Measurements of the direction of arrival of short radio waves reflected at the ionosphere

Abstract

Using a phase-comparison method described in another paper, measurements have been made of the direction of arrival of waves reflected at the ionosphere during normal daytime conditions, both over an oblique path corresponding to a range of 700 km. and at nearly vertical incidence. The frequency range covered was 4 to 15 Mc./see., and using pulsed signals the direction of arrival of discrete rays could be studied. The work so far carried out has been confined principally to first-order F -layer reflexions, but a few observations of second-order reflexions and of reflexions from the E region have also been made. The results show that a typical first-order F echo exhibits both rapid (second to second) fluctuations in direction and slow changes of a quasi-period of many minutes (ranging from a few up to more than 30 min.). These results confirm earlier deductions from direction-finding studies and are interpreted as indicating that large-scale and apparently random tilts occur in the ionosphere. The F layer at any point is to be regarded as usually tilted in a random direction, the r. m. s. magnitude of the tilt being of the order of 1·5 to 2° and the tilt changing at a rate corresponding to the slow directional changes observed. Some indication as to the extent of these tilts is given by experiments using two direction-measuring stations separated by 27 km. These two stations, operated simultaneously, exhibit extremely close correlation so far as the slowly changing directional deviations are concerned, both for oblique paths and at nearly vertical incidence. The results are consistent with others obtained in Australia by different methods which suggest that there are compressional waves in the atmosphere at the height of the F layer, and that these waves have a wave-length of several hundred kilometres and a velocity of some 5 to 10 km./min.