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 implementation, GPS has also become an integral asset in numerous civilian applications and industries around the globe, including recreational used (e.g., boating, aircraft, hiking), corporate vehicle fleet tracking, and surveying.
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.