Modulation and Signal-Processing Tradeoffs for Reverse-GPS Wildlife Localization Systems

Reverse-GPS wildlife localization systems are emerging as a key technology for regional high-throughput wildlife tracking. Two such systems have been designed, implemented, and deployed (in six sites on three continents). Both of the existing systems suffer from limitations due to the modulation that is used by transmitters, which are attached to wild animals, and due to the detection and estimation algorithms that they use to detect transmissions and estimate their arrival times. This paper investigates key tradeoffs associated with three different modulation schemes that wildlife tags can plausibly use. The factors that we investigate include the ability to detect weak signals from distant tags, the ability to accurately estimate the time-of-arrival at a given SNR, and the computational cost of these detection and estimation algorithms. Our key contributions are (1) evidence that BPSK modulation is superior in essentially all relevant metrics, except perhaps chip availability, to FSK and OOK; (2) evidence that OOK is a second-best choice and its main drawback is poor performance under interference from other tags; (3) algorithms to inexpensively search the frequency-delay space at moderate and high SNRs. We also report on implementation efforts designed to integrate robust processing of BPSK tags into a wildlife tracking system.