The satellites go "ping" on a regular basis, announcing where they are, and the exact time. Your Sat Nav listens to the satellites and notices that the pings arrive at different times. That's because it takes time for the signals to travel, and the further the satellite, the longer it takes.
Not surprising really.
But how does that help? Well, if the signal from satellite A arrives 5 millionths of second before the signal from satellite B, we are about a mile closer to A than B. That puts us on a funny looking surface, which intersects the Earth in a great curve (although not a Great Circle!) We can compute that curve because the satellites have told us where they are.
Now do this with another satellite, and another, and the possible places you could be get more and more limited. Until there's only one place you can be.
And that's where you are.
Purely passive, you don't tell the satellite anything, it doesn't know where you are, and you know where you are without transmitting anything. Ideal for the military, who first designed it.
The maths isn't hard, but it's non-trivial, and very clever.
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