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Why do GPS satellites sometimes need thruster adjustments from ground control?

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Cruise ship captain in uniform studies navigational charts with a compass on a table in the ship's bridge. — navigation Officers on a passenger ship using charts for navigation.

GPS satellites require periodic thruster adjustments from ground control because their orbits are subject to various external forces that cause them to drift from their intended configurations.1

These orbital variations are primarily caused by:
* Gravitational influences: The gravitational pulls exerted by the Moon and the Sun perturb the satellites' paths.1
* Earth's shape: The Earth is not a perfect sphere; its nonspherical mass distribution, including the equatorial bulge, affects satellite motion.21
* Solar radiation pressure: The pressure exerted by solar radiation on the satellite's surface creates a force that can alter its trajectory over time.13

Monitoring stations track these changes and relay corrected orbital information to the satellites. On average, every 18 months, the satellites drift sufficiently far from their original configuration that they must be "nudged" back into position using onboard thrusters fired by ground control to maintain the precision of the GPS constellation.1