Calibrating a 90-kHz multibeam sonar

Quantitative use of multibeam sonar requires calibration. Protocols for calibrating multibeam sonar in an absolute sense by means of a standard target are being developed. These protocols are illustrated for the Simrad SM2000 Multibeam Echo Sounder, with 90-kHz operating frequency and 300 beams over a 90-deg sector (/spl plusmn/45 deg), each of nominal beamwidth 1.5/spl times/1.5 deg. In a sea well on Iselin Dock at the Woods Hole Oceanographic Institution, directional characteristics of the sonar were measured at the farfield range of 23 m in a sea well. At the large freshwater indoor tank at the University of New Hampshire, similar characteristics of directivity were measured at the nearfield range of 11.7 m. Beamforming was accomplished by digital signal processing. Beam patterns are presented and compared at both ranges. In addition, measurements of the on-axis response were made for each available time-varied-gain function, enabling linearity and dynamic range to be assessed.