text: space-time : what is the "length" difference of 1meter at 2000 km from earth ground and 1m on ground ? ~2000 km is the gps satellites altitude. i know gps system use the einstein theory that say there is a curvature of space-time due to earth mass, to adjust data.  if i take something we consider to be 1 meter length on groud to 2000km altitude, it will be (a bit) bigger : how much ? and same question for 2000km from ground.  the reference to measure length is the lightspeed. a meter has the same length no matter where you are.  the gps system might use the curvature of the earth to modify the length of what the system describes as a meter, because a curved meter spans a shorter distance along one axis.  the length of a meter is immutable and is defined as: the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second where the speed of light, c, is 299,792,458 m/s.