International Comparisons of Network Time Protocol Servers

This paper describes a recently designed system that measures the time transmitted by network time protocol (NTP) servers located in North, Central, and South America. Direct measurements of the time transmitted by each server are obtained by comparing the received time stamps to the Cordinated Universal Time scale, UTC(NIST), located at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. International comparisons of the time differences between servers are obtained by utilizing UTC(NIST) as a common-view observation signal. Results of both direct and common-view measurements are published in real time via the web site of the Sistema Interamericano de Metrologia (SIM) Time and Frequency Metrology Working Group, providing verification of a given server’s accuracy. The paper describes the NTP measurement system, and presents results from direct comparison and common-view measurements. It also discusses factors that contribute uncertainty to NTP measurements; in particular network asymmetry caused by inconsistent routing and/or network congestion.