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Time Difference of Arrival Wildlife Radiolocation System

Shane Nelson, Cary Logan, and Annabelle Colestock, Applied Research Laboratories, University of Texas at Austin, P.O. Box 8029, Austin, TX 78713-8029 USA

The Applied Research Laboratories, the University of Texas at Austin, have developed and tested a Time Difference Of Arrival (TDOA) wildlife radiolocation system using lightweight, low power, spread spectrum transmitters. This system, being developed in cooperation with the Texas Parks and Wildlife Department (TPWD), is specifically intended to improve the radio-tracking of mountain lions in southwest Texas, and is generally suited to applications in which accurate and temporally dense positions are required on a moderate to large numbers of animals.

Current direction finding radiolocation systems suffer from two primary disadvantages. First, these systems are usually labor intensive and therefore not suited for continuous tracking (although there are existing fixed network directional systems). Second, they are inherently inaccurate at all but short ranges since the position error is a direct function of the signal transmit distance. The primary advantage of direction finding systems is that they require only small inexpensive transmitters to be worn by the animal. Satellite tracking systems such as Argos also require only small transmitters and are useful over very wide ranges but must suffer even larger positional errors and ongoing costs. GPS collars which offer the highest level of accuracy are not suited to all applications. Many environments (tropical, for example) preclude successful GPS tracking. Finally, the size of GPS collars limits the size of animals that can be tracked and the cost of the collars, as well as associated ongoing telemetry costs, make them prohibitively expensive for applications involving many animals.

The TDOA system we have developed requires only lightweight inexpensive transmitters on the animals to be tracked (design goal: <1 lb; <$100; >1 year continuous operation). Three or more receiving stations located around the tracking environment (and not required to be static during tracking) relay Time Of Arrival measurements to one of the stations, designated as a master, where multiple animal positions can be computed and recorded at a user defined update rate. This configuration suits the system to applications requiring the tracking of many animals, since the cost of tracking additional animals is low once the infrastructure is in place. The system is also suited to any application requiring continuous monitoring, since it is not manpower intensive and does not require the use of external telemetry systems (communication satellites, for instance) which represent ongoing costs.

This presentation outlines the design of the current system as well as future development goals, describes the potential benefits to the wildlife management community, and details testing results, including positioning accuracy, position update rates, and extrapolated operational parameters such as the lifetime of the transmitters and the performance in various environments.

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