The algorithm used to realize UTC(NIST)

The National Institute of Standards and Technology (NIST) maintains a real-time estimate of Coordinated Universal Time (UTC), which is called UTC(NIST). UTC(NIST) is realized by steering the output of a hydrogen maser to minimize the difference between UTC(NIST) and UTC, a time scale computed by the International Bureau of Weights and Measures (BIPM). The difference between UTC(NIST) and UTC is published in the BIPM monthly Circular T. The reference for UTC(NIST) is AT1, a free-running time scale computed at NIST in real-time from an ensemble of cesium clocks and hydrogen masers maintained at the NIST laboratory in Boulder. The steering that implements UTC(NIST) is realized using a commercial phase stepper that produces an output that can be offset either in time or in frequency from the input signal from the maser. The current algorithm, which has been used for many years, maximizes the frequency smoothness of UTC(NIST) at the expense of its time accuracy. Since both AT1 and UTC have improved significantly in the last few years as a result of better clock hardware, this algorithm may no longer be optimum. We are investigating other choices which may be able to preserve the desirable goal of frequency smoothness while at the same time improving the RMS time accuracy.