How GPS works: how your phone knows where you are
Why GPS needs four satellites, not three
Three distances should be enough to find a place. Yet your phone will not give you a full position from fewer than four GPS satellites. The fourth one has a job you would not guess.
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1
Your phone has three distances, and they do not agree. Its clock is a little off, so the three circles miss each other.
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2
Rono has an idea: put an atomic clock in the phone. But an atomic clock costs far too much for a phone. The fix must come from somewhere else.
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