Global Positioning System (GPS) satellites transmit signals to receiving equipment on the ground. GPS receivers are passive in their operational nature, they only receive signals, never transmit. GPS receivers require an unobstructed view of the sky, so they are only used outdoors. For this very same reason, their functionality is limited when used in forested areas or near tall buildings. GPS operations are dependent on a very accurate time reference. This reference is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite carries an atomic clocks on board.
GPS satellites function by transmitting data that indicates their location and the current time. The satellites synchronize their operations so that the repeating signals are transmitted simultaneously. The signals travel at the speed of light, but reach ground GPS receivers with slight time variations since the satellites are at different distances from the earth’s surface. Using the time stamp of the signal and the actual time of its reaching the GPS receiver, the distance of the GPS satellite can be determined. Once the GPS receiver estimates the distance to at least four GPS satellites, it can make a three dimensional calculation of its position.