Ionospheric models are mostly unable to correctly predict the effects of space weather events on the ionosphere. This is especially true for the International Reference Ionosphere (IRI) which by design is a monthly median climatology model [Bilitza et al., 2011]. The IRI electron density profile critically depends on the correct values of the F2 layer peak height and density, hmF2 and NmF2 (or foF2) which are directly affected by most space weather events. Real time data from the global ionosonde network GIRO [Reinisch and Galkin, 2011] are assimilated in the IRI electron density model in an effort to create the IRI Real Time Assimilative Model IRTAM. First results for the foF2 assimilation are available at http://giro.uml.edu/RTAM [Galkin et al., 2012]. New GIRO data arrive nominally every 15 min –possibly faster during campaigns- and new electron density profiles are generated. Using the new Vary-Chap profile model [Nsumei et al., 2012] for the topside profiles leads to more realistic total electron content estimates.

Refernces
Bilitza D., L.-A. McKinnell, B. Reinisch, and T. Fuller-Rowell (2011), The International Reference Ionosphere (IRI) today and in the future, J. Geodesy, 85:909-920, DOI 10.1007/s00190-010-0427-x
Galkin, I. A., B. W. Reinisch, X. Huang, and D. Bilitza (2012), Assimilation of GIRO Data into a Real-Time IRI, Radio Sci., 47, RS0L07, doi:10.1029/2011RS004952.
Nsumei, P., B. W. Reinisch, X. Huang, and D. Bilitza (2012), New Vary-Chap profile of the topside ionosphere electron density distribution for use with the IRI Model and the GIRO real time data, Radio Sci., doi:10.1029/2012RS004989.
Real time assimilative foF2 maps for IRI

Reinisch, B. W. and I. A. Galkin (2011), Global Ionospheric Radio Observatory (GIRO), Earth, Planets and Space, 63(4), 377-381.

Knowledge of the state of the upper atmosphere, and in particular its ionized part, is very important in several applications affected by space weather, especially the communications and navigation systems that rely on radio transmission. To better classify the ionosphere and forecast its disturbances over Europe, a data and model infrastructure platform called the European Digital Upper Atmosphere Server (DIAS) has been established in the National Observatory of Athens by a European consortium formed around eight ionospheric stations, and funded by the European Commission.
The DIAS system operates since 2006 and the basic products that are delivered are real-time and historical ionograms, frequency plots and maps of the ionosphere on the foF2, M(3000)F2, MUF and bottomside electron density, as well as long term and short term forecasting up to 24 hour ahead. The DIAS system supports more than 500 subscribed users, including telecommunication companies, satellite operators, space agencies, radio amateurs, research organizations and the space weather scientific community. In 2012 the system has been upgraded, in close collaboration between NOA, INGV and BAS, with funding from the ESA/SSA Programme.
The first group of new products results from the implementation of the TaD model (Topside Sounder Model assisted by Digisonde) that makes possible the generation of maps of the electron density at heights up to GNSS orbits, and of TEC and partial TEC maps (topside and plasmaspheric) over Europe. The TaD is based on the simple empirical functions for the transition height, the topside electron density scale height and their ratio, based on the Alouette/ISIS database, and models separately the oxygen, hydrogen and helium ions density profiles. The model takes as input the plasma characteristics at the height of maximum electron concentration that are provided in real-time by the DIAS Digisondes. To further improve its accuracy, we adjust the modeled TEC parameter with the GNSS-TEC parameter calculated at the Digisondes location. This adjustment forces the model to correctly reproduce the topside scale height, even in cases when the scale height at hmF2 is not available. This adjustment is very important for the application of TaD in an operational environment.
The second group of new products consists of long term prediction and of nowcasting maps of the foF2 parameter that cover the whole European region - including Scandinavia. Long term prediction maps have been extended to 80 deg N applying the CCIR coefficients for the region above 65 deg N, while from 32 to 60 deg N we continue to apply SIRM (Simplified Ionospheric Regional Model), as in the case of middle latitude maps that are released routinely by the DIAS system. Between 60 and 65 deg N there is a buffer zone where an interpolation routine is applied. Nowcasting maps are based on the SIRMUP (SIRM updated in real-time) concept, however, a different effective sunspot number (Reff) is estimated for each latitudinal zone, from which a synthetic Reff is calculated.

Solar flares influence the Earth's space up to its surface for several minutes and are often precursors of space weather storms. These phenomena are often related to coronal mass ejections (CMEs) and gain more and more interest. Hence, the DLR Neustrelitz started recently the project 'Global Ionospheric Flare Detection System - GIFDS' to establish an operational near real time ground based warning system.
If a solar flare hits the Earth, the ionization of the bottomside ionosphere increases which impacts the propagation of VLF waves during daytime by a sudden significant change in signal strength and phase. In order to receive permanent information from the dayside sector the warning system requires a network of VLF receivers around the globe. DLR is just establishing the first two receivers at DLR Neustrelitz and in US at the University of Alabama Huntsville.
Operational measurements are realized by Perseus SDR (Software Defined Radio) receivers enabling the reception of multiple frequency channels in a range of 10 to 60 kHz. As simultaneous measurements at different channels show the same characteristic variation, the impact of a solar flare is assumed and a warning message will be automatically generated and distributed. In comparison to X ray data of GOES satellite, further analyses are accomplished concerning the reliability, strength and time resolution of derived warnings. Preliminary results are reported.

The forecast of sporadic E layer occurring locally and sometimes nontransparent is the crucial topic for radiocommunication.
We propose the method of forecasting sporadic E layer appearance. The method is based on magnetic data and the changes of magnetic Eta parameter defined as the square root of a ratio of the energy of the external part of the vertical component to that of the horizontal components. We present the correlation of sporadic E layer appearance 1-2 hours after the increase of Eta value. The correlation between data from different European ionosondes and data from magnetic observatories lying close to the selected ionosonde was taking into account. We apply autocovariance method for prediction of the Eta index variations and in this connection the sporadic E layer appearance. Forecasting method and statistics for Warsaw ionosonde and data from Belsk Magnetic Observatory placed 50 km South-West from Warsaw are presented. Near real time magnetic data from Belsk (every 30 minutes) give the possibility to predict the Eta index variations and the sporadic E layer appearance 2 hours after the increase of the eta value.

On October 2012 ESA funded the competence surveys titled GINESTRA (Ground-based Ionosphere monitoring Networks in SoutheasTeRn Asia: a survey), MEDSTEC (Towards Mapping of Electron Density, Scintillation and Total Electron Content) and MImOSA (Monitoring the Ionosphere over South America) in the frame of the ALCANTARA initiatives (http://www.esa.int/SPECIALS/GSP/SEMDZ9NLM5H_0.html). The three projects started between October and November 2012 and ended on May-June 2013.
The competence surveys were addressed to the assessment of the current capabilities on ionospheric monitoring in the areas of interest (South-eastern Asia, Africa and South America, respectively), in order to understand how the existing facilities can be integrated with new installations, to effectively support and/or improve space weather activities oriented to assist GNSS operations. Over the considered regions, the presence of the Equatorial Ionospheric Anomaly and of the South Atlantic Magnetic Anomaly produces important peculiarities in the distribution of the plasma irregularities. Consolidating the operational monitoring of these irregularities, by taking also into account possible improvement on existing monitoring networks, can positively impact models and space weather activities. The surveys were made through: a careful analysis of the existing literature, a close interaction with public and private local entities and a dissemination activity. The paper presents the results of the three surveys and the possible follow-up actions in the field.

Space weather may cause disruptions to satellite-based telecommunication and navigation services owing to degradation of radio signals when propagating through the ionised portion of the Earth’s atmosphere. Trans-ionospheric radio waves may be scattered when propagating through large-to-small scale plasma density inhomogeneities. The net result of this scattering process is to disperse the energy carried by the signal and manifest as random fluctuations in amplitude and phase (i.e. scintillation), associated with lower tracking C/N0 conditions. The appearance of plasma density inhomogeneities varies with solar and magnetic activities, local time, latitude and season, in response to space weather conditions. The impact of adverse space weather conditions on satellite navigation systems and services include accuracy degradation in code and carrier measurements as well as in differential positioning, losses of lock and cycle slips, unavailability of SBAS messages. Recent developments are presented on the following aspects: (1) further understanding of the scattering mechanisms leading to radio wave scintillation, (2) space weather impact on different GNSS frequencies and corresponding signal tracking, (3) occurrences of losses of lock at both high and low equatorial latitudes.

The free electrons present in the earth's ionosphere affect GNSS (Global Navigation Satellite Systems) signals by introducing delays in their propagation that may be corrected in part or entirely by the use of several known techniques. But the ionosphere is not a regular medium and electron density irregularities may appear that can strongly affect wave propagation, causing rapid fluctuations in the amplitude and phase of the GNSS signals called scintillations. Scintillations can cause serious degradation problems to the performance of GNSS receivers by inducing cycle slips, losses of lock of the signals and decreasing the accuracy of the PVT solution. Scintillation activity depends on solar and geomagnetic activity, season, local time, geographic location and frequency. The approach of the next solar maximum predicted for 2013 will bring an increase of ionospheric disturbances, with possible degradations of the services relying on GNSS. This have driven the research interest both from the academy and industry to improve the robustness of GNSS to the threats posed by ionospheric activity.

Ionospheric Scintillation Monitoring Receivers (ISMR) are specialized receivers able to track and monitor scintillations in order to collect data that can be used to model the phenomenon, study its affects at receiver level and possibly predict its occurrence in the future. They make use of correlation data from the tracking processing blocks to measure the amount of scintillation affecting a satellite signal in both amplitude and phase. This is normally done by computing the S4 and phase deviation indexes in a minute by minute basis.

Within this work we deal with a specific environment of an ISMR where the monitoring of scintillation activity is threatened by the presence of radio frequency interference in the operation area. Given the crowded telecommunication environment where ISMR are likely to work in the next years, it is of interest to study the effects that other telecom systems transmitting at carrier frequencies close to the GNSS bands may have on the estimation of scintillation indexes due to unintentional leakages of power out of their allocated bandwidth. Robust tracking of GNSS signals under such conditions must be guaranteed and it must also be ensured as best as possible that the typical scintillation indices output by these receivers are not affected by the additional error source. RFI is, among the different error sources that corrupt satellite navigation waveforms, a particularly harmful error since in some cases it cannot be mitigated by a simple correlation process. This is indeed a problem that may affect the detection of ionospheric scintillation when monitored by GNSS signals, and will be analyzed in several interference scenarios.

Ionospheric disturbances at high latitudes can affect modern satellite based navigation and positioning systems. These effects can be complex and has not been studied as extensively as the effects at lower latitudes. The Norwegian Mapping Authority (NMA) monitors ionospheric activity trough our Real Time Ionospheric Monitor (RTIM) and are establishing a network of scintillation receivers for a real-time monitoring of scintillations in this area.
We present the ionospheric observations from a recent geomagnetic storm, and the observed consequences for a network RTK positioning service. Using data from monitor stations, we show the positioning errors that a user would experience during such an event. Although the storm in question only reached a Kp index of 4, which is below the NOAA scale for geomagnetic storms, it caused serious disturbances of the network RTK positioning at high latitudes (65-70 degrees N).

Dynamics Explorer 2 retarding potential analyzer (RPA) plasma density data is used as input to de Boor B-spline function. The satellite was on a nearly polar orbit. Various parameters derived from DE 2 satellite data needed for the scintillation index calculation have been modeled. Results are presented in the form of maps, of those parameters and scintillation index S4 in the geographic coordinates for various geophysical conditions.
Key words: scintillation index, spline model, high latitude etc.

Ionospheric influence on GNSS satellite signals delay, phase and amplitude changes can be deduced from information about various drifting ionospheric density structures. Large and small scale structures behaviour in the ionosphere can describe changes of the signal propagating in ionospheric plasma. This is the main source of problems for satellite positioning systems that has not been satisfactory solved yet.

We present a case study of ionospheric irregular structures patterns during geomagnetic storm event occured on 5th and 6th April 2010, measured by GPS monitors set on Svalbard and other diagnostic tools. In purpose of comparison we included data of quiet ionosphere period as reference point. Analyses of spatial and temporal phase gradients has been used as a basic tool for calculations of the properties of ionospheric electron concentration irregularities. The properties derived depend on the model of propagation of waves in ionosphere. We validated estimated properties of ionospheric irregularities using additional measurements.

Purpose of this investigation is to recognize ionospheric scintillation diffraction pattern, provide relation between phase measurements and properties of ionospheric irregularities for further forecasting and mitigation of ionospheric effects.

A method to retrieve the main thermospheric parameters (Tn, [O], [N2], [O2]) from electron density profile is applied for the first time to middle latitude daytime COSMIC and CHAMP ionospheric radio occultation (IRO) electron density profiles (EDP). It is shown that COSMIC IRO EDP can be used with the method under solar minimum (2007-2008) conditions to give neutral gas density with 10% inaccuracy. This agrees with the declared absolute inaccuracy of (10-15)% of CHAMP observations. However about 50% of the tested profiles from COSMIC either did not provide solutions at all or gave incorrect solutions due to their insufficient accuracy. The large percent of rejections indicate that IRO EDP must be carefully treated to eliminate some artificial results that are not consistent with the mid-latitude daytime F2-layer formalism that our method relies on. Consequently further filtering is required to be applied to the near-real time IRO EDP in order to be ingested as in put to our method for future on-line implementation. For solar maximum conditions the method was tested with IRO EDP from CHAMP and its performance was found to be quite stable. However CHAMP EDP are confined around 400 km in altitude and this is too low for getting correct solutions under solar maximum conditions.

High Frequency (HF) system is absolutely depending upon the ionosphere because ionosphere acts as a radio reflector. The Sun has an enormous impact on HF radio propagation because it affects the ionosphere which gives rise to most of the long distance effects that enable long distance radio communications on the HF bands. Survival of the ionosphere is directly related to radiations emitted from the sun, thus the movement of the Earth around the Sun or changes in the Sun's activity will result in variations in the ionosphere. Hence the variation of ionosphere gives an important role to the HF propagation that is a frequency which may provide successful communication now may not achieve an hour later. There are two general types of variations that is regular and irregular. Both regular and irregular variations have important effects on radio wave propagation. The regular variations that affect the extent of ionization in the ionosphere can be divided into three main classes: daily, seasonal, and sunspot variations. This paper is to study the variation of HF radio frequency and find the better HF channel for transmission in West Malaysia. The study will be limited to daily and sunspot variations. The study of HF in Malaysia is not very explored, so this study is the onset of HF study. Maximum usable frequency (MUF) is the highest frequency that allows reliable long-range communication between two points on the earth. In this study lower decile MUF, which is the optimum working frequency (OWF), has been selected to be predicted and transmitted because it provides a successful communication for 90% of the month. The transmission tests are conducted between Kajang (Lat. 2°097'N, Long. 101°079'E) Batu Arang (Lat. 3°032'N, Long. 101°047'E) and Lumut (Lat. 4°022'N, Long. 100°063'E), which is located at West Malaysia. The tests have been conducted from April 2009, at the beginning of solar cycle 24, to November 2011, the smoothed sunspot number (SSN) varying from low levels in 2009 (Rz12 = 4.18) to much higher levels in 2011 (Rz12 = 50.37), making it suitable to observe the variation of HF frequencies employed. Several predicted HF frequencies (OWF), whose value was obtained by the Advanced Stand Alone Prediction System (ASAPS) software produced by the Ionospheric Prediction Service (IPS) Australia, were selected for the transmission tests. As a result of this study it has been found that HF frequencies variations subsist in the transmission test. Occurrence of daily variations in the ionosphere are a result of the 24 hour rotation of the Earth about it axis and different layers of the ionospheric region. 27-day sunspot variations and the 11-year sunspot cycle have major effects on atmospheric ionization. It is observed that as SSN values increase, the range of operating HF frequencies that can be used also increases. SSN values also have effect on frequencies that can be used for daily HF communications. The occurrence of HF frequencies variations shows that there are need to find the best HF channel for transmission of HF signal. Moreover it is significant to choose the right frequencies for communication because to obtain better quality signals, better quality communication channels must be selected.

The conversion of the oblique ionogram to vertical ionogram is a very useful inversion technology. The vertical ionogram can provide the important ionospheric parameters, such as critical frequency, vertical height and electron density, for ionospheric research. The oblique ionosonde has the ability to detect the ionoshpere over sea and other terrain where it is not practical to deploy vertical sounder and provide more ionograms with less transmitting and receiving devices.

There are two ionosonde stations in Korea named Jeju and Icheon. Jeju station is locate in 420km south from the Icheon station. The converted vertical ionogram from the oblique iongrom is a estimated vertical ionogram of midpoint between two stations.

In this paper we discribe ionogram conversion algorithm and results of ionogram conversion to estimate vertical ionogram of midpoint between two stations. The converted ionogram was compared with vertical ionogram measured at two stations for the verfication. This paper also including impact analsys of ionogram conversion by the solar storm.

GNSS position errors due to ionosphere are partially linked to scintillations. This study focuses on scintillation activity observed under high latitude regions.. Three ISM (Ionospheric Scintillation Monitor) have been running measurements at 50Hz since end of 2012 in Norway in the frame of a collaboration between CNES and NSC/NMA for the ionosphere scintillations modelling. As the period of ISM data collection is not long enough for a valuable modelling, we first attempt to model the ROTI, the index of TEC rate of change which is among the observables affected by ionosphere disturbances, using data collected over many years (since around 1994) from the NMA GNSS network stations. In this study we have integrated electron flux obtained on the NOAA POES satellites to get a good statistics of the electron flux. Taking advantage of the 5 current POES satellites (POES 15 to 19) scanning at the same time several local times, and the fact that the first spacecraft has spent more than one solar cycle in orbit, the obtained statistics is very good, as for example more than 60,000 points with 8s resolution were obtained at extreme magnetic activity (for magnetic index Kp>8+). Averages of the electron energy flux in a McIlwain parameter L versus magnetic local time map were obtained and correlated to the ROTI measurements made at two latitude distant GNSS stations: Tromsoe and Tronheim. Therefore a model of ROTI was developed with as input parameters the station location and the time in day, the magnetic activity Kp, and the viewing direction (elevation and azimuth). This model can be used, associated with a Kp prediction, to forecast the ROTI, anywhere in the Scandinavian Norway. Intermediate analysis made to obtain it will be presented as well. Possible improvements will also be discussed.

It is known that the performance of global navigation satellite systems (GNSS) can be significantly perturbed during space weather events. Propagation of GNSS signals depends directly on the state of the ionosphere, since intensive irregularities and/or gradients of electron density modify the parameters of propagating waves. Ionospheric perturbations during geomagnetic storms are known to be the major source of such intensive ionospheric irregularities, indicating on large impact on GPS/GNSS performance. A part from the ionospheric storms, intense solar radio-bursts can significantly disrupt the operation of GNSS communication, as such solar events are the source powerful radio noise.

In this work, we analyze global distribution of GPS cycle slips and of GPS positioning errors during ionospheric storms and solar radio-bursts of different intensity. For our analysis we used data of GPS receivers from global networks IGS and UNAVCO, as well as numerous regional networks (in New Zealand, Australia, North and South America, Africa, Eurasia, and including Greenland and Antarctica), in order to obtain better global coverage and to better understand the effects of the space weather events in different longitudinal and latitudinal regions. The total number of stations used in our study is about 2500-4000 for each event.

The performance of GPS was estimated from several parameters: 1) ratio of GPS cycle slips; 2) ratio of count omissions in GPS output files; 3) GPS positioning errors. GPS cycle slips and count omissions were calculated from RINEX files for all satellites and for each GPS frequencies L1 and L2. The positioning errors were calculated as standard deviation between the known precise coordinates of a ground-based GPS receiver and coordinates computed by a receiver at each moment of time.

This year, a new service was integrated into the SSA SWE portal. The main products that is offered by the service are plots and data files of VTEC, ROTI and scintillation measured in Scandinavian region.
In addition to the real-time data, archived data is also available. Here we give an overview of the different data products and show some examples of data measured during various ionospheric activity levels.

High Frequency (3-30 MHz) (HF) Ionospheric Channel is used for military, civilian and amateur communications. By using Ionosphere, communication for distances beyond the line of sight is achieved. The main advantage of this type of communication is that it does not to require a satellite to communicate with a point beyond the line of sight. Actually the Ionosphere is used instead of a satellite. To use Ionosphere but not a satellite means independent communication for a country.

The European Digital Upper Atmosphere Server (DIAS) provides since 2006 at a routine basis long term prediction (up to 3 months ahead) and nowcasting maps of the foF2 parameter over the European middle latitudes (34° - 60°N), based on the implementation of the Simplified Ionospheric Regional Model (SIRM) and the real time updating of SIRM (SIRMUP) methods, respectively. Recently, the DIAS middle latitude maps were extended to cover the whole European region up to 80°N, as part of the integration of DIAS services into the ESA/SSA Space Weather Service Network. To this effect, the background SIRM and SIRMUP methods were upgraded to expand the DIAS prediction capabilities to the high latitude ionosphere. In particular, the SIRM output at middle latitudes is now combined with the CCIR results for the region above 60°N. For the collaboration of the two distinct modeling approaches, special consideration is given to the buffer zone between the global CCIR and regional SIRM models in an attempt to avoid large gradients due to the complex behavior of the ionospheric conditions at high latitudes. For this purpose, an interpolation routine is applied to formulate the ionospheric predictions between 50° and 60°N and the final map is obtained as the combination of the three grids: the middle latitude, the high latitude and the buffer zone grid. The real time updating of SIRM&CCIR grids is then achieved through the SIRMUP concept that is now adjusted to incorporate the estimation of the effective sunspot number (Reff) separately at middle and high latitudes. The paper reports the results from the development of the new models and their implementation in DIAS, as well as indicative results on their performance in each latitudinal zone.

A solar flare is a burst of light occurring in the chromosphere near a sunspot and is observed at a wide band of wavelengths (radio to X-rays). Together with coronal mass ejections (CMEs), a flare is an explosive event that releases high energy protons and electrons, including intense radiation in all wavelengths which can affect the Earth's upper atmosphere. The solar radio bursts are intense radio emissions from the Sun that are usually associated with solar flares. The recent development of ground-based networks of GPS as well as satellite systems has opened a new means to study the ionospheric effects during the solar flares. The effects of radio bursts associated with x-ray flare also known as extreme space weather events is of practical importance as being a cause of interference in wireless communications, disruption of HF and GNSS communication and other radio systems which has not yet being explored much. In this regard, we found 34 events of radio bursts (>1000 sfu at 1GHz) using Nobeyama observations and are found to be closely associated with x-ray flare and CMEs during 2000-2012. We found 2 C-, 18 M- and 14 X-class flare respectively associated with it. The preliminary results of the effects of these events on the earth’s ionosphere using satellite observations will be presented.

Single-station ionospheric parameter models offer more accurate results for a particular ionosonde station than global models and are easy to update. The models employ decomposition of the measured data and its correlation with different solar and geophysical parameters like sunspot number, geomagnetic index, etc. Their purpose is to provide short and long term prediction of the monthly-median f₀F₂. In this work we have investigated deterministic and non-deterministic methods for analysis of the f₀F₂ from the Dourbes Digisonde, Belgium and determination of diurnal, seasonal and solar cycle dependence of the F-layer critical frequency.

The three-dimensional (3-D) electron density mapping of the ionosphere given as output by the assimilative IRI-SIRMUP-P (ISP) model for three different geomagnetic storms is described. The goodness of the model results is tested by comparing the electron density profiles given by the model with the ones measured at two testing ionospheric stations: Roquetes (40.8 N,0.5 E), Spain, and San Vito (40.6 N,17.8 E), Italy. The reference ionospheric stations from which the autoscaled foF2 and M(3000)F2 data as well as the real-time vertical electron density profiles are assimilated by the ISP model are those of El Arenosillo (37.1 N,353.3 E), Spain, Rome (41.8 N,12.5 E), and Gibilmanna (37.9 N,14.0 E), Italy. The representation of the ionosphere made by the ISP model is on the whole better than that made by the IRI-URSI and the IRI-CCIR models. However, a few cases show that the assimilation of the autoscaled data from the reference stations causes either a strong underestimation or a strong overestimation of the real conditions of the ionosphere, which is in these cases better represented by the IRI-URSI model. This misrepresentation made by ISP is mainly due to the fact that the reference ionospheric stations covering the region mapped by the model turn out to be few, especially for disturbed periods when the ionosphere is very variable both in time and in space and hence a larger number of stations would be required.

GNSS is making a significant impact in support of operations where high accuracy is required, as in precision agriculture, where the meticulous application of pesticides and fertilizers translates into efficiency gain and profit. Other examples are surveying, geodesy, land management, off-shore operations. GNSS high accuracy techniques such as RTK (Real Time Kinematic), WARTK (Wide Area RTK) and PPP (Precise Point Positioning), exploiting the precision of GNSS signals carriers, are at the core of these applications and are especially sensitive to ionospheric perturbations, in particular scintillation phenomena, which are latitude and solar cycle dependent. Brazil sits in one of the most affected regions of the Earth and can be regarded as a test-bed for worst case scenarios. Problems with ambiguity fixing, crucial for GNSS carrier phase based techniques, have even before the rise of the solar cycle impeded the levels of availability expected by industry. A risk exists that the impact of high solar activity leads not only to disruption but even to disbelief on GNSS to support such applications. This issue is particularly relevant for the establishment of Galileo, and due to the technical challenge it poses. CALIBRA (Countering GNSS high Accuracy applications Limitations due to Ionospheric disturbances in BRAzil), a project funded by the European Community under the call FP7-GALILEO-2011-GSA-1a, aims to develop algorithms to be applied to the highly precise GNSS carrier phase observables, which will be implemented in GNSS receivers in order to counter the adverse ionospheric effects. The project has a two-year duration and started on November 2012. This paper presents initial progress on the project, with focus on the scientific and computational challenges that must be tackled to translate the assessment of temporal and spatial TEC gradients typical of the perturbed Equatorial ionosphere into tools that can support the development of algorithms capable to effectively counter the space weather threats to GNSS high accuracy positioning.

We present an approach to assimilating GPS total electron content (TEC) measurements into an empirical model of the ionosphere in near real time. The goal of this project is to extract maps of HF radio relevant parameters, such as foF2 and hmF2, from this model. We present our results in comparison to ionosonde measurements and discuss the techniques used including an ensemble Kalman filter and the role of ionospheric models such as NeQuick as part of the assimilation process.

The project "Short Wave critical Infrastructure Network based on a new Generation high survival radio communication system", SWING, deals with the study and design of a system of HF radio connections to link European Critical Infrastructures (ECIs). This system will replace broad band internet transmission when the latter fails. The HF network will withstand adverse conditions such as those encountered in case of a terrorist attack, guaranteeing the communication between ECIs and the transmission of necessary data for the survival and minimum operability of ECIs. SWING will be designed to evaluate the threat and increase the security awareness, as well as the level of protection, of analogous and/or interdependent ECIs. The project has to develop the standard software and hardware tools necessary for implementing communication protocols suited for a reliable and interoperable Short Wave (High Frequency) radio network back up. Therefore, SWING must also analyze the HF network requirements necessary for alerting and controlling ECIs in case of threat or attack, understand the particular characteristics of the ionospheric channel in order to establish a suitable control system for the frequencies to be employed, and design a radio-communication architecture for a HF radio network over Southern Europe. The use of proper ionospheric channels for data communication requires, in fact, the support of a geomagnetic and ionospheric awareness to provide information on the terrestrial effects generated by the arrival of interplanetary disturbances. Particularly intense solar events can affect the geomagnetic field and ionospheric plasma, changing the ionospheric structure especially in the HF band. The activities, organized in Working Packages (WPs), are distributed among four partners (INGV-Italy, CNIT-Italy, NOA-Greece, OE-Spain).

The ionosphere is influenced by a broad spectrum of waves propagating in ionized gas as well as in the neutral atmosphere. Periods of the waves extend from seconds to days. High frequency continuous wave Doppler shift sounding is an effective method for the monitoring of ionospheric oscillations in the period range from tens of seconds to tens of minutes. One of the frequent sources of such oscillations is geomagnetic micropulsations. The micropulsations are magnetohydrodynamic waves of periods of seconds to minutes that originate in the magnetosphere. Ionospheric disturbances caused by geomagnetic micropulsations occupy similar period range like infrasound. Infrasound is mechanical waves that propagate in the neutral atmosphere and it is coupled with the ionospheric plasma via collisions between neutral and ionized gas particles.
A method is presented here which may help to distinguish ionospheric response to geomagnetic micropulsations from ionospheric infrasound. It is based on the comparison of multipoint ionospheric Doppler type measurements with records of local geomagnetic fields at several stations close to the Doppler sounder. This way of data evaluation reduces the chance that an incidental local disturbance of geomagnetic record at a single station leads to misinterpretation of studied Doppler records containing infrasound on one hand. On the other hand, a chance is reduced that ionospheric response to geomagnetic micropulsations will be misinterpreted as infrasound, which is relevant particularly when evaluating Doppler data of lower signal quality.
To demonstrate the method, an event is analysed here when suitable conditions for observations of ionospheric response to geomagnetic micropulsations occurred together with a potential tropospheric source of infrasound - intense convective storms in the region of ionospheric Doppler sounding.

The morphology of ionospheric storm has been investigated across equatorial and low latitude of Indian region. The deviation in F2 region parameters at equatorial station Thiruvananthapuram (8.5°N, 76.8°E) and low latitude station Delhi (28.6°N, 77.2° E) have been studied during five geomagnetic storm periods. The southward polarity reversal of the z component of the interplanetary magnetic field, Bz, is found to be highly correlated with the storm sudden commencement (SSC). Both positive and negative phases have been noticed during the study and it is observed that in spite of local time variation in Dst, the corresponding deviation in F layer parameter vary with the intensity of the storm as well as latitude of the observing stations. The positive storm phase over equatorial stations are found to be more frequent while the drop in ionization in most of the cases have been noticed at low latitude station of varying amplitude of deviations from the mean quiet day value. Due to disturbed electric field the simultaneous height rises have been noticed at these stations, with higher amplitude at Delhi in between 0000 to 0600 EMT. Positive deviation in foF2 is also observed across low latitude station during the storm which is attributed to daytime eastward electric field penetrating promptly from high to low latitudes. It may also be concluded that the reaction as seen at different ionospheric stations may be quite different during the same storm depending on the station coordinates, local time of the magnetic disturbance beginning.

The extended solar minimum and new solar cycle gives an opportunity for comparative study of the ionosphere disturbances at background of extremely low and medium solar activity. For analysing of the global structure and dynamics of ionospheric disturbances we used data provided by different ground-based and satellite ionosphere measurements. It was processed the data from European, American, Japanese, and Australian ionosonde networks as a benchmark data source. The ionosphere modification on a global scale have been checked with use of Global Ionospheric Maps, provided by international GNSS Service, and data from FORMOSAT-3/COSMIC RO mission. Additionally for estimation of the electron density dynamic at high latitudes there were analyzed TEC fluctuations maps, created by IGS/EPN, PBO and POLENET data.

As case study events there have been selected geomagnetic disturbances, occurred during the years 2008-2013, with significant ionospheric responses. The global maps of TEC were used in order to estimate large scale storm effects, ionosonde data gives possibilities to study the local peculiarities of the ionosphere disturbances (two parameters have been processed - the NmF2 and hmF2). The ionospheric slabthickness parameter was calculated for corresponded ionosones location. Additionally for analysis of the height ionospheric structure we combined ionosonde-derived data with the Ne profiles from FORMOSAT-3/COSMIC RO measurements and global distribution of electron density at selected altitudinal intervals. It was resulted that selected moderate geomagnetic storms (Kp ~ 6) lead to the different ionospheric response (positive and negative) over European, American, Japan and Australian areas.

The global pattern and local temporal and quantitative characteristics of the ionosphere disturbances during selected storms were revealed. For example geomagnetic storm October 11, 2008 lead to short time positive ionospheric disturbance over Europe in TEC values with factor 2, foF2 - with factor 1,5-1,8 and uplifting of F2 layer maximum up to 100 km. Additionally it was carried out the comparison of the ionosondes-derived foF2 values with IRI-2012 model, that have the storm-time option. It was obtained the qualitative agreement between the ionosonde-derived foF2 values and model calculations for cases of negative ionospheric storms. The best agreement between model and observations results corresponds to the Northern Hemisphere mid-latitude stations.

We acknowledge the Australian IPS Radio and Space service and the National Institute of Information and Communications Technology (NICT) in Japan for providing ionosonde data. The authors would like to thank B.W. Reinisch and the Center of Atmospheric Research, University of Massachusetts Lowell for the ionogram data of DIDBase. We are also grateful to International GNSS Service (IGS) for GPS TEC products.

NeQuick2, International Reference Ionosphere (IRI) and Multi-Instrument Data Analysis System (MIDAS) are ionospheric electron density models used in positioning, imaging and assimilation algorithms. In this work, using the three models, we thoroughly analyzed the "Onion-peeling" algorithm which is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density (Ne) distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density vertical profile Ne(h). In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods.

Using simulated RO Total Electron Content (TEC) data computed by means of ideal RO geometries, we tried to formulate and evaluate an asymmetry level index for quasi-horizontal TEC observations. This asymmetry index is based on the Ne variations that a signal may experience along its propagation path (satellite to satellite link) during a RO event. This index is strictly dependent on RO geometry and azimuth of the occultation plane and is able to provide us indication of the errors (in particular those concerning the peak electron density (NmF2) and the vertical TEC (VTEC)) expected in the retrieval of Ne(h) using standard Onion-peeling algorithm. On the basis of the outcomes of our work, for a given geometry of a real occultation event and using NeQuick, IRI and MIDAS, we will try to investigate the possibility to predict the ionospheric asymmetry expected for the particular RO geometry considered. We could also try to evaluate, in advance, its impact on the inverted electron density profile, providing an indication of the product quality.