Calibration Schema for a Long-Range, Fishery-Research Side Scan Sonar

The NOAA National Marine Fisheries Service owns two Long-Range, Fishery-Research Side Scan Sonars (Klein model 7180). Having recognized a number of serious shortcomings in the original design and implementation of the 7180, NOAA embarked on an upgrade of the 7180 to bring the 7180's performance closer to the requirements of fish-habitat researchers for radiometrically adjusted backscatter observations from a calibrated sonar. With three-meter long mechanically curved arrays, it is difficult / impractical to calibrate the 7180 side scan sonar using conventional methods. However, given the recognized importance of calibrated and radiometrically adjusted backscatter data for fishery research, a schema was developed for the 7180 that built on, and extended, the protocol for calibration of multibeam sonar [1]. This paper describes the cascade calibration schema and presents results from applying it to the upgraded 7180 during a recent survey in the eastern Bering Sea. With this method, it is now possible to trace the 7180 backscatter levels to a recognized standard and to detect the onset of discrepancies among the several spatially overlapping measures of backscatter. This advances the fishery-research goal of consistent seabed characterization across wide swaths of the seabed.