Effects of radio‐collar position and orientation on GPS radio‐collar performance, and the implications of PDOP in data screening

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Abstract
Summary: Global positioning system (GPS) radio‐telemetry has become an important wildlife research technique worldwide. However, understanding, quantifying and managing error and bias in raw GPS radio‐telemetry data sets requires much more work. In particular, error and bias resulting from position (angle away from vertical) and orientation (compass direction) of GPS radio‐collars on free‐ranging animals is currently unknown. We tested the effects of collar position and orientation on GPS radio‐collar performance using five stationary GPS radio‐collars. We also investigated the use of positional dilution of precision (PDOP) as a method for screening data with high location errors. Orientation had no statistical effect on fix rates or location errors. The biggest source of variation was attributed to collar position, which resulted in significantly lower performance at angles below 90° from vertical. PDOP‐based screening was effective and can be used to lower location error, but the trade‐off between higher location accuracy and data loss (potentially leading to new bias) must be assessed. Synthesis and applications. The results of this study refine our understanding of error and bias in GPS radio‐telemetry data. We suggest that collar orientation can safely be disregarded, whereas radio‐collar position remains a large potential source of error and bias. This finding has major implications regarding animal activity and GPS radio‐telemetry research. Researchers need to quantify and account for biases resulting from animals moving through heterogeneous terrain and habitats.