How GPS finds a location — Technology, 14–17 years
How timing signals from satellites help a receiver estimate where it is. Technology, 14–17 years.
Position from timing
GPS receivers listen to signals from satellites whose positions and clocks are known. Each signal says when it was sent, so the receiver estimates how far it travelled by measuring the delay. Several distance estimates meet at one region, giving a position on Earth, usually with a small error.
Why satellites and time?
A phone cannot simply ask the ground where it is, because no single local marker covers the whole planet. Satellites provide known reference points above us, and radio signals travel at a nearly known speed. Measuring their travel time turns an invisible delay into distances that can locate the receiver.
Three distances on a map
Imagine three landmarks. You are 5 km from the first, so you are somewhere on its 5 km circle. You are 4 km from the second, leaving only the circle intersections. A third distance, say 3 km, usually selects one intersection. GPS does the same in three dimensions, using very precise timing and an extra signal to correct the receiver’s clock.
The blue dot is not exact
It is reasonable to treat the blue dot as your exact position: the map draws one clear symbol. But measurements contain delays, reflections from buildings, atmospheric effects and clock errors. The phone is estimating a best position, often shown with a circle of uncertainty. Indoors or between tall buildings, that circle can become much larger.
More than driving directions
GPS supports navigation, emergency location, aircraft and ship routes, farming machines and the timing used by communication networks. A running app can estimate speed and distance by comparing positions over time. It is valuable when its uncertainty is understood; a map position should not be treated as proof of exactly where a person is.
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