If you have ever used a smartphone to navigate an unfamiliar city, you have been relying on a Cold War weapons-navigation system. The Global Positioning System was developed by the U.S. Department of Defense starting in 1973, with the first satellite launched on 22 February 1978; it reached initial operational capability in 1993, full operational capability in 1995, and the current constellation has 31 operational satellites at about 20,200 km altitude.
GPS works on the principle of trilateration: each satellite broadcasts a signal that includes the time of transmission (from an atomic clock) and the satellite’s position; the receiver measures the travel time and computes its distance to the satellite. With four or more distances, the receiver computes its three-dimensional position and the time.
The system was originally operated with two levels of service. The Precise Positioning Service (encrypted, military) provided about 10 m accuracy. The Standard Positioning Service (civilian) was deliberately degraded by selective availability to about 100 m. Selective availability was turned off on 1 May 2000 by executive order, making the full accuracy available to civilian users.
The first operational use of GPS in combat was the 1991 Gulf War, where it was used for navigation, precision weapons, and unit coordination. The system’s effectiveness drove other countries to develop their own systems: the Russian GLONASS, the European Galileo (initial services 2016), and the Chinese BeiDou (global coverage 2018). The 2017 GSA study estimated the global GNSS market at about 150 billion euros per year.
For the rest of the military space article and the space race overview.