A GPS satellite has a very accurate clock, and transmits its position and time. The signal expands out at the speed of light.

When you receive a signal from one satellite, if you know what time it is *now*, then you know that you're somewhere on a sphere of a given radius

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If you receive signals from two satellites, then there are two spheres that you know that you're on, so you must be somewhere on the intersection of them, which is a circle

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And if you receive three signals, the three spheres intersect in two points, and only one of them is near the Earth's surface, so we know where we are.

But in reality, you need four GPS signals to calculate your position. Why is that?

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Your GPS *receiver* doesn't have an accurate clock; you might receive three signals now, but you don't know when *now* is!

It could be any time where the three expanding spheres intersect. Here I wind time back and forth, and paint our possible positions in red.

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We need to add a fourth GPS signal. The four expanding spheres will intersect at exactly 1 time (and position).

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