From: root{at}mauve.demon.co.uk
To: georgea{at}cam.org
Cc: atm{at}shore.net
Reply-To: root{at}mauve.demon.co.uk


>
> Survey grade units with a differential transmitter located at a known
> position.
> There is a wobble signal that is purposely placed on the GPS signal,
> this is to prevent "undesireables" from buying a cheap GPS unit and

This is incorrect, since several years ago. The dither has been removed.
http://www.mauve.demon.co.uk/gps.html
shows some graphs I did of the errors. 99% of the points are within 12m,
averaging for 4 hours takes this to 4m.
This is with a rather old reciever, a new one may get a factor of a few
better, as there is now a transmitted ionospheric correction IIRC.
> using it to guide something nasty to a pre-programmed target. I believe

This is also done by requiring (at least in the US, and as everyone wants
their GPSs to be able to be sold there) limits on altitude and velocity,
beyond which the GPS will not report positions.

> the accuracy for non P-code GPS units is around 50 metres (or more).
> P-code GPS units are available but even then I believe that the accuracy
> is within a few metres.

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