On 16/03/12 12:22, Bruce wrote:
It also takes time to do; your car satnav or hand held GPS receiver
gives you a near instant fix to within a few metres, but differential
GPS takes hours* to give an accuracy of millimetres.

That's not differential GPS. DGPS is just as fast as normal GPS and uses
a secondary transmission of local GPS error derived from a GPS receiver
at a known position. It's good to about 10cm.

Higher degrees of accuracy come from techniques like long term averaging
or carrier phase tracking.

Ian