No abstract

[1] The presence of the midlatitude trough can severely impact on HF radio systems since the electron density depletion within the trough reduces the maximum frequency which can be reflected by the ionosphere along the great circle path. Furthermore, the associated horizontal gradients in the electron density distribution frequently result in propagation well displaced from the great circle path. The signal characteristics associated with this type of propagation have been investigated for a 1400 km link oriented along the midlatitude trough between Sweden and the UK. As anticipated, the observed delay and Doppler spread characteristics are strongly dependent upon time of day and season since the trough is a nighttime feature which occurs predominantly during the winter. In particular, the Doppler spread is often large when great circle propagation has been suppressed and reflections are from the north of the great circle path (i.e., from the poleward wall of the trough or from gradients and/or irregularities associated with the auroral zone).

[1] Measurements of the time-of-flight, direction of arrival, and Doppler spread are presented for HF radio signals radiated on six frequencies between 4.6 and 18.4 MHz received over a subauroral path oriented along the midlatitude trough between Sweden and the UK. During the day, the signals usually arrived from the great circle direction whereas at night, especially during the winter and equinoctial months; the signals on frequencies between 7.0 and 11.1 MHz often arrived from directions well displaced from the great circle direction. In summer the deviations tended to be smaller (<5°) than those observed during the other seasons (several tens of degrees). The deviations were mainly to the north and often lasted all night, with the time of flight initially decreasing and then increasing, showing an approach and then recession of the reflection point. Southerly deviations were much less coherent and less frequent.

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.

[1] Off-great circle HF propagation effects are a common feature of the northerly ionosphere (i.e., the subauroral trough region, the auroral zone, and the polar cap). In addition to their importance in radiolocation applications where deviations from the great circle path may result in significant triangulation errors, they are also important in two other respects: (1) In systems employing directional antennas pointed along the great circle path, the signal quality may be degraded at times when propagation is via off-great circle propagation modes; and (2) the off-great circle propagation mechanisms may result in propagation at times when the signal frequency exceeds the maximum usable frequency along the great circle path. A ray-tracing model covering the northerly ionosphere is described in this paper. The results obtained using the model are very reminiscent of the directional characteristics observed in various experimental measurement programs, and consequently, it is believed that the model may be employed to enable the nature of offgreat circle propagation effects to be estimated for paths which were not subject to experimental investigation. Although it is not possible to predict individual off-great circle propagation events, it is possible to predict the periods during which large deviations are likely to occur and their magnitudes and directions.

[1] Observations from an HF radio experiment on a subauroral path between Sweden and the UK near sunspot maximum in 2001 are compared with the position of the midlatitude trough according to a statistical model. Periods of off-great circle propagation, occurring predominantly in winter and equinoctial nights at frequencies 7-11 MHz, show characteristics consistent with scattering from field-aligned irregularities in the northern trough wall and/or auroral oval. Very little reflection and/or scattering was apparent from directions to the south of the great circle path. These results are in marked contrast with those from a similar experiment conducted near sunspot minimum in 1994 in Canada, during which both southerly and northerly deviations were observed in the 5-15 MHz range. The contrasting results were simulated using ray tracing through a model ionosphere incorporating a model of the trough and, optionally, precipitation. The observed off-great circle propagation features on the European path could only be reproduced when precipitation within the northern trough wall/auroral zone was included, whereas features of the northerly and southerly deviations observed in the Canadian experiment could be simulated by the presence of the trough walls and without the need for precipitation.

No abstract

[1] Signal strength measurements at 2 GHz have recently been made on three over-sea paths in the British Channel Islands. This paper focuses on explaining the propagation characteristics during periods of normal reception and periods of enhanced signal strength with particular emphasis on a 48.5 km transhorizon path between Jersey and Alderney. Evaporation ducting and diffraction appear to be the dominant propagation mechanisms at most times. The influence of the evaporation duct during periods of normal propagation has been confirmed by modeling the over-sea propagation conditions using Paulus-Jeske evaporation duct refractivity profiles as input to the parabolic equation method. During periods of enhanced propagation, which occur approximately 8% of the time on the longest path (48.5 km), the presence of additional higher-altitude ducting/super-refractive structures has been verified and their influence has been modeled with reasonable success.