An improved multiple-frequency method for measuring in situ target strengths

Refinements have been made to the multiple-frequency method for rejecting overlapping echoes when making target-strength measurements with split-beam echosounders described in Demer et al. (1999). The technique requires that echoes, simultaneously detected with two or more adjacent split-beam transducers of different frequencies, pass multiple-target rejection algorithms at each frequency, and characterize virtually identical three- dimensional target coordinates. To translate the coordinates into a common reference system for comparison, the previous method only considered relative transducer positions and assumed that the beam axes of the transducers were parallel. The method was improved by first, optimizing the accuracy and precision of the range and angular measurements of the individual frequency detections; and second, precisely determining acoustically the relative positions and angular orientations of the transducers, thus completely describing the reference-system transformation(s). Algorithms are presented for accurately and precisely estimating the transformation parameters, and efficiently rejecting multiple targets while retaining measurements of most single targets. These improvements are demonstrated through simulations, controlled test-tank experiments, and shipboard measurements using 38- and 120-kHz split-beam transducers. The results indicate that the improved multiple- frequency TS method can reject more than 97% of multiple targets, while allowing 99% of the resolvable single targets to be measured.