Improvements to the multiple-frequency method for in situ target strength measurements

Refinements have been made to the multiple-frequency method that was recently developed to improve the accuracy and precision of in-situ target strength (TS) measurements using split-beam echosounders (Demer et al., J. Acoust. Soc. Am. 105(4) 2359:2376 (1999)]. The multiple-frequency method improves the rejection of unresolvable and constructively interfering target multiples by combining synchronized signals from two or more adjacent split-beam transducers of different frequencies which are not integer multiples of each other. In this study, the method itself was improved by: 1) optimizing the accuracy and precision of the angular and range measurements of the individual frequency detections; 2) more precisely determining the relative three-dimensional (3-D) locations (x, y, and z) and angular orientations (pan and tilt) of the transducers and thus the positional transformation; and 3) increasing the range resolution of one or more of the frequencies. These improvements are demonstrated through controlled test tank experiments using 38 and 120 kHz split-beam transducers and a 200 kHz single-beam transducer. Tolerances for matching target positions at two or more frequencies were determined to be ±0.5 m in radial range and ±0.7° off-axis (compared to ±l.0 m ±l.5° in the previous study). These experimental results indicate that such careful application of the multiple-frequency TS method can reject 100% of multiple targets while allowing 90% of the resolvable single-targets to be measured