It ties all the satellite measurements into a solid local reference. Why Differential GPS.Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements. 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.Ī fourth measurement does come in very handy for another reason however, but we'll tell you about that later. To decide which one is our true location we could make a fourth measurement. If we then make a measurement from a third satellite and find that we're 13, miles from that one, that narrows our position down even further, to the two points where the 13, mile sphere cuts through the circle that's the intersection of the first two spheres. Or in other words, we're somewhere on the circle where these two spheres intersect. That tells us that we're not only on the first sphere but we're also on a sphere that's 12, miles from the second satellite. Next, say we measure our distance to a second satellite and find out that it's 12, miles away. Knowing that we're 11, miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11, miles. First consider how distance measurements from three satellites can pinpoint you in space.
Forget for a moment how our receiver measures this distance. That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth. Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth.
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