If we then make a measurement from a third
satellite and find that we're 13,000 miles from that one, that narrows our position down
even further, to the two points where the 13,000 mile sphere cuts through the circle
that's the intersection of the first two spheres.
So by ranging from three satellites we can narrow our position to just two points in
space.
To decide which one is our true location we could make a fourth measurement. But
usually one of the two points is a ridiculous answer (either too far from Earth or moving
at an impossible velocity) and can be rejected without a measurement.
A fourth measurement does come in very handy for another reason however, but we'll tell
you about that later.
Next we'll see how the system measures distances to satellites.
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