Prevention: Navigating with GPS is not always as accurate as it seems. Set one down so it is not moving and just watch the readings keep changing. According to the U.S. Department of Defense, which maintains the system, the GPS signals we get should be accurate to within 50 feet 95 percent of the time, while the other 5 percent can be out as much as 300 feet. In practice, however, the average tends to be around 30 feet. To improve on this, WAAS (wide area augmentation system) brings it to an official 23 feet for 95 percent of the time but seems to average closer to 10 feet.