That isn't at all how the ephemeris data is represented in GPS. The satellite locations/headings are specified in earth-fixed geocentric coordinates. So, it's a

That isn't at all how the ephemeris data is represented in GPS. The satellite locations/headings are specified in earth-fixed geocentric coordinates. So, it's as if the earth was sitting still and the satellites were swarming around it. The effect of earths very slow changes in rotation speed would effect the accuracy, but the ephemeris data is periodically updated based on observations of satellite locations by ground stations, there are many larger sources of positioning error than changes in the earth's rotation. This is also why first fix from a cold (memory wiped or long offline) receiver takes a long time even when it knows your location, unless you have AGPS (e.g. via the cell network): it doesn't know what satellites are overhead and it has to do a brute force search before it can find the signals and download the ephemeris data. (There is a side channel that carries info on leap seconds, so that civil timing stuff can derrive UTC from the GPS time. ... but that isn't used by GPS itself at all). Here is a nice page that shows the current GPS time and notes that it does not include leap seconds: http://www.leapsecond.com/java/gpsclock.htm http://www.leapsecond.com/java/gpsclock.htm