Clive D. W. Feather wrote:
In article , Tom
Anderson writes

Unlikely as this may sound, it works by picking up
synchronised radio signals from members of a family of 31 satellites
orbiting 20 000 km above the earth, measuring the time differences
between them with an accuracy of a few nanoseconds, which tells you
the differences in distances to the satellites with an accuracy of a
few metres (out of twenty million - not bad!), then doing geometric
calculations to work out where that means the receiver must be. It's
the kind of thing that if it didn't exist, you'd think it was an
absurd idea.

I don't know if it'll help, but here's how I explain it to my kids.

You're lost because it's foggy or pitch black. You have a map of the
area but can't see any landmarks. You've lost your watch. However, you
know that the local church clocks are accurate, they strike exactly
one second between beats, and each has a different set of chimes so
you can tell which one you're hearing.

You hear a clock chime and strike the hour. 20 seconds later (using
the first set of strikes to time things) you hear another clock.
Since sound travels at 300m/s you know that you're 6km closer to the
first clock than the second one. Some careful thought allows you to
draw a curve on your map which is all the places 6km closer to the
first church than the latter - you are somewhere on that curve.

Meanwhile, 12 seconds after the second clock you heard a third one.
You are therefore 3.6km closer to the second than the third and 9.6km
closer to the first than the third. These let you draw two more lines
that you're also on and, hopefully, all three lines cross at exactly
one place, which is where you are.

GPS uses radio (which moves much faster) rather than sound, and the
transmitters keep moving; however, the signal coming from them says
exactly where they are, so that isn't a problem. Nonetheless the
principle is the same.

How do the receivers cope with dozens of satellites all broadcasting on the
same frequencies? Time splicing?