A GPS Clock, or GPS disciplined oscillator (GPSDO) is a combination of a GPS receiver and a high-quality, stable oscillator such as a quartz or rubidium oscillator whose output is controlled to agree with the signals broadcast by GPS and GNSS satellites.[1][2] GPSDOs work well as a source of timing because the satellite signals must be accurate in order to provide positional accuracy for GPS in navigation. These signals are accurate to nanoseconds and provide a good reference for timing applications.[3][4]
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GPSDOs serve as an indispensable source of timing in a range of applications, and some technology would not be practical without them.[5] GPSDOs are used as the bases for Coordinated Universal Time (UTC) around the world. UTC is the official accepted standard for time and frequency. UTC is controlled by the Bureau International des Poids et Mesures (BIPM). Timing centers around the world use GPS to align their own time scales to UTC.[6][7] GPS based standards are used to provide synchronization to wireless base stations[8] and serve well in standards laboratories as an alternative to cesium based references.[3]
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. The disciplining mechanism works in a similar way to a Phase Locked Loop (PLL), but in most GPSDOs the loop filter is replaced with a microcontroller that uses software to compensate for not only the phase and frequency changes of the local oscillator, but also for the ‘learned’ effects of aging, temperature and other environmental parameters.[3][9]
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 propagation effects such as multipath, atmospheric conditions and other impairments. On the other hand a quality oven controlled oscillator has better short term stability but is susceptible to aging and other long term effects. A GPSDO aims to utilize the best of both sources, combining the short term stability performance of the oscillator with the long term stability of the GPS signals to give a reference source with excellent overall stability characteristics.[10]
When the GPS signal becomes unavailable, the GPSDO goes into a state of holdover, where it tries to maintain accurate timing on its own.
Sophisticated algorithms are used to compensate for the aging and temperature stability of the oscillator while the GPSDO is in Holdover.[11]
The use of Selective Availability (SA) prior to May 2000 restricted the accuracy of GPS signals available for civilian use and in turn presented challenges to the accuracy of GPSDO derived timing. The turning off of SA resulted in a significant increase in the accuracy that GPSDOs can offer.[12]