Retained Satellite Information Influences Performance of GPS Devices in a Forested Ecosystem

Global Positioning System (GPS) units used in animal telemetry often suffer from nonrandom data loss and location error. GPS units use stored satellite information to estimate locations, including almanac and ephemeris data reflecting satellite positions at weekly and at <4-hr temporal scales, respectively. Using the smallest GPS collars (45-51g) available for mammals, we evaluate how satellite information and environmental conditions affected GPS performance in 27 mobile trials, and field reliability during 56 deployments on Pacific marten (Martes caurina). We conducted trials in Lassen National Forest, California, USA, during March 2011-January 2012. We programmed GPS units to retain or remove satellite data (i.e., continuous or cold-start mode) before attempting a location (fix), thereby mimicking differing fix intervals. In continuous mode, fix success was 2.2x higher, was not influenced by environmental obstructions, and improved after a location with >= 4 satellites (3-D). In cold-start mode, fix success was negatively correlated with vegetation cover. Location error was lower for 3-D fixes. Censoring cold-start fixes with only 3 satellites (2-D) and 2-D locations prior to the first 3-D fix in continuous mode decreased location error by 91% and 55%, ensuring <50m accuracy. The significance of previous fix success and reduced battery expenditure underscores the benefits of ephemeris data and short fix intervals. Only 66% of 56 units functioned upon delivery for field deployment. Once tested and deployed, 28% malfunctioned. This study demonstrates that GPS tests should use the same fix schedules as field deployments, and GPS data quality in dense cover improves with short fix intervals. Miniature GPS units are a promising tool, but the study design should be carefully considered. (C) 2015 The Wildlife Society.