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 or other GNSS satellites. GPSDOs work well as a source of timing because the satellite time 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. Applications GPSDOs serve as an indispensable source of timing in a range of applications, and some technology applications would not be practical without them. GPSDOs are used as the basis for Coordinated Universal Time (UTC) around the world. UTC is the official accepted standard for time and frequency. UTC is controlled by the International Bureau of Weights and Measures (BIPM). Timing centers around the world use GPS to align their own time scales to UTC. GPS based standards are used to provide synchronization to wireless base stations and serve well in standards laboratories as an alternative to cesium-based references. GPSDOs can be used to provide synchronization of multiple RF receivers, allowing for RF phase coherent operation among the receivers and applications, such as passive radar and ionosondes. Operation 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. 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) 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. On the other hand, a quality oven-controlled oscillator has better short-term stability but is susceptible to thermal, aging,