GPS satellite positions

"Your GPS unit measures your exact position relative to the satellites which are exactly stationary with the earth." Not quite: they complete their orbit 2x every days. It would be more accurate to say "Your GPS unit measures your exact position relative to the satellites who's positions above the earth are exactly known and transmitted with the time stamp received by your receiver." (Some) Communication satellites are in geostationary orbit (the ones that require you to point a dish at them), GPS satellites are not: they are a system of 24-32 satellites orbiting on 6 different planes.