White Rabbit-Based Time Distribution at NIST

The National Institute of Standards and Technology (NIST) produces a real-time realization of UTC(NIST) which is used to contribute to Coordinated Universal Time (UTC) and as a source for accurate time in the USA. The atomic clocks contributing to the time scale ensemble, the time transfer systems used to contribute to UTC and the distribution system used to disseminate UTC(NIST) to remote users are located in different parts of the NIST campus, far from each other and from the UTC(NIST) reference point. Since the physical inputs to these systems are not collocated within the campus, an accurate and stable infrastructure for time signal distribution is required. Currently, the local delays need to be known with an uncertainty of a few hundreds of picoseconds to avoid compromising the ultimate accuracy of the time transfer link's calibrations. Previously, coaxial cables or a commercial fiber-based frequency transfer system implemented by amplitude-modulation of a laser source were used to distribute signals between on-site locations, and clock trip calibrations were performed to measure the delays experienced by these signals [1]. The capability of WR-based time transfer systems to provide an on-time, accurate remote copy of its input pulse-per-second (PPS) signal made it a very appealing alternative to our previously implemented distribution system, which required time consuming re-calibration following instances of temporary signal interruptions. In this paper, we evaluate the use of WR-based time and frequency transfer within the NIST campus and verify its calibration procedure using a clock trip protocol [1].