How GPS works (was Re: Bus Information Signs)

Clive D. W. Feather wrote:
> In article <Pine.LNX.4.64.0804121354430.17885@urchin.earth.li>, Tom
> Anderson <twic@urchin.earth.li> 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?