Abstract :
Standard frequency transmissions were first used in 1923, and since then their importance has steadily increased. It is intended to give them a world-wide continuous coverage, and the frequencies of 2.5, 5, 10, 15 and 25 Mc/s have been allocated to this service. Transmissions from the United Kingdom have been resumed and extended since the end of the 1939¿45 War and give good coverage over the important area within a radius of about 1000 miles, in which reception of the United States WWV service has been poor, particularly during the last few years. The accuracy required in the measurement of frequency can for many purposes be expressed in cycles per second rather than as a proportion of the frequency, and it has in consequence increased as the higher-frequency radio bands have come into use. A close study of the accuracy that can be achieved has therefore been made at the National Physical Laboratory. It has been found that, although an accuracy of frequency comparison of ±2 parts in 108 can be relied upon when conditions are such that the wave is reflected from the E-layer of the ionosphere, random variations of several parts in 107 occur at other times, in addition to a systematic effect at dawn and sunset. An accuracy of ±2 parts in 109 can, however, be achieved with the experimental transmission on 60 kc/s. Accuracies of ±2 parts in 109 can also be achieved by using the timing pulses of the high-frequency transmissions, but this method involves the use of high-grade local standards and fairly elaborate auxiliary equipment.
