>The GPS system was designed for full coverage at a certian number
>of satellites in the air (24, IIRC).  The original spec called
>for many more for redundancy and additional accuracy, but there
>are only a few 'extra'.

My understanding too, except I believe they have extra ones in orbit, but
idle rather then active so they have immediate availability of a spare if an
active satellite should fail.

>When extra precision is needed in a particular portion of the
>world, I have no doubt that they'll change their orbits slightly
>so there are more in the area needed at one given time.  This
>would necessarily mean fewer in other areas.

I suspect that any extra accuracy or coverage required would be done by just
making a standby satellite active, without shifting the orbit of any others.

For those with Motorola GPS units, it is worth using the Motorola utility on
their website to track the satellites in view. It produces a nice polar
picture of where it believes the satellites are.

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