Developments in an HF Nowcasting Model for Trans-Polar Airline Routes

HF communications can be difficult in the polar regions since they are strongly influenced by space weather events. Airline communications within the polar regions rely on HF communications and improved nowcasting and forecasting techniques in support of this are now required. Previous work has demonstrated that ray tracing through a realistic, historical ionosphere provides signal coverage in good agreement with measurements. This paper presents an approach to providing a real-time ionospheric model by assimilating TEC measurements and validates it against observations from ionosondes

Progress towards a propagation prediction service for HF communications with aircraft on trans-polar routes

Commercial airlines began operations over polar routes in 1999 with a small number of proving flights. By 2014 the number had increased to in excess of 12,000 flights per year, and further increases are expected. For safe operations, the aircraft have to be able to communicate with air traffic control centres at all times. This is achieved by VHF links whilst within range of the widespread network of ground stations, and is by HF radio in remote areas such as the Polar regions, the North Atlantic and Pacific where VHF ground infrastructure does not exist. Furthermore, the Russian side of the pole only has HF capability. Researchers at the University of Leicester and at Lancaster University have developed various models (outlined below) that can be employed in HF radio propagation predictions. It is anticipated that these models will form the basis of an HF forecasting and nowcasting service for the airline industry. Propagation coverage predictions make use of numerical ray tracing to estimate the ray paths through a model ionosphere. Initially, a background ionospheric model is produced, which is then perturbed to include the various ionospheric features prevalent at high latitudes (in particular patches, arcs, auroral zone irregularities and the mid-latitude trough) that significantly affect the propagation of the radio signals. The approach that we are currently adopting is to start with the IRI and to perturb this based on measurements made near to the time and area of interest to form the basis of the background ionospheric model. This is then further perturbed to include features such as the convecting patches, the parameters of which may also be informed by measurements. A significant problem is the high variability of the high latitude ionosphere, and the relative scarcity of real-time measurements over the region. Real time measurements that we will use as the basis for perturbing the IRI include ionosonde soundings from, e.g. the GIRO database, and TEC measurements from the IGS network. Real-time modelling of HF radiowave absorption in the D-region ionosphere is also included. The geostationary GOES satellites provide real-time information on X-ray flux (causing shortwave fadeout during solar flares) and the flux of precipitating energetic protons which correlates strongly with Polar Cap Absorption (PCA). Real-time solar wind and interplanetary magnetic field measurements from the ACE or DSCOVR spacecraft provide geomagnetic index estimates used to model the location of both auroral absorption (on a probabilistic basis) and the proton rigidity cutoff boundary that defines the latitudinal extent of PCA during solar proton events (SPE). Empirical climatological models have been uniquely adapted to assimilate recent measurements of cosmic noise absorption (at 30 MHz) from a large array of riometers in Canada and Scandinavia. The model parameters are continuously optimised and updated to account for regional and temporal variations in ionospheric composition (and hence HF absorption rate (dB/km)) that can change significantly during the course of an SPE, for example. Real-time optimisation during SPE can also improve estimates of the proton rigidity cutoff and improve the modelled ionospheric response function absorption vs. zenith angle) at twilight

Developments in HF Propagation Predictions to Support Communications with Aircraft on Trans-polar Routes

Commercial airlines began operations over polar routes in 1999 with a small number of proving flights. By 2014 the number had increased to in excess of 12,000 flights per year, and further increases are expected. For safe operations, the aircraft have to be able to communicate with air traffic control centres at all times. This is achieved by VHF links whilst within range of the widespread network of ground stations, and by HF radio in remote areas such as the Polar regions, the North Atlantic and Pacific where VHF ground infrastructure does not exist. Furthermore, the Russian side of the pole only has HF capability. This has created a demand for improved HF nowcasting and forecasting procedures to support the polar operations, which are the subject of this paper

Predictions and observations of HF radio propagation in the northerly ionosphere:the effect of the solar flares and a weak CME in early January 2014

We have previously reported on a significant new multi-national project to provide improved predictions and forecasts of HF radio propagation for commercial aircraft operating on trans-polar routes. In these regions, there are limited or no VHF air-traffic control facilities and geostationary satellites are below the horizon. Therefore HF radio remains important in maintaining communications with the aircraft at all times. Space weather disturbances can have a range of effects on the ionosphere and hence HF radio propagation - particularly in the polar cap. While severe space weather effects can lead to a total loss of communications (i.e. radio blackout), less intense events can still cause significant disruption. In this paper we will present the effect of a series of M and X class solar flares and a relatively weak CME on HF radio performance from 6 to 13 January 2014. This is an interesting interval from the point of view of HF radio propagation because while the solar effects on the ionosphere are significant, except for an interval of approximately 12 hours duration, they are not so intense as to produce a complete radio blackout on all paths. Observations of the signal-to-noise ratio, direction of arrival, and time of flight of HF radio signals on six paths (one entirely within the polar cap, three trans-auroral, and two sub-auroral) will be presented together with riometer measurements of the ionospheric absorption. Global maps of D-region absorption (D-region absorption prediction, DRAP) inferred from satellite measurements of the solar wind parameters will be compared with the HF and riometer observations. In addition, a ray-tracing model using a realistic background ionosphere and including localised features found in the ionospheric polar cap (e.g. polar patches and arcs) will be used to model the expected and observed HF radio propagation characteristics

Modelling of off-great circle propagation effects for HF radiowaves received over northerly paths

Both the high latitude ionosphere and mid --latitude trough region are essentially inhomogeneous and non-stationary media containing a multitude of large-scale irregularities. Electron density gradients associated with these irregularities form a diverse range of tilted reflection surfaces for high frequency (HF) radio waves. There are also a large number of smaller scale inhomogeneities that scatter the signal.;A series of measurements in the polar cap, auroral and sub-auroral zones of the Earth have been made by researchers at the University of Leicester over a number of years. It have been established that HF radio signals propagating through the high latitude ionosphere often arrive at the receiver over paths well displaced from the great circle direction, sometimes by up to 100°. Another common feature of high latitude HF propagation is the large Doppler and delay spreads imposed on the signal. These together with directional spread of the received signal energy are important parameters to be considered in the design of communication system and the associated signal processing methods.;The main outcome of the project described in this thesis is the design of a model of the high latitude ionosphere providing numerical investigation of off-great circle propagation of HF signals based on three-dimensional ray tracing. A large number of simulations were carried out for different types of propagation paths, and a comparison of the model results with observations indicates that the model is capable of reproducing the main features of HF signals propagating in the high latitude ionosphere.;A major outcome of the ray-tracing simulations is that paths other than those subject to experimental investigation can be assessed. It is anticipated that the results of this research will be incorporated into prediction tools for forecasting the effects of off-great circle propagation on any path impinging on the northerly ionosphere

Near real-time input to an HF propagation model for nowcasting of HF communications with aircraft on polar routes

The authors have previously reported on the development of an HF propagation model for signals reflected from the northerly regions of the ionosphere, and its validation by comparison with measurements made over a number of paths within the polar cap, crossing the auroral oval, and along the mid-latitude trough. The model incorporates various features (e.g. convecting patches of enhanced plasma density) of the polar ionosphere that are, in particular, responsible for off-great circle propagation and can lead to propagation at times and frequencies not expected from on-great circle propagation alone. Currently, the model drivers include ionosonde measurements and geomagnetic data from a period of several days spanning the time of interest. We have previously only examined the propagation effects on a historical basis, and have achieved good agreement between measurements and simulations

Observations of HF radio propagation at high latitudes and predictions using data-driven simulations

Researchers at the University of Leicester, Lancaster University and St Petersburg State University have developed various models that can be employed in HF radio propagation predictions. Signal coverage predictions make use of numerical ray tracing to estimate the ray paths through a model ionosphere that includes various ionospheric features prevalent at high latitudes (in particular patches, arcs, ionisation tongue, auroral zone irregularities and the mid-latitude trough). Modelling of D-region absorption is also included. GOES satellites provide information on X-ray flux (causing shortwave fadeout during solar flares) and precipitating energetic proton flux which correlates strongly with Polar Cap Absorption (PCA). Solar wind and interplanetary magnetic field measurements from the ACE or DSCOVR spacecraft provide geomagnetic index estimates used to model the location of both auroral absorption and the proton rigidity cutoff boundary that defines the latitudinal extent of PCA during solar proton events (SPE). This paper presents measurements and associated modelling for a 9 day period

Observations of HF radio propagation at high latitudes and predictions using data-driven simulations

Researchers at the University of Leicester, Lancaster University and St Petersburg State University have developed various models that can be employed in HF radio propagation predictions. Signal coverage predictions make use of numerical ray tracing to estimate the ray paths through a model ionosphere that includes various ionospheric features prevalent at high latitudes (in particular patches, arcs, ionisation tongue, auroral zone irregularities and the mid-latitude trough). Modelling of D-region absorption is also included. GOES satellites provide information on X-ray flux (causing shortwave fadeout during solar flares) and precipitating energetic proton flux which correlates strongly with Polar Cap Absorption (PCA). Solar wind and interplanetary magnetic field measurements from the ACE or DSCOVR spacecraft provide geomagnetic index estimates used to model the location of both auroral absorption and the proton rigidity cutoff boundary that defines the latitudinal extent of PCA during solar proton events (SPE). This paper presents measurements and associated modelling for a 9 day period

Aspects of HF radio propagation

The propagation characteristics of radio signals are important parameters to consider when designing and operating radio systems. From the point of view Working Group 2 of the COST 296 Action, interest lies with effects associated with propagation via the ionosphere of signals within the HF band. Several aspects are covered in this paper