Understanding the Causes of Missed Global Positioning System Telemetry Fixes

The use of Global Positioning Systems (GPS) on wildlifetracking collars has increased with improved technology. These collars obtain more locations per animal and potentially eliminate temporal biases caused by nonrandom radiotelemetry schedules (Rodgers et al. 1996). They have reduced potential biases in analyses of habitat use resulting from animals moving into thicker cover in response to the presence of researchers. The GPS technology has also increased the spatial accuracy of locations so that, for most applications, the error in locations is lower than the error of most habitat maps (Rodgers et al. 1996, Rempel and Rodgers 1997). One remaining potential bias is due to missed fixes, which occur when the GPS unit on the collar fails to receive signals from 3 or more satellites and thus, cannot calculate and record a position estimate. Previous studies have documented that terrain (D’Eon et al. 2002, Frair et al. 2004) and vegetation (e.g., Rempel et al. 1995, Edenius 1997, D’Eon et al. 2002) can affect fix success. Other authors have documented temporal effects due to deciduous canopy development and leaf-drop (Moen et al. 1997, Dussault et al. 1999), a changing satellite constellation throughout the day (Moen et al. 1997), and diminished success rates as batteries weaken (Gau et al. 2004). Few published studies have directly assessed behavioral influences on fix success. Only moose (Moen et al. 1996) and deer (Bowman et al. 2000) have been observed during fix attempts. Both species had reduced fix success when animals were bedded. Researchers have used activity sensors on elk (Rumble et al. 2001) and moose (Moen et al. 2001) to determine that inactive animals had lower fix success. We found no studies that examined whether there were differences in fix success between individual animals of the same species or that incorporated animal characteristics with terrain and vegetative characteristics into analyses of fix success. As bears exhibit high individual heterogeneity in behavior and in fix success, we felt it was important to include variables representing behavior and look for potential explanations for any patterns between individual bears and fix success. Location error for successful fixes has a number of causes including: 1) poor satellite geometry, typically expressed as high values of position dilution of precision (PDOP); 2) satellite clock errors; 3) satellite ephemeris errors; 4) unmodeled atmospheric delays; 5) multipath signals, when the collar receives multiple signals reflected from water or other surfaces; 6) receiver internal noise; and 7) selective availability (SA; Wells 1986). Many of these errors can be removed through differential correction (Puterski et al. 1992). The primary source of location error before May 2000 was SA, a U.S. Department of Defense policy under which random clock errors were introduced to satellite transmissions and reduced GPS location accuracy to 100 m 95% of the time (Wells 1986) but which could be as high as 200 m (Puterski et al. 1992). Since SA was discontinued in May 2000, a growing number of researchers no longer employ differential correction. Differential correction can be a time-consuming process, but it is still the only easy way to reduce errors (Adrados et al. 2002). Janeau et al. (2001) and Adrados et al. (2002) compared errors preand post-SA removal in 2 types of collars (one brand with differential correction and one without), and D’eon et al. (2002) addressed the overall error in coordinates since SA was turned off (98.5 m 95% circular error probability [CEP] for 2-dimensional (2D) locations and 26.2 m 95% CEP for 3D locations), but we found no research that examined how much additional error would be removed if the same locations were differentially corrected. Our objective was to determine which of the multiple factors affecting fix success were most important in our study area. We compared the influence of satellite availability, terrain, vegetation, animal size, and animal movement rates on fix success using data from GPS-collared grizzly bears. We compared the best models from collars on bears with tests of fix success at stationary sites and with error rates from collars placed at random test sites. We also compared error rates for corrected and uncorrected coordinates relative to the true location of GPS collars at random test sites.