Acoustic Tracking of Neutrally Buoyant Float.

摘要

This report summarizes work done on the development and testing of the acoustically tracked drifter. The overall goal of the drifter is to acquire Lagrangian measurements of water motion so as to study mixed layer processes such as Langmuir circulation. The float was developed jointly with Eric D'Asaro (Applied Physics Laboratory, University of Washington), with D'Asaro focusing primarily on the mechanical aspects of the float and our group focusing primarily on the acoustical tracking. Tests were carried out in Haro Strait in March 1991 and in Georgia Strait in November 1991, in addition to some preliminary open ocean tests. Basically, the tests were very successful, although they did point out a number of changes that must be made to subsequent versions. The basic approach adopted was to set up the float in the neighbourhood of our drifting imaging/tracking sonar platform. This sonar, at preset intervals, transmits an 11kHz interrogation pulse. The transponder on the float replies with a sequence; first, a response to allow range tracking, and then time-coded signals with pressure and temperature information. The most difficult part of the tracking involves azimuthal direction measurement, which is carried out using four hydrophones at the end of each of four 4.25m arms on the sonar platform. Time-delay processing is carried out to determine the direction with appropriate corrections being made for instrument orientation.