On Fri, 29 Aug 2008, Tom Hughes wrote:
> He was referring to a combine using a DGPS system which has a base
> station at a fixed point on the farm whose location is well known. It
> then compares that known location to one calculated from the satellites
> in the normal and broadcasts the difference to the mobile received on
> the tractor/combine which uses the difference to correct it's own
> calculated position.
A DGPS station actually works out how big an error each received
satellite signal has and transmits that data to the (mobile) GPS receiver,
which then applies the correction _before_ calculating the location.
(i.e. the DGPS signal contains the timing errors for each satellite rather
than the errors in the coordinates, since the errors in the calculated
coordinates would depend on which satellites the GPS is using, which is
something the DGPS transmitter doesn't know).
I suspect that a system that accurate is probably not just using DGPS
though - it probably has a set of ground-based transmitters at known
locations that it uses for ranging as well.
- Steve
xmpp:[EMAIL PROTECTED] sip:[EMAIL PROTECTED] http://www.nexusuk.org/
Servatis a periculum, servatis a maleficum - Whisper, Evanescence
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