Abstract
FOR many years standards of frequency have been made available to users over a wide area by means of radio transmissions, the carrier waves of which are controlled by the standard; and the frequencies of 2.5, 5, 10, 15, 20 and 25 Mc./s. have been allocated to this purpose by international agreement. Frequency standards can now be maintained in operation with a stability of a few parts in 109 per month, and they can be intercompared in the laboratory with a precision of ± 1 part in 1010. There is, however, a loss of accuracy due to transmission, and although it has been found that ± 1 part in 108 can be achieved under suitable conditions, the accuracy in general is limited to about ± 2 parts in 107 because progressive movements of the ionospheric layers at which the waves are reflected cause a steadily increasing or decreasing path-length and a consequent Doppler-effect frequency change. In view of this limitation, an experimental transmission on 60 kc./s. was included in the United Kingdom service when this was resumed in 1950, with the call sign MSF. The ionospheric layer at which such low-frequency waves are reflected is known to be comparatively stable, and the ground-wave of the transmission, which is, of course, not affected by changes in the ionosphere, is received over a useful area. Results1 obtained at the National Physical Laboratory show that this transmission can be used in the United Kingdom with an accuracy of ± 1 part in 109 in an observation time of a few minutes; and in spite of its low power of 10 kW., it has been received and measured at Harvard University with the same precision by a somewhat different technique in a time of about 10 min. Higher accuracies of comparison can be obtained by averaging the observations over longer periods.
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References
Essen, L., Proc. Inst. Elect. Eng., 101, Pt. III, 249 (1954).
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PIERCE, J., MITCHELL, H. & ESSEN, L. World-wide Frequency and Time Comparisons by Means of Radio Transmissions. Nature 174, 922 (1954). https://doi.org/10.1038/174922a0
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DOI: https://doi.org/10.1038/174922a0
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