Why do GPS satellite orbits need to be monitored and corrected?

GPS satellite orbits require constant monitoring and correction because they are subject to various perturbations that cause them to drift from their intended paths. These variations are primarily caused by the gravitational influence of the Moon and the Sun, the nonspherical shape of Earth, and the pressure exerted by solar radiation.12

The control segment of the GPS system, which includes a Master Control Station and several global monitoring stations, tracks these satellites to detect such orbital changes. Once identified, corrected orbital data is relayed back to the satellites. Additionally, because satellites within a given orbital ring eventually drift too far from their original configuration, ground control must periodically fire onboard thrusters to nudge them back into the correct position.2