Atomic Clocks and Time Scales

Abstract

This chapter presents atomic frequency standards with high-stability frequency of oscillations. Also discussed is the Allan variance used for measuring the frequency fluctuations in such standards. We present the design of the 9.2 GHz caesium standard, which is presently the most important atomic standard. We discuss the directions of development and the metrological parameters of caesium atomic standards: more complicated and expensive, but providing better accuracy caesium fountain standards, and much cheaper miniature caesium standards with the size of a matchbox. Also discussed are other frequency standards: hydrogen masers, rubidium standards and the currently implemented standards with a visible wavelength. Optical frequency standards are potentially 10⁵ times more stable than the caesium standard, though at present their actual stability is only 10 times higher. Together with the optical frequency standards we discuss the optical comb, used for mapping frequencies of the order of 10¹⁴ Hz to MHz frequencies. Also discussed are the major time scales, the International Atomic Time (TAI) and the Coordinated Universal Time (UTC), as well as the role of satellite navigation systems (GPS, GLONASS, BeiDou) in the dissemination of the standard time signal.