This article is from the Astronomy FAQ, by Joseph Lazio (jlazio@patriot.net) with numerous contributions by others.
By Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
Navstar GPS (global positioning system) is a satellite based
navigation system operated by the US Air Force. The signals broadcast
by GPS satellites, contain all information required by a GPS receiver
in order to determine both UTC and TIA highly accurately. Commercial
GPS receivers can provide a time reference that is closer than 340 ns
to UTC(USNO) in 90% of all measurements, classified military versions
are even better.
 
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