VLF timing: Conventional and modern techniques including omega

VLF signals from 10 to 30 kHz play an important role in long-range time dissemination, communications, and navigation. A brief history of VLF transmissions is given with particular emphasis on the time dissemination capabilities of the Omega navigational system. Traditional methods are compared with modern capabilities. Traditional lead-edge time dissemination by VLF normally can provide an accuracy of a few milliseconds, although stabilized transmitters can permit an accuracy of better than 100 µs. Frequency comparisons to an accuracy of a few parts in 10
¹¹
per day typically are made with commercially available equipment. The present four-station Omega system will be expanded to an eight-station worldwide network in the near future. With implementation of this global network, carrier ambiguities can be resolved at a receiving station with a cesium frequency standard over intercontinental ranges; phase can be maintained to better than 3 µs and frequency to about 1 part in 10
¹²
. Continuous maintenance of phase to better than 1 µs is indicated when using specialized techniques. Propagation aspects of VLF transmissions limit received accuracy; however, this is tempered by low attenuation of stable signals to long ranges.