Influence of satellite geometry and differential correction on GPSlocation accuracy

摘要

The location accuracy of Global Positioning System (GPS) units depends on location type (3-dimensional, 3-D>2-dimensional, 2-D) and satellite geometry (indexed by horizontal dilution of precision, HDOP). To determine the best computation method for positions collected by CPS telemetry collars in hilly terrain, we used a stationary GPS collar that attempted to calculate a position every 10 minutes for 5 days, and we evaluated the relationship between horizontal and vertical location accuracy and HDOP (range of HDOP, 3-D=1.4 to 489.1, 2-D=1.4 to 4,891). The 50th and 95th percentiles of horizontal and vertical location error were related linearly to HDOP. Location error depended mainly on accuracy of the collar altitude estimate used to compute the position in 2-D. Most forced 2-D locations not differentially corrected were more accurate than 3-D locations when collar altitude error was less than or equal to 50 m. It was better to force the computation of 2-D positions from differentially corrected 3-D locations with HDOP>15 when the collar altitude error was less than or equal to 10 m. We also used data collected on 10 free-ranging moose (Alces alces) for 12 months to examine whether moose altitude could be estimated accurately using prior 3-D locations. Accuracy of moose altitude estimation was related inversely to the time elapsed since the first 3-D location used to make the estimate. When animal altitude is likely to vary greatly within small time periods, we suggest running the differential correction program twice for a single time period, using an HDOP cutoff of 20-25 in the first run and 10-12 in the second. All 3-D positions computed during the first processing of the data should be kept bur only the 2-D locations calculated in the second pass should be used because they were calculated using more accurate estimates of animal altitude. When applying this method to our data, only 9% of successful locations are discarded and we estimate that horizontal location error is <35 m 95% of the time.