Time varying gain (TVG) measurements of a multibeam echo sounder for applications to quantitative acoustics

Protocols of calibration procedures of a multibeam echo sounder have been suggested previously [Foote et al., 2005, J. Acoust. Am. 117: 2013-2027], in which the time-varying-gain (TVG) of the multibeam system was assumed to follow the nominal system settings and was not investigated thoroughly. In the current study, the influence of the measured TVGs on system performance was investigated quantitatively. Specifically, measured TVGs were compared to those derived from the nominal system settings for the Simrad SM20 90-kHz multibeam echo sounder (formerly the SM2000 90-kHz),. It was found that for resolved targets, the difference between the theoretically predicted TVG and the measured TVG averaged over all 128 beams was less than 25% for ranges greater than 8 m, equivalent to about 1.0 dB uncertainty in target strength estimates. For unresolved targets, the TVG error is approximately 8 dB at a range of 20 m, resulting in a possible change by a factor of 6 in abundance estimates. For ranges greater than 120 m, the error reduces to within 1.0 dB, corresponding to about a 25% error in abundance estimate