GPS is a worldwide network of 24 satellites orbiting Earth at an altitude of 11,000 miles. Each satellite carries an atomic clock that allows users on the ground to calculate distances from the length of time signals take to travel from satellite to ground. The satellites send down the time information in radio signals telling us exactly where we are on the face of the planet. Our civilian GPS radio receivers tell us where we are on the planet within a few yards. The system was built by the United States Department of Defense.
Military receivers have special technology providing far more accurate position data for navigation.
Yes, there is a GPS wristwatch. The Casio Computer Company, a Japanese electronics firm, has built a GPS receiver into a watch. They say it's the world's smallest GPS receiver. It displays the wearer's latitude and longitude by degrees, minutes and seconds in addition to the usual clock functions.