The clocks on board the satellites have their rates corrected so that they run at the same rate as ground clocks--i.e., the satellite clocks' effective rate is not the "natural" rate of a clock at their altitude and moving at their orbital velocity. The ground stations then send corrections to the satellite clocks to compensate for the fact that the rate correction that's built in to the clocks on the satellites isn't exactly right, because the satellites are not in perfectly circular orbits at constant altitude.