Tracking acoustic transmitters by code division multiple access (CDMA)-based telemetry

Abstract

This paper describes a new acoustic telemetry system based on the principles of code division multiple access (CDMA) whereby signal coding enables the simultaneous sub-meter monitoring of potentially hundreds of animals at high sampling rate (e.g. 5 s) and on a single frequency channel. The technology permits reliable operation under a variety of conditions including high ambient noise, multipath and in the presence of multiple tagged animals. The concepts of dilution of precision (DOP) and reliability index (RI) as performance measures of CDMA position solutions are introduced. DOP and RI are used in the design phase of a study to develop hydrophone array geometries that maximize position solution precision and minimize the occurrence of system-induced data outliers. Computed position solutions qualified by DOP and RI assist in data interpretation and provide an objective means of separating system induced outliers from animal behavior. To assess the suitability of CDMA telemetry for monitoring tagged animals under shallow water conditions, experiments were conducted to track the movement of blue crab (Callinectes sapidus) in a salt marsh pond. During the study, data from 32-tagged animals was collected by a CDMA telemetry system and used to provide preliminary information on individual animal movement. Performance results obtained from the study indicate that the precision of position solutions consistently achieved sub-meter levels under shallow water conditions (depth <0.5 m). Accumulated detection statistics indicate that the nominal probability of detection exceeded 80% at individual hydrophone channels. The ability to simultaneously monitor the tagged population of 32 animals at a sampling rate of 5 s was verified.