Validating ATLAS: A regional-scale high-throughput tracking system

Fine-scale tracking of animal movement is important to understand the proximate mechanisms of animal behaviour. The reverse-GPS system—ATLAS—uses inexpensive (~€25), lightweight (<1 g) and low-power (~0.4 mJ/transmission) tags. Six systems are now operational worldwide and have successfully tracked over 50 species in various landscape types. The growing use of ATLAS to track animal movement motivates further refinement of best-practice application and an assessment of its accuracy. Here, we test the accuracy and precision of the largest ATLAS system, located in the Dutch Wadden Sea, using concurrent GPS measurements as a reference. This large-scale ATLAS system consists of 26 receivers and covers 1,326 km² of intertidal region, with almost no physical obstacles for radio signals, providing a useful baseline for other systems. We compared ATLAS and GPS location estimates for a route (mobile test) and 16 fixed locations (stationary test) on the Griend mudflat. Precision was estimated using standard deviation during the stationary tests. We also give examples of tracked red knots Calidris canutus islandica to illustrate the use of the system in tracking small shorebirds (~120 g). ATLAS-derived location estimates differed from GPS by a median of 4.2 m (stationary test) and 5.7 m (mobile test). Signals that were collected by more receiver stations were more accurate, although even three-receiver localisations were comparable with GPS localisations (~10 m difference). Receivers that detected 90% of the 1 Hz transmissions from our test tag were within 5 km of their furthest detection but height of both receiver and tag seemed to influence detection distance. The test tag (1 Hz) had a fix rate of >90% at 15 of 16 stationary sites. Tags on birds (1/6 Hz) on the Griend mudflat had a mean fix rate of 51%, yielding an average sampling rate of 0.085 Hz. Fix rates were higher in more central parts of the receiver array. ATLAS provides accurate, regional-scale tracking with which hundreds of relatively small-bodied species can be tracked simultaneously for long periods of time. Future ATLAS users should consider the height of receivers, their spatial arrangement, density and the movement modes of their study species (e.g. ground-dwelling or flying).