In article , Tom
Anderson writes
I have no idea how GPS works but if it uses any form of cellular
pattern to locate a vehicle I did wonder if we had crossed from one
cell to another in the second example.

It doesn't. 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.

--
Clive D.W. Feather | Home:
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