Anomalous interference in Omega VLF wave propagation on east-to-west equatorial paths

Abstract

The phase of Omega Haiku (Hawaii U.S.A.) signals has been measured at 13.6 and 10.2 kHz on-board ship from Tokyo to Fremantle, Australia, during the early parts of the winters of 1979 and 1980. Short-term (∼- 1 h) fluctuations are observed on the phase of the Haiku signals as received around the geomagnetic equator at a distance of 8000 km west of the transmitter. Phase cycle slippings take place frequently in association with the phase fluctuations, the occurrence frequency of which is a maximum at 6 S geomagnetic latitude. These propagation anomalies are a consequence of the passage of the ship through an interference pattern the spacing distance of which is about 11 km at 13.6 kHz —one-half of the wave-length of the transmitted wave. It is concluded that the interference is caused by the signal propagated directly from the transmitter and the long-path signal propagating a distance of 32.000 km in the west-to-east direction. This result implies that the attenruation of the east-to-west propagating Omega wave is anomalously great at the equator, in pood agreement with calculated values based on the Galejs' anisotropic waveguide model.