The effect of the mid-latitude trough on the direction of arrival and time-of-flight of HF radio signals

The effect of the mid-latitude trough on the direction of arrival and time-of-flight of HF radio signal

The influence of high latitude off-great circle propagation effects on HF communication systems and radiolocation

The influence of high latitude off-great circle propagation effects on HF communication systems and radiolocatio

The simulation of off-great circle HF propagation effects due to the presence of patches and arcs of enhanced electron density within the polar cap ionosphere

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

Observations of HF propagation on a path aligned along the mid-latitude trough

Observations of the direction of arrival and time of flight of HF signals propagating on a 1400 km path oriented along the mid-latitude trough are presented. At night, the signal commonly arrives from directions offset from the great circle bearing by up to 80° and these events have been categorised into five main types. Statistics indicating how often these categories of propagation were observed in the period August 2006 to September 2007 are presented. The physical mechanisms which result in the off great circle propagation are also discussed

Time of flight and direction of arrival of HF radio signals received over a path along the midlatitude trough: Theoretical considerations

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

Propagation of HF radio waves over northerly paths: measurements,simulation and systems aspects

Large deviations in the direction of arrival of ionospherically propagating radio signals from the Great Circle Path (GCP) have serious implications for the planning and operation of communications and radiolocation systems operating within the HF-band. Very large deviations are particularly prevalent in the polar and sub-auroral regions where signals often arrive at the receiver with bearings displaced from the great circle direction by up to ±100° or more. Measurements made over several paths are presented in this paper, and the principle causes of off-great circle propagation outlined. Significant progress has been made in modelling the propagation effects and work is now in hand to incorporate the results into tools to aid the planning and operation of HF radio systems operating at northerly latitudes

Comparison of Modelled and Observed Ionospheric HF Radio Propagation over the Polar Cap in Response to Solar Flares and a Weak CME of January 2014

Space weather events can have a range of disruptive effects on the ionosphere, especially in the polar cap. This region is of growing importance for intercontinental air travel, lying across the shortest path between significant destinations, e.g, Washington-Beijing. Following these great-circle routes is increasingly desirable as travel time, cost and pollution is reduced. However, in the polar cap geostationary satellites lie below the horizon and both geographic and geopolitical considerations mean there are at best limited VHF radio air-traffic control facilities. Thus HF radio propagation via the ionosphere is of critical importance in maintaining communications with aircraft flying transpolar routings. Hence adverse space weather conditions, leading to ionospheric disruption which in turn affects HF radio propagation is of critical importance when considering whether a polar routing is viable in the days and hours in advance of a flight. [opening paragraph

Aspects of HF radio propagation

<p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">radio systems. From the point of view Working Group 2 of the COST 296 Action, interest lies with effects associated</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">with propagation via the ionosphere of signals within the HF band. Several aspects are covered in this paper:</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">a) The directions of arrival and times of flight of signals received over a path oriented along the trough have</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">been examined and several types of propagation effects identified. Of particular note, combining the HF observations</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">with satellite measurements has identified the presence of irregularities within the floor of the trough that</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">result in propagation displaced from the great circle direction. An understanding of the propagation effects that</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">result in deviations of the signal path from the great circle direction are of particular relevance to the operation</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">of HF radiolocation systems.</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">b) Inclusion of the results from the above mentioned measurements into a propagation model of the northerly</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">ionosphere (<em>i.e. </em>those regions of the ionosphere located poleward of, and including, the mid-latitude trough)</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">and the use of this model to predict the coverage expected from transmitters where the signals impinge on the</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">northerly ionosphere.</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">c) Development of inversion techniques enabling backscatter ionograms obtained by an HF radar to be used</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">to estimate the ionospheric electron density profile. This development facilitates the operation of over the horizon</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">HF radars by enhancing the frequency management aspects of the systems.</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">d) Various propagation prediction techniques have been tested against measurements made over the trough</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">path mentioned above, and also over a long-range path between Cyprus and the UK.</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">e) The effect of changes in the levels of ionospheric disturbances on the operational availability at various</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">data throughput rates has been examined for the trough path mentioned earlier.</p> <p style="margin: 0.0px 0.0px 0.0px 0.0px; font: 9.0px Times;">The topics covered in this paper are necessarily brief, and the reader is referred to full papers referenced</p> <br /&gt