Thank you 404t, I didn't know that.
I just follow in the general direction that the arrow thingy on my Etrex is pointing - which can be a problem if I'm not holding it absolutely flat, because then I climb or descend a bit
Seriously though, last week a mate was delivering a vessel to Adelaide and the GPS went "strange" for about half an hour. Way off course. Haven't checked RAIm ephermises etc., but the only thing he could put it down to was a dud satellite.