Horizontal positioning error derived from stationary GPS units: A function of time and proximity to building infrastructure.

This investigation quantified Horizontal Positioning Error (HPE) from stationary Global Positioning System (GPS) units. In experiment 1, GPS units were placed within close proximity of each other and data were collected at three times within the same day. In experiment 2, the GPS units were configured in a straight line from the edge of the field (nearby to a stadium), towards the centre. Circular Error Probable of 95% (CEP95) was used to quantify HPE, mean numbers of satellites recorded. CEP95 and the number of satellites were inversely related in both experiments. Changes in satellite availability throughout the day led to significant variability in HPE across trials in experiment 1. In experiment 2, the largest CEP95 was found among units located closest to the stadium. Collectively, these findings identify important considerations for using GPS to map athletes' positions that have not been adequately addressed in the sports science literature. The number satellites, time between repeated measures testing, and the proximity of nearby buildings, can cause unpredictable changes in measurement error and should be considered in the interpretation of data. Further, we recommend that CEP95 be reported in GPS-based experiments, which may help prevent the misleading interpretation of inaccurate or unstable data.