States; it can be shut down at any time.
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.
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