The Global Positioning System uses a constellation of satellites that go "ping" regularly, giving a location and time stamp. Any receiver can therefore compute the difference in path lengths to the different satellites. And since it knows their positions, thereby compute an approximation to its own location.

The position is only an approximation because the time of flight varies with atmospheric conditions, reflections from surfaces and accuracy of the timing. However integration over time improves the accuracy to within a few meters.

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