RESULTS OF AIRBORNE FIELD MEASUREMENTS IN THE ANTIPODAL REGION OF RADIO STATION NPM, HAWAII

The U. S. Naval Research Laboratory is conducting a comprehensive investigation of very-low-frequency vlf radio wave propagation. The objective of this investigation is to determine the parameters necessary for predicting the reliability of vlf communications in the ocean areas of the world. It is believed that an investigation of the electromagnetic fields at and near the antipode of a vlf transmitter would provide much propagation information. Since all the antipodes of the Navy vlf transmitter stations, except for station NPM in Hawaii, are located in remote ocean areas, this makes them operationally difficult to investigate, especially with respect to navigation. The NPM antipode, however, is located on land near Ghanzi, Bechuanaland Protectorate, in the Kalahari Desert in southern Africa. Navigation in the vicinity of the NPM antipode could conceivably be more precise than in other antipodal areas. An analysis of the data obtained in July 1963 by the Naval Research Laboratory via aircraft flights in the vicinity of the antipode of radio station NPM, Hawaii 19.8-kHz transmitter is presented. Definite evidence of convergence of field strengths was found, as expected a peak value of 2.0 mvm for the field was measured during the experiment. Indications of in inverse square root dependence on distance for the field strength were observed. The expected diurnal pattern of highest field strengths at local sunrise, lowest fields at local sunset, and intermediate fields at local noon and midnight was found to be true.