Bum, I thought I'd deleted this thread, not posted it.

TA-t3: There are two different types of differential GPS. Most people recognise the type you mention. But you can buy commercial surveying equipment that achieves much greater accuracy by using two or more GPS receivers and processing the data from them together - either in real time over a radio link to a computer or in post-processing. Accuracy can be less than a centimetre (I am a volcanologist and we use this type of GPS data to measure deformation of volcanoes.)

"Proper" differential GPS won't work on something like the N810. Commercial units use the L2 frequency (no, I don't know what it is either) and information that isn't available in NMEA sentences.

So I think all could be done is just differencing the two GPS measurements. I haven't tested this, but it might give accuracy of about 1m under the right conditions. Good enough to find a car, but you'd have to be within 10m for the bluetooth to work anyway.

Another idea for greatness dead in the water.

Darius2006, I am not a coder. I can script, but the last programming I did was in FORTRAN, in the days when it was in capital letters. The n800 might just get me back into coding - I am tempted to do something based on gps-saver.