The military first came up with the idea for the GPS system in 1973. The first satellite, called NAVSTAR 1 (an acronym for Navigation Satellite Timing and Ranging), was launched in 1978.
In 1993, the U.S. Air Force launched the final satellite needed to complete the system of 24 satellites orbiting the Earth. Several additional satellites have been launched since then to enhance and refine the GPS system.
I would go with 1993 as the answer.