The GPS satellite clock bias, drift and drift-rate are explicitly determined in the same procedure as the estimation of the satellite ephemeris. The behaviour of each GPS satellite clock is monitored with respect to GPS Time, as maintained by an ensemble of atomic clocks at the GPS Master Control Station in Colorado Springs. The offset, drift and drift-rate of the satellite clocks are available to all GPS users as clock error coefficients broadcast in the Navigation Message (section 3.3.2).
What is available to users is actually a prediction of the clock behaviour for some time into the future (24 hours or more ahead). As the random deviations of even cesium and rubidium oscillators are not predictable (section1.3.2), such deterministic models of satellite clock error are accurate to about 20 nanoseconds, or six metres in equivalent range, depending upon the time since the last Navigation Message update. Selective Availability is a further artificial dithering of the satellite clocks causing several dekametres error in the range (or phase-range equivalent).