G'day Jethro,
The guys at
www.gps-speedsurfing.com/default.aspx have been looking at this quite extensively and that is why they have standardised on only a few GPS's.
They also have standardised on the software used to analise the data to also limit irregularities from coming out.
A few of the sailors have also done quite some research by taking out two different types of GPS at the same time to compare data. This information can be picked up in the forums (sorry I couldn't find the thread for you)
Setup of the units to take identical readings ie 2 sec as a aginst 1 sec can affect data as well.
Positioning of the units is also important so as they have a relatively clear acces to sky.
If they set up and analyse the data as described at GPS speed then the results should be reasonably consistant.
For more information Slowboats the man as he lives and breathes this stuff.
Alby
.