Results of the two-frequency VLF transmission experiments from Criggion GBZ

During the period August–December 1965, time-duplex transmissions in the frequency range 11.6–13.8 kHz were radiated from the VLF station GBZ at Criggion, Wales. Measurements on these transmissions were made at a few selected sites at large distances from the transmitter, and also by monitoring the signals aboard a high-speed aircraft. A study of the rms variation of the phase delay of the difference frequencies in the range 200 to 2200 Hz, indicate that the phase-delay fluctuations of two frequencies separated by 200 Hz are not independent. The implications of this in terms of navigation requirements are discussed, it being shown that on the available experimental evidence, for rms errors of 5 nm, the use of two-beat frequencies (200 Hz and 1.2 kHz) would be sufficient for navigation assuming a common frequency near 11 to 12 kHz. Further, the measurements of the propagation characteristics obtained by monitoring the transmissions as a function of distance in an aircraft indicated that modal interference out to distances of order 3000 km contributes an important part to the overall rms phase-delay fluctuations throughout a 24-hr period.