Transhorizon radiowave propagation due to evaporation ducting: the effect of tropospheric weather conditions on VHF and UHF radio paths over the sea

A detailed description of evaporation ducts and their relevance to radiowave propagation over the sea has been presented. The constantly changing weather conditions over the sea mean that marine and coastal environments, in particular, are prone to these unusual tropospheric phenomena that facilitate radio waves to have higher signal strengths and to travel longer distances than expected. Therefore, the influence of evaporation ducts on over-sea radiowave propagation needs to be thoroughly investigated. Research in this area will have implications for maritime communication systems used in coastal cellular telephone networks, commercial shipping, naval radar operations and sea-rescue

Signal strength variations at 2 GHz for three sea paths in the British Channel Islands: observations and statistical analysis.

Measurements of signal strength are reported for a study of UHF propagation on three transhorizon sea paths in the British Channel Islands. Enhancements of up to 30 dB from the mean occur for periods of hours or days, especially in the summer, and constitute about 5% of the overall data. The probability distribution of received power is tabulated for the three paths and various antenna heights, and is compared with predictions of signal strength using ITU-R Recommendation P.1546-2. The difference between median and upper decile in the data is much less than predicted, whereas the difference between upper decile and the upper percentile is much greater than predicted

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

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

A comparison between the measured and predicted parameters of HF radio signals propagating along the midlatitude trough and within the polar cap

Prediction of the propagation characteristics of HF signals is an important aspect in the planning and operation of radio systems operating within that frequency band. Various computer codes have been developed by a number of organizations for this purpose. These prediction techniques assume that propagation is along the great circle path and ignore the effects of various large-scale ionospheric structures that can be present in the northerly ionosphere and result in propagation well displaced from the great circle path. This paper reports on a statistical analysis of observations of the direction of arrival and signal strength, and their comparison with VOACAP predictions for four paths, two roughly tangential to the midlatitude trough, one trans-auroral, and one entirely located within the polar cap

Measurement and modelling of HF channel directional spread characteristics for northerly paths

The northerly ionosphere is a dynamic propagation medium that causes HF signals reflected from this region to exhibit delay spreads and Doppler shifts and spreads that significantly exceed those observed over midlatitude paths. Since the ionosphere is not perfectly horizontally stratified, the signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and elevation. Such large directional spreads may have a severe impact on radio systems employing multielement antenna arrays and associated signal-processing techniques since the signal environment does not comprise a small number of specular components as often assumed by the processing algorithms. In order to better understand the directional characteristics of HF signals reflected from the northerly ionosphere, prolonged measurements have recently been made over two paths: (1) from Svalbard to Kiruna, Sweden, and (2) from Kirkenes, Norway, to Kiruna. An analysis of these data is presented in this paper. The directional characteristics are summarized, and consideration is given to modeling the propagation effects in the form of a channel simulator suitable for the testing of new equipment and processing algorithms

Signal strength variations at 2 GHz for three sea paths in the British Channel Islands: Detailed discussion and propagation modeling

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

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