Time and Frequency Metrology in the Context of Relativistic Geodesy

Abstract

A status report is given on current practice and trends in time and frequency metrology. Emphasis is laid on such fields of activity that are of interest in the context of relativistic geodesy. In consequence, several topics of a priori general relevance will not be dealt with. Clocks and the means of comparing their reading are equally important in practically all applications and thus dealt with in this contribution. The performance of commercial atomic clocks did not change significantly during the last 20 years. Progress is noted in the direction of miniaturization, leading to the wide-spread use of chip-scale atomic clocks. On the other hand, research institutes invested considerably into the perfection of their instrumentation. Cold-atom caesium fountain clocks realize the SI-second with a relative uncertainty of close to \(1\times 10^{-16}\), and with a relative frequency instability of the same magnitude after averaging over a few days only. Optical frequency standards are getting closer to being useful in practice: outstanding accuracy combined with improved technological readiness can be noted. So one necessary ingredient for relativistic geodesy has become available. Satellite-based time and frequency comparison is here still somewhat behind: Time transfer with ns-accuracy and frequency transfer with \(1\times 10^{-15}\) per day relative instability have become routine. Better performance requires new signal structures and processing schemes, some appear on the horizon.