Five Years of VLF Worldwide Comparison of Atomic Frequency Standards

Abstract

The VLF radio broadcasts of GBR(16.0 kHz), NBA(18.0 or 24.0 kHz), and NSS(21.4 kHz) have enabled worldwide comparisons of atomic frequency standards to parts in 1010 when received over varied paths and at distances up to 9000 or more kilometers. This paper summarizes a statistical analysis of such comparison data from laboratories in England, France, Switzerland, Sweden, Russia, Japan, Canada, and the United States during the 5‐year period 1961–1965. The basic data are differences in 24‐hr average frequencies between the local atomic standard and the received VLF radio signal expressed as parts in 1010. The analysis of the more recent data finds the receiving laboratory standard deviations, âi;, and the transmission standard deviation, τ, to be a few parts in 1011. Averaging frequencies over an increasing number of days has the effect of reducing âi and τ to some extent. The variation of the âi; with propagation distance is studied. The VLF‐LF long‐term mean differences between standards are compared with the recent portable clock tests, and they agree to parts in 1011.