The effects of large scale ionospheric gradients on H.F. direction finding at high latitudes

The high latitude ionosphere contains a multitude of large-scale ionospheric density structures. These act as tilted reflecting surfaces, which cause HF (3-30 MHz) radio signals to deviate from the plane of the great circle between the transmitter and receiver. There are also a large number of smaller scale electron density irregularities that scatter the signal out of the plane of the great circle. Consequently, measurements of the bearing of incoming HF signals are often found to be displayed from the great circle bearing of the transmitter, sometimes by as much as 100°. This thesis is a study of bearing measurements obtained during the period 1993 to 1996 over seven high latitude paths of various lengths and orientation. Characteristics of the bearing deviations are found to be strongly related to geophysical parameters such as geomagnetic activity (Ap) indices and the orientation of the interplanetary magnetic field since these also parameterise the morphology of large scale ionospheric density structures. In addition, the daily times of near-great circle propagation are found to relate strongly to Ap on paths lying in the region of the mid-latitude trough. Relationships are identified between bearing deviations and signal parameters such as the signal strength and the Doppler spread of the signal spectrum. Ionograms have been recorded on two paths, and exhibit characteristic features that identify modes of propagation associated with reflections from tilted ionospheric layers and scattering from ionospheric irregularities. The associated bearings have been examined to determine the orientation of these tilts and/or the approximate location of scattering centres. The results of this research may be used as a basis for devising diagnostic tests to determine the level of confidence in the accuracy of bearing measurements made of HF signals at high latitudes.