The Global Positioning System is one of the greatest innovations of modern times.  As a Government Accountability Office study put it in September, “GPS is an essential national security asset, and a key component of economic growth, national infrastructure, and transportation safety.”  That’s because it provides precise location, velocity and timing data to receivers anywhere on earth, day or night, rain or shine.  There are currently 40 GPS satellites operating in mid-earth orbit (12,540 miles up), and a user on the ground is typically within range of signals from nine of them.  You need to receive signals from at least four to establish your position accurately.

from Forbes – Tech http://ift.tt/1i9GYUE
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