International comparison of atomic frequency standards via VLF radio signals

Abstract

A study was mad e of data obtained over an 18·month period (July 1961to December 1962 on th e co mpari so n of atomic frequency s tandard s located in se ve n laboratories in the Un ited States, Europe, and Ca nada, using th e VLF signals of eBR (16 kc/s), Rugby, England, and NBA (18kc/s), Balboa, Canal Zone. Eac h laboratory observes th e accu mulated diffe ren ce in phase over a 24-hr period (the sa me for a ll laboratories, or nea rly so) be twee n its own sta ndard (e ith e r laboratory or commerciaUy co ns tru cted) and the receive d VLF signal. A stati sti cal analysis was des igned to se parate the obse rvation s at eac h laboratory into three components: (a) lon g-term mean diffe rences a mong the atomic standard s; (b) es timates or the standard deviations , a;, at eac h receiving stati o n; and (c) es timates of the tran s mitt e r standard deviations, 7. Each a; inc lud es rece iver flu ctuations, propaga tion effects pe rc uliar to th e pat h, and meas ure me nt unce rtainties; T in clud es the tran smitter flu c tuation s and propagation effects co mmon to all pat hs.Th e study shows that aLat each .receiver varied from a low of 0.39 X 10-1 0. units of fractional frequency (that is, 0.39 parts in 10 10 ) (GBR data) at LSRH to a high of 1.97 X 10-10 (GBR data) at NRC with an average for all s tations of 1.01 X 10-10 meas ured against eBR and 0.99 X 10-10 when meas ured against NBA. Also, the average T for eBR is 1.26 X 10-10 and for NBA is 0.68 X 10-10 • Fi na ll y, it is shown that: (1) the mea ns of the frequ e nc ie s of th e seven individual laboratories agreed with the grand mean of these seven laboratories to within ± 2 parts in 10 10 for the 18-month period , and (2) the laboratory-type standards agreed with th eir grand mean to within ± 1 part in 10 10

Five Years of VLF Worldwide Comparison of Atomic Frequency Standards

“…Since the Twelfth General Conference of Weights and Measures authorized a temporary atomic designation for the physical measurement of time in October 1964 (General Conference of Weights and Measures XII, 1964), these comparisons of atomic frequency standards have taken on added significance. This paper supplements a previous 18-month comparison of atomic frequency standards (Morgan, Crow, and Blair, 1965) and covers the period 1961 up through part of 1966. Many previous comparison studies through VLF and LF radio transmissions have been reported.…”

“…The methods of comparison and statistical analysis have been described by Morgan, Crow, and Blair (1965 • z BodUy (1966}: .Boctl.ly, Hartke, andHyatt (1966) . Initial analysis of these data yields the long-term mean of the daily observations at each receiving laboratory and the variance ST (or standard deviation Si) of these observations.…”

“…The means give immediately the long-term mean differences between the atomic standard frequencies of the receiving laboratories. By averaging the daily observations from all receiving laboratories and a little further statistical analysis (see Morgan et al, 1965), the variance r\ common to all receiving laboratories, is obtained. Thus 7' includes variations of the transmitter oscillator, the transmitting system, and propagation variations common to all receiving stations.…”

“…These and other substantial variations are easily accounted for by the uncertainties in the estimates. Even 95 percent confidence intervals based ori independence of daily observations place upper limits for these two values of ai at five parts in 10 11 , and intervals taking account of dependence would ' See Morgan et al (1965) for a more complete explanation of the theory and notation. In brief, a, and T are standard deviations of theoretical distributions that could be known place them even higher.…”

Distribution of standard frequency and time signals

“…There are about seven stations broadcasting standard frequency and time signals in the LF band, with transmitted frequency and time interval accuracies ranging from 0. propagation and the improvements in the accuracy of the atomic standards and the VLF receiving equipment, frequency comparisons over long distances with precisions better than one part in 10' ' are now possible. For example, an analysis of data on the comparison by VLF for an 18month period of several widely separated atomic frequency standards [25], [26] indicated that long-term precisions of this order are realizable. Although the analyses did not separate the fluctuations due to the propagation medium from those due to the transmitter or receiving system, the internal consistency of results indicated that the propagation effects were not limiting the measurements.…”