Global Satellite Navigation Constellations

The United States GPS program traces its origin to the 1973 merger of several U.S. military satellite navigation efforts, with the first Block I satellite launched on February 22, 1978, from Vandenberg Air Force Base. The full constellation of 24 satellites was completed in 1993 and the system was declared to have achieved Full Operational Capability on July 17, 1995. A pivotal policy shift occurred on May 1, 2000, when President Bill Clinton ordered the discontinuation of Selective Availability, a deliberate degradation of civilian signal accuracy that had been in place since the system opened to civilian use, instantly improving typical civilian positioning accuracy from around 100 meters to roughly 15–20 meters, and to a few meters with today's modernized signals and receivers. GPS is operated and maintained by the U.S. Space Force and typically comprises around 31 operational satellites distributed across six orbital planes at an altitude of approximately 20,200 kilometers, each completing an orbit in about 11 hours 58 minutes.