Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Evaluating the Frequency and Time Uncertainty of GPS Disciplined Oscillators and Clocks



Michael A. Lombardi


Global Positioning System (GPS) disciplined oscillators and clocks serve as standards of frequency and time in numerous calibration and metrology laboratories. They also serve as trusted frequency and time references in many industries, perhaps most notably in the telecommunication, electric power, transportation, and financial sectors. These devices are inherently accurate sources of both frequency and time because they are adjusted via the GPS satellites to agree with the Coordinated Universal Time (UTC) time scale maintained by the United States Naval Observatory (USNO). Despite their excellent performance, it can be difficult to evaluate their uncertainty and even more difficult for metrologists to prove their claims of uncertainty and traceability to skeptical laboratory assessors. This paper is written for metrologists and laboratory assessors who work with GPS disciplined oscillator (GPSDOs) or a GPS disciplined clock (GPSDCs) and need to assess their uncertainty and establish traceability to the International System (SI) units. It describes the relationship between GPS time and Coordinated Universal Time (UTC), explains why GPS time is traceable to the SI, and provides methods for evaluating the frequency and time uncertainty of signals produced by a GPSDO or GPSDC.
Measure: The Journal of Measurement Science


frequency, GPS, time, uncertainty


Lombardi, M. (2016), Evaluating the Frequency and Time Uncertainty of GPS Disciplined Oscillators and Clocks, Measure: The Journal of Measurement Science (Accessed May 23, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created December 1, 2016, Updated March 26, 2018