GPS tracking devices contain a sensor, which receives signals continuously broadcast from satellites maintained by the U.S. Government’s Global Positioning System to ascertain a user’s exact location at various points of time. Using that data, the watch is able to determine a runner’s pace and distance traveled. There are at least 24 satellites circling the planet at all times. Once the watch locates at least three, its software is able to calculate a user’s position. The U.S. Airforce claims the GPS system can be accurate to about 3.5 meters, though Garmin, one of the best-known GPS watch manufacturers, reports their receivers to be accurate to within about 15 meters. Certain features and some conditions (such as terrain, tall buildings or tree cover, or atmospheric effects) can make finding satellites difficult, and the receivers less accurate.
How accurate do watches need to be?
No GPS watch is perfectly accurate. However, for training purposes, absolute accuracy is unnecessary. Most watches are consistent enough to give users a general idea of their distance and pace; that information is usually sufficient to provide useful feedback during individual runs and to track progress over time.