Accuracy of a 10 Hz GPS Unit in Measuring Shuttle Velocity Performed at Different Speeds and Distances (5 - 20 M).

The aim of this study was to validate the accuracy of a 10 Hz GPS device (STATSports, Ireland) by comparing the instantaneous values of velocity determined with this device with those determined by kinematic (video) analysis (25 Hz). Ten male soccer players were required to perform shuttle runs (with 180° change of direction) at three velocities (slow: 2.2 m·s; moderate: 3.2 m·s; high: maximal) over four distances: 5, 10, 15 and 20 m. The experiments were video-recorded; the "point by point" values of speed recorded by the GPS device were manually downloaded and analysed in the same way as the "frame by frame" values of horizontal speed as obtained by video analysis. The obtained results indicated that shuttle distance was smaller in GPS than video analysis (p < 0.01). Shuttle velocity (shuttle distance/shuttle time) was thus smaller in GPS than in video analysis (p < 0.001); the percentage difference (bias, %) in shuttle velocity between methods was found to decrease with the distance covered (5 m: 9 ± 6%; 20 m: 3 ± 3%). The instantaneous values of speed were averaged; from these data and from data of shuttle time, the distance covered was recalculated; the error (criterion distance-recalculated distance) was negligible for video data (0.04 ± 0.28 m) whereas GPS data underestimated criterion distance (0.31 ± 0.55 m). In conclusion, the inaccuracy of this GPS unit in determining shuttle speed can be attributed to inaccuracy in determining the shuttle distance.