GPS disciplined oscillator

[10] A GPSDO works by disciplining, or steering a high quality quartz or rubidium oscillator by locking the output to a GPS signal via a tracking loop.

[3][11] One of the keys to the usefulness of a GPSDO as a timing reference is the way it is able to combine the stability characteristics of the GPS signal and the oscillator controlled by the tracking loop.

GPS receivers have excellent long-term stability (as characterized by their Allan deviation)[7] at averaging times greater than several hours.

However, their short-term stability is degraded by limitations of the internal resolution of the one pulse per second (1PPS) reference timing circuits, signal propagation effects such as multipath interference, atmospheric conditions, and other impairments.

When the GPS signal becomes unavailable, the GPSDO goes into a state of holdover, where it tries to maintain accurate timing using only the internal oscillator.