On Observations of Modal Interference of the North Dakota Omega Transmission

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CURRENTLY, THE RESEARCH TRIANGLE INSTITUTE is under contract to NASA to provide support in evaluating OMEGA phase data being collected by NASA personnel. Phase data from Norway, Trinidad, and North Dakota are being recorded at all three OMEGA frequencies at various locations within approximately 500 km (300 n.mi.) of Hampton, Virginia. At most of the receiver sites modal interference of the North Dakota signal has been observed. Previous studies indicate that modal interference generally is significant only within approximately 1500 km of the transmitter, is more prevalent at night, usually is more predominant at 13.6 kHz than at 10.2 kHz, and generally manifests itself as a shift in the average phase measurement at a given receiver site. The temporal effects of modal interference other than the presunrise phase shift have not been reported and apparently are only rarely experienced. This paper discusses time varying modal interference experienced during several phase measurement periods in the winter of 1973–1974. Interference was observed at different receiver sites all located approximately 2 Mm from North Dakota. Large temporal phase fluctuations were noted in the nighttime phase measurements on several occasions. Average nighttime phase levels in some cases corresponded very closely to the daytime values over large portions of the nighttime period. In other cases this occurred for approximately two hours between the middle of the nighttime period and sunrise. In still other cases the shift from the normal nighttime level to near daytime level and back has been more sinusoidal with a period of from one to four hours. There have been isolated situations when peak to peak fluctuations have been on the order of 50 cec and greater. Amplitude fluctuations generally accompany these large phase fluctuations. In some cases modal interference during the daytime has appeared as a more conventional level shift with little time variation. An analysis of these temporal phase perturbations due to modal interference is presented using a theoretical model. Conditions which may have caused this interference at these relatively long distances from the transmitter are discussed.