Determining the Uncertainty of Frequency Measurements Referenced to GPS Disciplined Oscillators

GPS disciplined oscillators (GPSDOs) are commonly used as references for frequency calibrations. Over long intervals, a GPSDO is an inherently accurate source of frequency because it is continuously adjusted to agree with the Coordinated Universal Time (UTC) time scale maintained by the United States Naval Observatory (USNO). However, most frequency calibrations last for intervals of one day or less, and it can be difficult for metrologists to determine the uncertainty of a GPSDO during a short interval, and even more difficult to prove their uncertainty claims to skeptical laboratory assessors. This paper can serve as a guide to metrologists and laboratory assessors who work with GPSDOs as frequency standards. It describes the relationship between GPS time and Coordinated Universal Time (UTC) and explains why GPS time is traceable to the SI. It discusses how a GPSDO utilizes the GPS signals to control the frequency of its local oscillator. It explains how to estimate frequency stability, and how to apply estimates of frequency stability to determine the uncertainty of a GPSDO used as the reference for a frequency calibration.