Metrological and legal traceability of time signals
Michael A. Lombardi, Demetrios Matsakis, Judah Levine
Metrological traceability requires an unbroken chain of calibrations that relate to a reference, with each calibration having a documented measurement uncertainty. In the field of time and frequency metrology, the desired reference is usually Coordinated Universal Time (UTC), or one or more of its official realizations, termed UTC(k), and traceability to UTC is a legal requirement for many entities. Traceability to UTC can be established in three areas – frequency, time interval, and time-of-day synchronization, but this paper focuses solely on the traceability of time signals used for synchronization. We first examine the definition of traceability, then discuss how traceability can be established via the reception of time signals transmitted by satellites and network time servers, followed by a discussion of how these signals can meet the synchronization and traceability requirements of the financial and electric power industries. Not all of the available UTC time signals are considered in this paper, as we primarily focus on direct broadcast and common-view Global Positioning System (GPS) signals, with uncertainties measured in nanoseconds, and Network Time Protocol (NTP) signals, with uncertainties measured in microseconds and milliseconds.