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.