Search

This paper presents the design, development, and performance evaluation of a compact wideband phased array active helical antenna for monitoring HF interference. Traditional multi-element HF antenna designs are often subjected to size restrictions. The front-to-back ratio of the proposed circularly polarized phased array exceeds 20 dB across the operating range. This array enables the simultaneous monitoring of vertically and horizontally polarized signals, offering a practical alternative to larger antennas, particularly in space-constrained scenarios. Through advanced low-noise amplifier design, efficiency and bandwidth are optimized while mitigating mutual coupling effects. Optimal beamformer design enhances array performance with respect to more traditional array design approaches. The test results verify exceptional performance with an average axial ratio of 2 dB and a high front-to-back ratio beyond the desired frequency range. This study offers valuable insights into the design and deployment of compact wideband phased array antennas, providing an effective solution for monitoring HF interference. [ABSTRACT FROM AUTHOR]

A new analytical formula for H0, one of the three parameters (H0, g, and r) on which the NeQuick model is based to describe the altitude profile of the electron density above the F2-layer peak height hmF2, has recently been proposed. This new analytical representation of H0, called H0,corr, relies on numerical grids based on two different types of datasets. On one side, electron density observations by the Swarm satellites over Europe from December 2013 to September 2018, and on the other side, IRI UP (International Reference Ionosphere UPdate) maps over Europe of the critical frequency of the ordinary mode of propagation associated with the F2 layer, foF2, and hmF2, at 15 min cadence for the same period. The new NeQuick topside representation based on H0,corr, hereafter referred to as NeQuick-corr, improved the original NeQuick topside representation. This work updates the numerical grids of H0,corr by extending the underlying Swarm and IRI UP datasets until December 2021, thus allowing coverage of low solar activity levels, as well. Moreover, concerning Swarm, besides the original dataset, the calibrated one is considered, and corresponding grids of H0,corr calculated. At the same time, the role of g is investigated, by considering values different from the reference one, equal to 0.125, currently adopted. To understand what are the best H0,corr grids to be considered for the NeQuick-corr topside representation, vertical total electron content data for low, middle, and high latitudes, recorded from five low-Earth-orbit satellite missions (COSMIC/FORMOSAT-3, GRACE, METOP, TerraSAR-X, and Swarm) have been analyzed. The updated H0,corr grids based on the original Swarm dataset with a value for g = 0.15, and the updated H0,corr grids based on the calibrated Swarm dataset with a value for g = 0.14, are those for which the best results are obtained. The results show that the performance of the different NeQuick-corr models is reliable also for low latitudes, even though these are outside the spatial domain for which the H0,corr grids were obtained, and are dependent on solar activity. [ABSTRACT FROM AUTHOR]

The sporadic E (Es) layer virtual height h'Es and the ordinary wave critical frequency f oEs are routinely measured ionogram parameters, used to characterize the Es altitude occurrence and intensity. It has become common practice to take the real height hEs to be about equal to h'Es by assuming that signal propagation delays in the E region plasma below the layer are small and can be neglected. Although this applies for nighttime, during daytime it may overestimate hEs significantly. The present paper relies on true height analysis theory to devise a simplified method and propose an algorithm that can estimate Es real heights reasonably well. The method relies on h'Es and f oEs ionosonde measurements and E region electron density profiles obtained from the International Reference Ionosphere model. The algorithm is applied to a typical set of Digisonde observations to compute hEs and examine real height variations and functional dependencies. Whereas hEs ≃ h'Es at nighttime, during daytime there are notable h'Es - hEs differences taking values less than 10 km for most of the observed layers. During the early morning and early afternoon hours, however, when weak layers appear at upper heights, the virtual to real height differences become larger reaching 20-25 km. The method proposed here for the estimation of hEs can be easily applied to improve the accuracy of the results of sporadic E layer studies. [ABSTRACT FROM AUTHOR]

One of the most popular indices for monitoring the occurrence and intensity of ionospheric L-band irregularities is the Rate of TEC Index (ROTI). Due to low TEC in the mid-latitude ionosphere, ROTI has received significantly less attention than the equatorial and polar ionosphere. On the other hand, spread F is an established ionogram irregularity signature. The present study aims to correlate ROTI and spread F activity over European Digisonde stations for a low-to-moderate solar activity year (2011). With a focus on the latitude-dependent occurrence, the analysis demonstrates that range spread F (RSF) has been identified for all notable ROTI (>0.15 TECU/min) cases which also coincide with MSTID activity over the stations, suggesting induced gravity waves or polarization electric fields as the driving mechanism for enhanced ROTI activity. The diurnal and seasonal features are also presented. Maximum irregularity occurrence was observed around the 45° N from 18:00 to 05:00 UT with the seasonal maximum occurrence in January. Over lower mid-latitude Digisonde stations (latitude < 45° N), the diurnal and seasonal occurrence was observed from 19:00 to 04:30 UT in July. [ABSTRACT FROM AUTHOR]

In this work, we investigate various types of ionospheric disturbances observed over Europe following the earthquake that occurred in Turkey on 6 February 2023. By combining observations from Doppler sounding systems, ionosondes, and GNSS receivers, we are able to discern different types of disturbances, propagating with different velocities and through different mechanisms. We can detect co‐seismic ionospheric disturbances close to the epicenter, as well as ionospheric signatures of acoustic waves propagating as a consequence of propagating seismic waves. Unlike the vast majority of past ionospheric co‐seismic disturbance studies that are primarily based on Total Electron Content variations, reflecting disturbances propagating around the F‐region peak, the focus of the present study is the manifestation of disturbances at different ionospheric altitudes by exploiting complementary ionospheric remote sensing techniques. This is particularly highlighted through ionospheric earthquake‐related signatures established as specific ionogram deformations known as multiple‐cusp signatures which appear as additional cusps at the base of the F‐region attributed to electron density irregularities generated by Rayleigh surface waves that generate acoustic waves propagating up to the ionosphere. Therefore this study underlines the advantage that multi‐instrument investigations offer in identifying the propagation of earthquake‐related ionospheric disturbances at different ionospheric altitudes and distances from the earthquake epicenter. Key Points: The 2023 Turkey earthquake induced a spectrum of ionospheric disturbances as shown by networks of GNSS receivers, ionosondes and HF DopplerDisturbances appeared as shock acoustic wave‐induced traveling ionospheric disturbances in the near field and Rayleigh wave‐induced ionogram multiple‐cusp signature at longer distancesMulti‐instrument remote sensing techniques can detect ionospheric disturbances at a range of altitudes‐distances from the epicenter [ABSTRACT FROM AUTHOR]

We present an analysis of the evolution of the ionospheric Total Electron Content (TEC) and its variability index Vσ during the Starlink launch disaster on 3–8 February 2022 two‐phase geomagnetic storm. JPL GIM‐TEC global maps with increased spatial (1° in latitude and longitude) and temporal (15 min cadence) resolution as well as SWARM satellite data have been used in the analysis. Comparison of data for five time periods including Starlink launches with geomagnetic Dst index demonstrates thermospheric mass densities increasing by about 50% during the development of the storm on 3–5 February. A spectrum periodogram analysis is applied on TEC at sunrise and sunset solar terminator (STh) at an altitude h = 300 km. Periodicities in the solar terminator TEC generated waves are estimated as 24 hr, 12 hr, 1 hr, 30 min, 20 min, and 15 min. A significant morning−evening asymmetry resulted in the density anomalies concentrated in a certain—evening—sector of terminator when plasma is shifted to the dusk side forming a global asymmetry. Key Points: Neutral and electron plasma density at the dawn−dusk terminator increased globally, during the storm on 3–5 February 2022Sunrise‐sunset Total Electron Content (TEC) oscillation was observed in all cases with asymmetry of morning−evening values with dominant evening magnitude maximizing during the stormSunrise‐sunset TEC exhibited oscillation with periods of 24 hr, 12 hr, 1 hr, 30 min, 20 min, and 15 min as extracted from JPL GIM‐TEC global maps [ABSTRACT FROM AUTHOR]

Midlatitude ionosonde observations show that there is a sporadic E (Es) diurnal cycle that starts in higher altitudes at sunrise. This property is labeled as the "Sporadic E sunrise effect." Given that sporadic E layers are composed of metal ions, the sunrise effect implies that metal atom solar photoionization could play a role in the diurnal variability of sporadic E occurrence. This possibility is endorsed by Arecibo's incoherent scatter radar observations, showing that weak ion layers at lowest E and uppermost D region heights appear at sunrise to live during the daytime, apparently coming out of nighttime metal atom mesospheric layers. The solar photoionization of metal atoms increases the abundance of metal ions available for Es layer generation during the daytime, whereas this effect is absent at nighttime. This can explain why sporadic E layers start or intensify at sunrise all year round and why Es activity maximizes during sunlit hours, as has been reported in many ionosonde and satellite radio occultation studies. The significance of metal atom solar photoionization on the regular diurnal variation of Es went unnoticed, despite existing evidence for a long time. The present paper provides a base for a better physical understanding of the pronounced 24-hr periodicity in Es layer intensity and places a step toward the improvement of simulation models for the predictions of sporadic E layer characteristics. [ABSTRACT FROM AUTHOR]

Nine years of ionograms from a higher mid-latitude ionospheric station (Moscow) are analyzed, by applying the 'height–time–intensity' (HTI) technique along with Spectrum (Lomb periodogram) analysis with the aim to investigate the daily and seasonal variability of sporadic E (Es) and intermediate descending layers (IDLs). Es and IDL traces are observed over Moscow, which are characterized by a 12-h periodicity prevailing throughout the year. Shorter periodicities in IDL and Es occurrence are also observed. A 6-h periodicity in Es and IDL dominates during November and December, while an 8-h periodicity is found mainly from October to February for IDL and in July for Es. These periodicities are primarily induced by the semi-, quarter- and terdiurnal thermospheric tides, respectively. Our results also establish the systematic and widespread manifestation of shorter-scale (4.8- and 4-h) periodicities observed mainly for IDL and less frequently for Es only during December and January, in the nine years considered, which is most probably linked to higher-order solar tides. [ABSTRACT FROM AUTHOR]

Using the topside electron density (Ne) measurements recorded over Cyprus and Russia, we investigate the latitudinal variation in the topside electron density during the interval 2014–2020, encompassing a period of high-to-low solar activity. The selected topside electron density dataset employed in this study is based on the in situ Langmuir probe data on board the European Space Agency (ESA) Swarm satellites, in the vicinity of the three Digisonde stations in Nicosia (35.14°N, 33.2°E), Moscow (55.5°N, 37.3°E) and Saint Petersburg (60.0°N, 30.7°E). Our investigation demonstrates that the ratio Ne_Swarm/NmF2 between the coincident Ne_Swarm and the Digisonde NmF2 observations is higher than one on various occasions over Nicosia during the nighttime, which is not the case over Moscow and Saint Petersburg, signifying a discrepancy feature of the electron density at Swarm altitudes which depends not only on the solar activity and time of day but also on the latitude. [ABSTRACT FROM AUTHOR]

We demonstrate a simple method to estimate the direction of propagation of travelling ionospheric disturbances using in-situ electron density measurements in the topside ionosphere by exploiting the side-by-side flying configuration of Swarm A and C satellites. Corresponding cases of TIDs recorded on detrended GPS total electron content (TEC) maps over different continents are used to validate this approach. This simple method could be suitable to detect the direction of propagation of TID wavefronts over the ocean and the polar regions where ground-based GNSS stations are unavailable. [ABSTRACT FROM AUTHOR]

A serious problem affecting the local radio industry in Cyprus is radio interference from the Middle East. This problem is especially acute on the highway along the coast during the summer months because interference degrades the sound quality in vehicle receivers due to chirping noise, hissing, fading and distortion. During the last decade the issue was explored by monitoring the field strength intensity of unwanted signals with respect to atmospheric conditions that excacerbate interference. According to the research outcomes presented in this paper, severe interference occurs due to "Tropospheric Ducting", i.e., radio energy becomes trapped between two boundaries in the lower layers of Earth's atmosphere. Thereby, this phenomenon acts as a waveguide that favors radio waves to propagate beyond the horizon with very strong field strength intensity where under certain atmospheric conditions exceeding the expected free space theoretical value. Because the commercial Band FM is already oversaturated for years, it was considered important to expand this research to explore the impact of interference on the new digital DAB, DAB+ radio, that will soon be launched in Cyprus, as it is discussed below. [ABSTRACT FROM AUTHOR]

Due to the long distance propagation characteristics of the HF spectrum, noise and interference are significant and challenging factors that have to be addressed in the context of HF system performance. Both aspects exhibit systematic diurnal, seasonal, and solar cycle variability that result in a number of corresponding measurement campaigns all over the globe, in an effort to characterise HF noise. Noise at HF consists of man-made and naturally occurring HF noise with atmospheric and galactic components. An understanding of HF noise variation in terms of time, azimuth, and filter bandwidth is crucial for a specific location, as it can provide a background knowledge based on which appropriate values of HF system parameters may be selected. This paper deals with the temporal, azimuthal, and bandwidth characteristics of HF noise based on a one-month dataset of electric field strength measurements recorded by a directional antenna in Cyprus. [ABSTRACT FROM AUTHOR]

Interference from other users, due to the long-distance propagation of HF signals, causes spectral congestion, which is considered a primary detrimental effect on HF (3–30 MHz) communication systems. More than a year of HF electric field measurements from a calibrated monopole HF antenna collected by a dedicated measurement system installed in Cyprus have been used to compare neural networks and non-linear regression for the development of single-allocation congestion models in the HF spectrum. These models can be used to advise operators on typical interference spectral congestion levels and assist in the planning and management of frequency usage by assessing the interference level to alleviate spectral congestion in the HF band over Cyprus. The performance of these alternative modeling approaches is discussed in the context of their ability to successfully capture the diurnal variability of HF interference at a certain incident field strength threshold. [ABSTRACT FROM AUTHOR]

For the first time, we reconstructed global distribution of both the total electron content disturbance W index and TEC values for eight extreme storms (Dst < -250 nT) occurred before the epoch of GNSS observations in solar cycle 22. We created a model based on superposed epoch analysis of the training set of GIM-W maps of nine SC23 extreme storms. Global GIM-W index maps are calculated from 15-min UPC GIM-TEC (UQRG) as the logarithmic deviation of instantaneous TEC from the monthly median GIMMTEC empirical model. We introduced the storm phase metrics for main and recovery phases of the positive ionosphere disturbance (the WU-index), the negative disturbance (the WL-index) and the ring current (the Dst-index). The probabilistic forecasting model (Pmodel) for SC22 GIM-Wx maps is developed based on GIM-W maps of the SC23 training set. The storm phase distribution Fx for the eight SC22 extreme storms is calculated from the proxy time shift (lag) of peak WUmax and WLmin relative to Dstmin. Proxy GIM-TECx maps are calculated by adjusting the GIM-MTEC median to the GIM-Wx prediction. Validation of the technique based on data of UPC and JPL for four intense ionospheric storms showed a root-mean-square error less than 3 TECU. The proposed technique can be applied for both the past and future forecasting of GIM-W index and GIM-TEC maps. [ABSTRACT FROM AUTHOR]

The design and construction of an upgraded HF quarter-wavelength two-dipole active array with 90° difference feed was implemented in the course of a research project to perform a directional (azimuthal) investigation of interference at HF. The lack of affordable compact antennas to meet the project requirements was the incentive to develop a compact unidirectional antenna, with the maximum possible front-to-back ratio at frequencies of 20–30 MHz, where the dimensions of traditional passive antennas are enormous. By installing a low-noise very-high-input impedance amplifier in each dipole of the array, the effect of the mutual coupling between the two dipoles was reduced, improving the front-to-back (F/B) ratio over a wide frequency range. Electronic steering, easy polarization adjustment, and fast and easy deployment were the key requirements for the construction of the antenna. Therefore, a light and compact design was of the utmost importance to meet the space limitations at the monitoring site, which did not allow the deployment of a traditional HF directional antenna that employs a very long boom and elements. [ABSTRACT FROM AUTHOR]

We examine systematic differences between topside electron density measurements and different topside model formulations including α-Chapman, NeQuick, and an improved NeQuick (NeQuick-corr) topside formulation, recently proposed. The global topside electron density data set used, was extracted from Radio Occultation (RO) topside electron density profiles on board Low-Earth Orbit satellites from the COSMIC/FORMOSAT-3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite) mission. By using RO topside electron density measurements collocated with digisonde stations, we ensure that our investigation is based on two independent data sets (RO and digisondes). A subset of these profiles, with matched (within 5%) peak RO-digisonde characteristics (foF2 and hmF2) is also exploited. This subset is exploited to extend the investigation on the basis of a higher quality validation data set. The comparison demonstrates that α-Chapman and NeQuickcorr underestimate, whereas NeQuick overestimates COSMIC topside electron density observations. The main outcome of this study is the significant NeQuick topside representation improvement that can be achieved near the F region peak, if the key parameter g, which controls the change of scale height with respect to altitude, is optimized to a value of 0.15 (compared to a currently adopted value of 0.125). The NeQuick-corr topside formulation using the optimized g value of 0.15 outperforms all other topside formulations. [ABSTRACT FROM AUTHOR]

The present study focuses on ionospheric F-layer critical parameters (foF2/hmF2) and Total Electron Content (TEC) by exploiting a long-term data (2009 to 2016) from Digisonde and Global Positioning System (GPS) receiver observations over a lower-middle latitude European location Nicosia, Cyprus (geographic 35° N, 33° E; magnetic dip. 32° N). Also, it adheres to evaluating the performances of global empirical standard International Reference Ionosphere (IRI-2016), Standard Plasmasphere Ionosphere Model (SPIM-2017/IRI-Plas 2017), and interpolated TEC from the IGS-Global Ionospheric Maps (GIMs) by comparing with the Digisonde and GPS observations. Our results confirm a noteworthy departure of estimated foF2/hmF2 and TEC in SPIM-2017, even larger than that of IRI-2016 as regards to the Digisonde and GPS observations, irrespective of season, solar activity, and geomagnetic activity conditions. This contradicts the general perception on an ideal behavior of the empirical climatological model over the middle latitude region, inferring that the transient region between the low and middle latitude does suffer from noticeable uncertainties. Also, we did not notice any convincing response of the storm sub-model in IRI except a rare flipping in its magnitude and absolutely no alteration in SPIM model estimations to the onset of geomagnetic storms over the region despite the storm sub-model was kept 'on'. This suggests a revisit into the underlying storm sub-model database in the models. Interestingly, the ingestion of Digisonde recorded foF2/hmF2 and GPS-TEC into the SPIM-2017 model results in a remarkable convergence of the parameters towards the local observations even during active geomagnetic storm conditions. It ascertains the flexibility in SPIM-2017 towards plausible improvements in the model estimations by ingesting GPS-TEC. Above all, such a scheme is in line with efforts towards reliable ionospheric error mitigations in navigation, communication, and satellite applications. [ABSTRACT FROM AUTHOR]

Tropospheric delay comprises one of the most important error sources in satellite navigation and is caused when radio signals broadcasted by GPS satellites propagate into the atmosphere. It is usually projected onto zenith direction by using mapping functions named as Zenith Tropospheric Delay (ZTD). ZTD is described as the sum of the Zenith Hydrostatic Delay (ZHD) and the Zenith Wet Delay (ZWD) and with the aid of surface pressure and temperature the integrated water vapor can be estimated. The main objective of this study is to evaluate the tropospheric delay performance for GNSS integrated water vapor estimation by using GPT2w model, ECMWF's IFS (ECMWF stands for the European Centre for Medium-Range Weather Forecasts) reanalysis model and ground meteorological data from two stations of the permanent network of Cyprus and Greece. The period from 27 May to 3 June 2018 is characterized by two different synoptic conditions: high pressure with fair weather in central Mediterranean (Greece), on the one hand, and high instability over the upper levels of the atmosphere that resulted in thunderstorms inland and mountainous areas during midday over the Eastern Mediterranean (Cyprus), on the other hand. In general, the results show that both the empirical blind model GPT2w and the ECMWF (IFS) operational model perform well in particular over Nicosia when used for the retrieval of Integrated Water Vapor (IWV) from GNSS measurements, although appreciable deviations were observed between ECMWF (IFS)-retrieved IWV and the one retrieved from GNSS observations by using meteorological measurements. A sharp increase of IWV prior to the abrupt rainfall events during noon on 30 and 31 May over Nicosia was also found. [ABSTRACT FROM AUTHOR]

The bottomside thickness and shape (B0 and B1) are the critical key elements for depicting a realistic electron density profile in the International Reference Ionosphere (IRI) model. We investigated their longitudinal variability using a large database of FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) electron density profiles covering the period 2006–2015 from which these parameters are derived using a simple analytic Chapman least squares fitting. The parameters are then compared with the IRI‐2012 submodels, that is, Bil‐2000, Gul‐1987, and ABT‐2009 during different seasons and solar activity. Besides apparent adherence of B0 to the equatorial region, the opposite occurrence pattern of maximum B0 to that of NmF2 is realized, manifesting semiannual distribution and hemispheric asymmetry. Unlike NmF2, the presence of single anomaly crest in B0 in all seasons except March equinox and their leading local time stamps are among the key observations. In spite of regular representation of IRI submodels during equinox, the performance of Gul‐1987 seems to be inferior during solstice with respect to geomagnetic field and equatorial electrodynamics, thereby depicting an erroneous distribution of B0. The striking wave‐like feature in B0 longitudinal variability is also revealed indicating its possible connection with the nonmigrating diurnal/semidiurnal tides. Moreover, the solar activity effect on B0 appears to be relatively higher over the Asian and Pacific longitudes (60–150°E). Although speculations are made over B1 variability, its complete longitudinal characteristics are indistinguishable in our results, except the subordinate magnitude as compared to IRI estimations. The important findings from this study may reinforce the understanding of bottomside variability and future improvements in IRI. Key Points: Longitudinal variability of B0 and B1 is investigated from FORMOSAT‐3/COSMIC occultation profilesLongitudinal wave‐like feature in B0 indicates nonmigrating tidal influences with latitudinal anticorrelation connection with NmF2Discrepancy among the bottomside submodels in IRI is noticed with the relatively improved performance in ABT‐2009 [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the characteristics of HF (1.6-30 MHz) spectral occupancy and describes the development of neural network models to predict the likelihood of interference in the HF spectrum over a significant part of Northern Europe. The analysis and developed models are based upon several years of 24-h measurements recorded at four sites in Northern Europe over a substantial part of the previous sunspot cycle. The measure of occupancy used is congestion, which is defined as the fraction of channels within a certain frequency allocation with signals exceeding a given threshold. [ABSTRACT FROM AUTHOR]

The critical frequency of ionospheric F2 layer (foF2) is a measure of the highest frequency of radio signal that may be reflected back by the F2 layer, and it is associated with ionospheric peak electron density in the F2 layer. Accurate long-term foF2 variations are usually derived from ionosonde observations. In this paper, we propose a new method to observe foF2 using a stand-alone global positioning system (GPS) receiver. The proposed method relies on the mathematical equation that relates foF2 to GPS observations. The equation is then implemented in the Kalman filter algorithm to estimate foF2 at every epoch of the observation (30-s rate). Unlike existing methods, the proposed method does not require any additional information from ionosonde observations and does not require any network of GPS receivers. It only requires as inputs the ionospheric scale height and the modeled plasmaspheric electron content, which practically can be derived from any existing ionospheric/plasmaspheric model. We applied the proposed method to estimate long-term variations of foF2 at three GPS stations located at the northern hemisphere (NICO, Cyprus), the southern hemisphere (STR1, Australia) and the south pole (SYOG, Antarctic). To assess the performance of the proposed method, we then compared the results against those derived by ionosonde observations and the International Reference Ionosphere (IRI) 2012 model. We found that, during the period of high solar activity (2011-2012), the values of absolute mean bias between foF2 derived by the proposed method and ionosonde observations are in the range of 0.2-0.5 MHz, while those during the period of low solar activity (2009-2010) are in the range of 0.05-0.15 MHz. Furthermore, the root-mean-square-error (RMSE) values during high and low solar activities are in the range of 0.8-0.9 MHz and of 0.6-0.7 MHz, respectively. We also noticed that the values of absolute mean bias and RMSE between foF2 derived by the proposed method and the IRI-2012 model are slightly larger than those between the proposed method and ionosonde observations. These results demonstrate that the proposed method can estimate foF2 with a comparable accuracy. Since the proposed method can estimate foF2 at every epoch of the observation, it therefore has promising applications for investigating various scales (from small to large) of foF2 irregularities. [ABSTRACT FROM AUTHOR]

The high frequency (HF) radio channel is highly variable and unpredictable. On the other hand, being largely utilized around the globe, the HF spectrum is densely occupied, characterized by significant levels of interference. Therefore, spectral congestion is always a severe issue to be addressed. One year data (June 2012–June 2013) of HF spectral occupancy measurements collected by a dedicated measurement system installed in Cyprus have been examined in order to characterize and model the HF interference environment over the eastern Mediterranean region. An investigation into the short-term temporal variability of HF spectral occupancy for selected indicative allocations has been conducted. With a view to extend the data analysis from descriptive to predictive and from qualitative to quantitative, time series analysis has been applied to model the HF spectral occupancy of the indicative ranges in the HF spectrum. We categorized different time series models for different occupancy patterns. Analysis for three broadcast allocations covering three different portions of the HF spectrum namely, lower (5.950–6.200 MHZ), middle (11.650–12.050 MHZ), and upper (17.550–17.900 MHZ), is presented. Results of this investigation are expected to be useful for radio system design and frequency management strategies. [ABSTRACT FROM PUBLISHER]

Ionospheric total electron content (TEC) variations prior to 2 large earthquakes in Nepal ( M = 7.8) and Chile ( M = 8.3) in 2015 were analyzed using measurements from global navigation satellite system network with the aim to detect possible ionospheric anomalies associated to these seismic events and describe their main features, by applying statistical and spectral analysis. It was shown that abnormal TEC variations appeared few days up to few hours before the events lasting up to 8 h, whereas intensified TEC wave-like oscillations with periods 20 and 2-5 min were also identified that could be linked to the impending earthquakes. An unusual modification of the equatorial ionospheric anomaly 5 days before the main shock was also detected. Spectral analysis on TEC satellite measurements proved an effective method for the discrimination between seismically induced ionospheric waves and those of different origin such as the solar terminator transition and geomagnetic storms. [ABSTRACT FROM AUTHOR]

This paper presents the development of Neural Network models to predict the likelihood of interference experienced by Broadcast users in the HF spectrum (3-30 MHz). The models are based upon several years of measurements recorded at Linköping (Sweden) across the HF band, covering a substantial part of a sunspot cycle. The dataset used for the model development is a result of a long-term project being undertaken jointly by the University of Manchester and by the Swedish Defence Research Establishment, to measure systematically and to analyze the occupancy of the entire HF spectrum. The measure of occupancy used is congestion, which is defined as the fraction of channels within a certain frequency allocation with signals exceeding a given threshold. The procedures for measuring and modeling congestion as a function of solar activity, time of day, day of year and incident field strength threshold are briefly presented. The accuracy of the predictions produced by the developed models demonstrate their ability to successfully capture the 24-hour, seasonal and long-term trend in the variability of congestion. These models can be used to advise operators on typical interference occupancy levels and assist the HF broadcast service in the planning of frequency usage and management by assessing the interference effect to short-wave, broadcasting in an effort to alleviate spectral congestion in the HF broadcast bands. [ABSTRACT FROM AUTHOR]

This paper presents a review on the PECASUS service, which provides advisories on enhanced space weather activity for civil aviation. The advisories are tailored according to the Standards and Recommended Practices of the International Civil Aviation Organization (ICAO). Advisories are disseminated in three impact areas: radiation levels at flight altitudes, GNSS-based navigation and positioning, and HF communication. The review, which is based on the experiences of the authors from two years of running pilot ICAO services, describes empirical models behind PECASUS products and lists ground- and space-based sensors, providing inputs for the models and 24/7 manual monitoring activities. As a concrete example of PECASUS performance, its products for a post-storm ionospheric F2-layer depression event are analyzed in more detail. As PECASUS models are particularly tailored to describe F2-layer thinning, they reproduce observations more accurately than the International Reference Ionosphere model (IRI(STORM)), but, on the other hand, it is recognized that the service performance is much affected by the coverage of its input data. Therefore, more efforts will be directed toward systematic measuring of the availability, timeliness and quality of the data provision in the next steps of the service development. [ABSTRACT FROM AUTHOR]

The objective of this article is to present a concept for single-frequency Global Navigation Satellite System (GNSS) positioning local ionospheric mitigation over a certain area. This concept is based on input parameters driving the NeQuick-G algorithm (the ionospheric single-frequency GNSS correction algorithm adopted by Galileo GNSS system), estimated on a local as opposed to a global scale, from ionospheric characteristics measured by a digital ionosonde and a collocated dual-frequency Total Electron Content (TEC) monitor. This approach facilitates the local adjustment of Committee Consultative for Ionospheric Radiowave propagation (CCIR) files and the Az ionization level, which control the ionospheric electron density profile in NeQuick-G, therefore enabling better estimation of positioning errors under quiet geomagnetic conditions. This novel concept for local ionospheric positioning error mitigation may be adopted at any location where ionospheric characteristics foF2 and M(3000)F2 can be measured, as a means to enhance the accuracy of single-frequency positioning applications based on the NeQuick-G algorithm. [ABSTRACT FROM AUTHOR]

The purpose of the present study is to investigate simultaneously pre-earthquake ionospheric and atmospheric disturbances by the application of different methodologies, with the ultimate aim to detect their possible link with the impending seismic event. Three large earthquakes in Mexico are selected (8.2 Mw, 7.1 Mw and 6.6 Mw during 8 and 19 September 2017 and 21 January 2016 respectively), while ionospheric variations during the entire year 2017 prior to 37 earthquakes are also examined. In particular, Total Electron Content (TEC) retrieved from Global Navigation Satellite System (GNSS) networks and Atmospheric Chemical Potential (ACP) variations extracted from an atmospheric model are analyzed by performing statistical and spectral analysis on TEC measurements with the aid of Global Ionospheric Maps (GIMs), Ionospheric Precursor Mask (IPM) methodology and time series and regional maps of ACP. It is found that both large and short scale ionospheric anomalies occurring from few hours to a few days prior to the seismic events may be linked to the forthcoming events and most of them are nearly concurrent with atmospheric anomalies happening during the same day. This analysis also highlights that even in low-latitude areas it is possible to discern pre-earthquake ionospheric disturbances possibly linked with the imminent seismic events. [ABSTRACT FROM AUTHOR]

The CyFFORS (Cyprus Flood Forecasting System) project aims at increasing flood risk awareness and promoting preparedness against flooding by developing and validating a pilot flood forecasting system targeted over three river/stream basins in the Larnaca region, Cyprus, and Attica region, Greece. The present study demonstrates the analysis of flood-associated information, based on ground-based and ERA5 model reanalysis data, which is a necessary procedure prior to the development of the hydrometeorological modeling tool, in one of the study areas, namely in the Rafina catchment in Attica, Greece. The analysis focusses on 12 stream flood episodes that occurred in the period 2008–2014. The results show that most events were associated with a typical, for the study area, wet-season cyclonic activity. The detailed investigation of two case studies highlighted important spatiotemporal differences in the generation and development of rainfall, as well as in the flooding magnitude, which were related to specific characteristics of the synoptic-scale forcing, topography and soil moisture preconditioning. Moreover, highly correlated positive relationships were found between the observed maximum stream discharge and the duration and maximum total accumulation of precipitation. A strong positive correlation was also evident between the peak discharge and the flooding impacts, leading to the identification of preliminary discharge thresholds for impact-based warnings, which can be applied to the pilot CyFFORS forecasting system. [ABSTRACT FROM AUTHOR]