My bet is that they position the satellite in a stable orbit, or in an orbit for which they know the rate of drift. Then the satellite can adjust the position it emits to account for its variation in movement.

Then the question becomes, how to know exactly where a satellite is with precision. This can be done from a few fixed point on Earth with high accuracy, I guess using a technique like described here (using reflectors and the speed of light): https://youtu.be/dsRsap2_RAc?t=3m37s