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Testing Releasable GPS Radiocollars on Wolves and White-tailed Deer

Introduction

Satellite technology has been used in wildlife studies for many years, but primarily with transmitting collars (Warner 1967, Buechner et al. 1971, Kolz et al. 1980, Fancy et al. 1988) rather than receiving collars (Rempel et al. 1995, Moen et al. 1996). Transmitting collars transmit to satellites which transfer the data to earth. Receiving collars calculate and store positions with data from an internal Global Positioning System (GPS) that scans 24 earth-orbiting satellites. The satellites continuously broadcast radio signals, and a GPS receiver must simultaneously receive signals from 4 such satellites to determine its 3-dimensional position (latitude, longitude, and height).

Stand-alone GPS receivers are usually accurate to within 40 m 50% of the time, and to within 100 m 95% of the time (Hurn 1989). This sub-optimal accuracy is a result of code dithering called "selective availability" introduced into the satellite signals by the U.S. Department of Defense. Much of this error can be removed through differential correction, which can increase accuracy to within 2-5 m (Trimble Navigation, Ltd. 1992). However, differential correction is an intensive process and may be worthwhile only when research questions cannot be answered with uncorrected data (Moen et al. 1997, Rempel and Rodgers 1997). Global Positioning Systems collars are about 10 times more expensive than conventional radiotelemetry collars, but for an equivalent data-collection protocol the overall cost is dramatically reduced (Rodgers and Anson 1994, Ballard et al. 1995), and the GPS collars we tested are reusable. Costs of GPS collars are roughly equivalent to costs of satellite collars.

Global Positioning Systems collars weighing 1.8 kg and allowing remote data downloading have been tested but they could only be used on large animals such as moose (Alces alces; Rempel et al. 1995, Moen et al. 1996). To retrieve the collar, one had to recapture the animal.

We tested a 920-gm GPS collar on free-ranging wolves and deer. It was designed to be remotely released but was incapable of remotely downloading the data. We chose this collar because it was the only GPS collar light enough (<1100 g) to be used on our study animals. It also had the following desirable characteristics: large numbers of possible locations per collar, high accuracy of locations obtained, regularity of data collection intervals, and flexibility of data collection programming. Also, drop-off collars have been demonstrated to be particularly useful for addressing some research questions (Mech and Gese 1992). This study evaluated success of the collar's longevity and numbers of locations obtained rather than accuracy of locations. Accuracy of GPS collars under a range of conditions has been discussed elsewhere (Rempel et al. 1995, Moen et al. 1996).

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