GPS

The Global Positioning System (GPS) is run by the US Air Force.  It consists of several dozen satellites, a half dozen ground stations, and a huge number of GPS receivers that calculate accurate time and position from data broadcasted by the satellites.  The ground stations track the satellites very accurately, and make adjustments their orbits and to their clocks due to relativistic effects of satellite speed and the force of gravity on the satellite.

Originally the GPS signals from the satellites could be degraded in a way only the military (who after all paid for the system) could correct.  This degradation turned out to be unpopular and was turned off by Presidential decree.  Newer satellites have separate frequencies for military use only whose data are encrypted.

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