These days, some people can't find their way across town without consulting the Global Positioning System (GPS) receivers in their vehicles. GPS data comes from at least 24 satellites in medium Earth orbit. These satellites are operated and maintained by the U.S. Air Force. Thus, the GPS program is owned by the U.S. Department of Defense -- and they can shut it down if they deem it necessary. However, GPS has not been deactivated since it became operational in 1995. Concerns about terrorists interfering with the GPS receivers in U.S. civilian aircraft are largely unfounded, since all aircraft are required to maintain a secondary means of navigation, and air traffic control would still be able to maintain proper flight operations.
The GPS constellation includes both old and new satellites, with newer satellites rotating into service as older ones become obsolete or inoperative. The newest satellites have a lifespan of 12 to 15 years, as opposed to older satellites, which have a lifespan of about seven years. As of 2015, the Air Force has 31 operational GPS satellites, just in case of an issue with one of the active units.
More about the Global Positioning System: