Sensitivity to changes in sound speed on passive bearing and range estimation

A passive towed array can easily measure the bearing to a target. Range can be found by at least two methods; measurements of time of arrival differences at three spaced hydrophone or hydrophone clusters or triangulation using bearings from three sub-arrays. The accuracy depends on good knowledge of the sound speed. In this report historical data have been analysed to find the change in recorded sound speed over a 5 meter depth interval, equivalent to measuring the sound speed in the tower of a submarine instead of the same depth as the towed array. The effect on bearing and range estimates from global sound speed variations as well as variation along the array have been analysed, and recommendations presented.