Nick wrote in
:

Is it as great as that? The relativistic effects would be the same
for all satellites, so, while a clock on Earth may disagree with a
clock on the satellite, all satellites would disagree by the same
amount. Therefore, while the uncompensated effect may well be several
kilometres, wouldn't it always be the _same_ kilometres?

No. They originally tried it without any corrections, because many of
the design engineers didn't think it would matter. Turned out it did
matter. The theoretical error is large, although I can't remember the
exact numbers. BTW, it was September 2004, not November.

The GPS receiver doesn't really know the time, it just synchronizes with
the time reported by the satellites. If it thinks the time is different
than what it is, then it thinks it's in the wrong position, because it
calculates position based on the difference in time it takes the signals
to travel from different satellites. The SciAm article has a fuller
explanation, and you can also find several explanations on the net. You
can start at http://www.gpsinformation.net.

--
Regards,

Stan

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." B. Franklin