In this paper, a statistical analysis of the observations of HF signals propagating over two paths, one that crosses the auroral oval and one in the polar cap, are compared to the predictions given by VOACAP. Large deviations in the azimuth of arrival were observed on both paths with upper-decile values of 90° for the trans-auroral oval path, and 70° for the polar cap path. For the trans-auroral path, VOACAP generally underestimates the signal strength in the presence of off-great circle propagation, while for the polar cap path, the signal strength is underestimated when VOACAP predicts propagation via a 2E mode that does not occur.

The ionosphere within and close to the auroral zones is a dynamic propagation medium that is not perfectly horizontally stratified, and consequently the signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and elevation. In order to better understand this type of propagation, measurements have recently been made over two paths: (a) from Svalbard to Kiruna, Sweden, and (b) from Kirkenes, Norway to Kiruna. An analysis of these data is presented in this paper. The directional characteristics are summarised, and consideration given to modelling the propagation effects in the form of a channel simulator suitable for the testing of new equipment and processing algorithms.

The high latitude ionosphere is a very dynamic and disturbed region containing irregularities which, on scales much greater than a wavelength, may be considered as providing a rough reflecting surface for obliquely propagating HF radiowaves. To improve our knowledge and understanding of the complex propagation mechanisms prevalent in the high latitude region, an experimental campaign is currently being conducted with a receiver system capable of measuring the delay and Doppler spread characteristics and the directional structure of the received signals at Kiruna in northern Sweden.

Observations over recent years have established that large scale electron density structures are a common feature of the polar cap F-region ionosphere. These structures take the form of convecting patches and arcs of enhanced electron density which form tilted reflection surfaces for HF radiowaves, allowing off great circle propagation paths to be established. Numerical ray tracing has been employed to simulate the effects of these structures on the ray paths of the radiowaves. The simulations have reproduced the precise character of experimental observations of the direction of arrival over a propagation path within the polar cap and of oblique ionograms obtained over the same path.

A new network of transmitters and receivers has been installed in northern Scandinavia (including Svalbard), capable of measuring the time delay and Doppler spread characteristics and the directional structure of received signals on three HF paths. Based on the large amount of data expected from this experiment, a statistical classification of signal directional characteristics under various geophysical conditions and for all seasons can be made. In this paper, cases of both quiet and disturbed geomagnetic conditions have been studied, and comparisons have been made with data from magnetic disturbance observatories in the same geographic region.

The performance of high frequency direction finding systems is known to be related to the frequency of operation and to the propagation mode content of the incoming signal, which is dependent upon the electron density distribution in the ionosphere. In addition to large scale tilts that cause gross deviations of the signal from the great circle direction, irregularities in the ionospheric electron density distribution may be considered as providing a rough reflecting surface for HF radio waves. As a result, signals associated with each propagation mode arrive at the receiver over a range of angles in both azimuth and elevation. Measurements are presented of HF radio signals received over a disturbed, high latitude path. Multiple, closely separated traces were apparent in the time history of the calculated bearings, the precise nature of which depends upon the array geometry and algorithm employed. The presence of these multiple traces is attributed to the behaviour of the DF algorithms when the signals arrive over a range of azimuth and elevation angles due to the rough reflector nature of this disturbed region of the ionosphere.

The mid-latitude ionospheric trough is a region of depleted electron density which forms at night just equatorward of the auroral oval and is more pronounced during the winter and equinoctial months. The latitudinal width of the trough is typically a few degrees and its position and opening time (sometime after sunset) are dependent on geomagnetic activity. Within the trough, critical frequencies are typically reduced to below half the value of those outside of the trough region. A number of models exist which predict the location of the trough. The model proposed by Halcrow and Nisbet (1977) has been used in this study. This model, which is based on the average of a number of satellite observations, provides information on the opening position of the trough, which is dependent on the level of geomagnetic activity, the closing position, which occurs at sunrise, and the location of the trough walls. The observed changes in bearing on a west-east path during a period of relatively low geomagnetic activity have been simulated by means of ray tracing. Two potential modes of propagation have been investigated: (a) a two hop propagation with the ground reflection via nonspecular scatter from the sea surface at locations to the south of the equatorward wall of the trough; and (b) reflection from the electron density gradients present at the equatorward wall of the trough. (6 pages)