"Blunkett u-turn on data privacy plans"

[Andrew McLean]

In message <20030303221547.GA28749@elara.ftel.co.uk>, Ian G Batten 
<I.G.Batten@ftel.co.uk> writes
>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.

This link covers the subject of GPS accuracy pretty well

http://jerba.oc.nps.navy.mil/~jclynch/gpsacc.html

and basically backs up Ian.



-- 
Andrew McLean