Plain English Meaning

GPS works by timing how long a signal takes to reach the receiver from a satellite, so the satellite's clock has to be extremely accurate. The clocks are very precise, but they drift slightly. Even a tiny timing error becomes a noticeable position error on the ground.

Full Definition

Small timing errors in the atomic clocks carried aboard GPS satellites. Because GPS position is calculated from how long a signal takes to travel from a satellite to the receiver, even tiny errors in the satellite's clock translate directly into errors in the calculated position. These clock drifts are monitored by ground control stations and corrected through updates uploaded to the satellites, but residual errors remain a recognized source of GPS position inaccuracy.