I have noticed if its not less then 6~7 SATs it gaves the correct reading but little late... like after few mili-seconds or in some cases may be seconds until it receives another signal echoed back by SAT, I tried to check it with CULTUS I noticed while slowly moving say at speed for 30~40kmh it was showing correct speed with in few mili-seconds but then I speeded up to 120+ kmh and noticed that car reached 120 and GPS showed 120 after few or more seconds late until it received the signal from SATs, so it give reading late on bike too as bike accelrate much faster then cars ( but as my bike merely reaches 100kmh very slowly
) so GPS stays correct because it gets enough time ... heh hehehh e ...
So all in all this is true that GPS requires a bunch of seconds to calibrate correct reading with at least 6~8 Sats in range as i think.