hmmh...still does not explain this calibration concept. Having read GPS's specs I don't remember such a concept. And as for number of sats, 3 sats won't give errors of 200 yards. I have never seen such a problem with mine. This sounds really strange...This is why I introduced the idea of the distance information. Maybe still a side effect. In certain conditions it *might* assumre it is in a tunnel while it's not...who knows :-)