>if two gps receivers are used one stationary an one on the mower the mower's
>position can be calculated to within 1cm.  you read both receiver at the
>same time an calculate the difference then apply this to the fixed and known
>location of the stationary receiver.
>

Two identical receivers, even if they are close to each other, do not
necessarily lock on to exactly the same set of birds. AFAIK, to get
positions down to the cm level you need a special differential receiver to
transmit corrections to the "ordinary" receiver.

For anyone who is interested, there is a newsgroup: sci.geo.satellite-nav

Reg Neale