An Expendable Acoustic Transponder

The design and development of an expendable, low-cost acoustic transponder is described. The transponder is interrogated with a 9 kHz pulse of short duration and responds with a longer pulse at the same frequency. When operated in conjunction with a long-baseline tracking system, the transponder package allows accurate measurements of its position to be made. The primary application of this device has been to measure water current profiles in the Arctic, where the device is trimmed to sink at a slow rate. It could also be placed on the bottom to measure the drift of an ice-mounted tracking system. In addition, it serves as a handy, lightweight device for measuring locations on an ice floe by inserting it through holes drilled in the ice. Low cost (about S30 for parts) was achieved by using inexpensive miniature components for the electronics and standard plastic-pipe end caps for the pressure case. The heart of the system is a transducer that is mounted inside the pressure case and mechanically coupled to the end bell. This transducer is based on a piezoelectric smoke alarm buzzer. The details of the electronics and transducer design are presented along with the results of field tests of the system. The output of the system is approximately one acoustic watt; in arctic tests, it was possible to track the transponder at ranges in excess of 1000 yd, the range being limited more by refraction than by the power or sensitivity of the transponder. It is possible to extend the system operation to other frequencies by using the design approach described in the paper.