Global Positioning System (GPS) was developed by the U.S. Department of Defense to provide all-weather round-the-clock navigation capabilities for military ground, sea, and air forces.
Since its introduction, GPS has also developed into a valuable tool for a wide range of civilian uses and sectors, including surveying, tracking corporate vehicle fleets, and outdoor enjoyment (such as sailing, flying, and hiking).
They used S-band (SGLS) communications for control and telemetry and Ultra high frequency (UHF) cross-link between spacecraft.
Each spacecraft carried 2 rubidium and 2 Cesium clocks and nuclear detonation detection sensors.
Built by Rockwell Space Systems for the U.S. Air force, the spacecraft measured 5.3 m across with solar panels deployed and had a design life of 7.5 years.