"Blunkett u-turn on data privacy plans"

[Ian G Batten]

On Mon, 03 Mar 2003, Fearghas McKay wrote:
> > >Don't forget specialised GPS receiving
> > >stations are used to track minute movements (down to the order of 100
> > >microns) in e.g. earthquake research.
> >
> >I have never heard of such a thing. Most claim even differential GPS is
> >only good to a metre.
> 
> it is differential movement that is being measured not absolute
> positioning. You have two antenna and you measure where thwy are in
> relation to each other. Similar tricks have been used on ships for years as

I don't believe GPS is that accurate.  ``Survey'' GPS, aka Carrier
Positioning, will get you about a centimetre of precision, but in
nothing like real time nor at any specified time.  You can't just take
positional differences, because the factors which affect GPS precision
aren't constant over either distance or time.  A claim that you can
measure to a precision of 100 microns with a 1.5GHz (of the order of
20cm) signal wavelength would be surprising, to say the least.

The fine-grain work on tectonic movements around the Bay Area is done
with various interferometry techniques, either radar or laser.  So far
as I know, the GPS systems are used to establish the long-term movement
of the whole plate, for which centimetre precision is fine.

ian