and circular SAS images created with the ONR small synthetic aperture minehunter. (Bot- tom) Fifty years of SAS Development at NSWC PCD. Technologies were developed with partners that include: Northrup Grumman Corp.; Applied Research Laboratory, Penn State Uni- versity; Bluefin Robotics; EdgeTech Inc.; Applied Research Laboratories, University of Texas; Florida Atlantic University; and Woods Hole Oceano- graphic Institution. (Credit: U.S. Navy) (Credit: U.S. Navy)

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