The first experimental satellite was launched in 1978. By 1994, a full 24 GPS satellites were orbiting Earth. GPS satellites have atomic clocks on board to keep accurate time. The original GPS system began as a project of the U.S. Military. The GPSs are programmed to orbit the Earth in just two full rotations allowing at least four satellites to be "visible". These satellites transmit radio signals to use and provide navigation and timing. GPS enhances civilian life by enabling aviation, search missions, surveying and mapping, trucking, fishing and tracking.