Target-tracking in a parametric sonar beam, with applications to calibration

The difference-frequency band of the Kongsberg TOPAS PS18 sub-bottom profiling parametric sonar is very directional. Calibration of this by the standard-target method requires precise positioning and knowledge of target location. This was apparent during the first calibration trial performed on 10 December 2008. The proximity of split-beam transducers of the Simrad EK60/18- and 38-kHz scientific echo sounder to the TOPAS transducer on the hull of R/V “G. O. Sars” has been exploited, with synchronous operation of the several sonars. While results with a single split-beam transducer would be unambiguous, the availability of data from two split-beam transducers allows quantitative comparison of performance. Results from this comparison are presented here. Significantly, agreement of the respective measurements is strong at an approximate target range of 90 m, fair at 220 m, but rather weak at 325 m. The context of a moving target is noted. The challenge of target-positioning and -tracking in a dynamic environment, more so in the presence of wind and underwater currents, may be met by use of a split-beam scientific echo sounder, if with a range limitation still to be defined.