Abstract-
Synthetic aperture radar (SAR) signals can be affected by ionosphere, leading to a phase delay in the topography and displacement maps generated by SAR Interferometry (InSAR). Ionospheric phase delay is a major error source in InSAR observations, especially in L-band SAR data due to their lower frequency. Several methods have already been proposed to mitigate the effect of ionospheric on InSAR measurements. In this study, we evaluate ionospheric correction for L-band SAR data using 3 methods: first we implement ionospheric phase correction by using existing global models. Global ionosphere map (GIM) is generated on a daily basis at Center for Orbit Determination in Europe (CODE) using data from GPS/GLONASS, and provides global coverage of vertical total electron content ( VTEC) map with a resolution of 5° in longitude and 2.5° in latitude. Second we derive VTEC data provided by SWARM satellites (launched by European Space Agency) and integrate them into CODE to increase spatial resolution before applying them. Based on integrated VTEC map, the ionospheric correction on interferograms can be applied. We also assess ionospheric phase delay by estimating the azimuth shift in interferograms due to the approximate linear relationship between the azimuth shift and ionospheric phase streaks (IPS). In order to generate azimuth offset, both multiple aperture interferogram (MAI) based method and an amplitude tracking method are implemented. To evaluate the performance of different methods, we utilize L-band SAR data from ALOS-1 and ALOS-2, which were launched by Japan Aerospace Exploration Agency (JAXA). A comparison between these three methods is discussed and the results are presented in the presentation.

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Soil Moisture and Ocean Salinity (SMOS) is an Earth Explorer (EE) European Space Agency (ESA) mission launched in November 2009, in excellent operational status with plans to continue its operational phase till 2021 and beyond. The payload of SMOS consists of the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) instrument, a passive microwave two dimensional interferometric full polarization radiometer, operating at 1.413 GHz (wavelength of 21 cm). The MIRAS payload comprises a central structure and three deployable arms holding the equally distributed 69 antenna elements. The SMOS mission is based on a sun-synchronous orbit (dusk-dawn 6am/6pm) with a mean altitude of 758 km and an inclination of 98.44°. SMOS has a 149-days repeat cycle with a 18-days sub-cycle and a revisit time of 3 days. Due to the satellite’s orbit geometry and the size of the MIRAS’s antennae the Sun appears in the antenna field of view and direct solar radio observation is captured by the MIRAS instrument. The retrieved L-band solar flux, even optimized for the Sun signal removal from the image, is a valuable information that can be further used for research in the fields of solar observation and space weather forecast services.
In particular solar radio proxy at other centimetric wavelengths (wavelength between 3.2 and 31 cm) have recently been shown to offer equivalent if not better performance (Dudok de Wit and Bruinsma, 2017) to the classical 10.7 cm radio flux from Penticton Observatory to model the upper atmosphere density at altitudes up to approximately 1000 km hence improving space weather nowcasting and forecasting service. L-band polarimetric solar flux is also a valuable information for assessing the impact of solar radio burst (intensity and polarization) at the specific Global Navigation Satellite Systems (GNSS) frequencies: L1 at 1575.41MHz and L2 at 1227.60MH (R. Crapolicchio et al., 2018).

The paper presents the results of the validation exercise between estimated polarimetric Sun solar flux from SMOS dataset derived with the new level 1 processor version v721 (which is now available also with Sun position on back of the antenna plane, extending the observation cycle up to 24 hours a day and for the full Stokes parameter) and various ground-based radio-telescope measurements and GNSS receivers measurements. The reference dataset used is from the Humain solar radio telescope operated by "Observatoire royal de Belgique", from USAF Radio Solar Telescope Network (RSTN) and from the Nobeyama Radio Polarimeter of the National Astronomical Observatory of Japan for both Sun flares events and quiet Sun condition. Real-time GNSS observations from the EUREF Permanent Network (EPN) has been used to estimate the fades of carrier to noise density (ΔC/N0) during intense solar radio burst.

The validation results for the new SMOS solar flux show:

i) a strong timing correlation and a good correspondence for the entire Stokes parameters during several major Sun microwave burst analysed (including the event of September 2017, so far the strongest solar event for current solar cycle 24);
ii) the capability of the SMOS measurements to follow the increase of the mean solar activity for the solar cycle 24 during a period of transition from quiet to active Sun, with a strong correlation with traditional proxies for solar activity such as sun spot number and 10.7 cm radio flux;
iii) The direct comparison of GNSS ΔC/N0fade and SMOS solar flux show very strong correlations in the case studies where the SMOS 4th Stokes vector component shows right hand polarized radiation (GPS-like polarisation). This on one side validate the SMOS dataset polarisation signal and on the other side makes SMOS dataset valuable for comparison with GNSS signal reception fades during solar radio bursts (see Figure-1).

Based on the validation result, the SMOS mission can provide a valuable contribution in collecting and delivering centimetric wavelength measurements from the Sun for space weather monitoring and application development. There are still some caveats that shall be investigated and mitigated like the dependency of solar flux estimation from antenna’s Sun position. Nevertheless advantages in the usage of SMOS data for solar observation in L-band and for possible applications in space weather are described and summarized as:

i) The SMOS dataset availability almost in near real time and with permanent coverage, apart from the situation of the Sun eclipsed by Earth (few minutes per day in the Nov-Jan period);
ii) The SMOS 21.2 cm wavelength can be a good UV proxy complementary/substitute to the F10.7 index;
iii) The homogeneity of the data set which does not require cross-calibration among different on Earth observatories;
iv) An independent source of information for the interpretation of the on-ground observatories measurements that from time to time can be affected by Radio Frequency Interference (RFI);
v) Unique opportunity to exploit full polarization data set (i.e. 4th Stokes parameter) to fill that gap in solar radio observations.

References:
Dudok de Wit T, Bruinsma S. 2017. “The 30 cm radio flux as a solar proxy for thermosphere density modelling”. J Space Weather Space Clim 7: A9. DOI: 10.1051/swsc/2017008.

Raffaele Crapolicchio et al., “Solar radio observation from Soil Moisture and Ocean Salinity (SMOS) mission: a potential new dataset for space weather services”. 15th European Space Weather Week, 5 - 9 November 2018, Leuven, Belgium.

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As a result of the coupling between the solar wind and the Earth’s magnetosphere-ionosphere system, plasma density irregularities are often developed in the polar ionosphere, which in turn can severely degrade satellite-based positioning and communication systems. High-latitude ionospheric irregularities are often associated with polar cap patches, blobs, particle precipitation, and the main ionospheric trough. These ionospheric phenomena can be studied by using in-situ data from polar orbiting satellites. Here, we present a statistical study based on observations from the European Space Agency’s Swarm mission. Swarm satellites are especially useful at high latitudes due to the good coverage in both hemispheres as compared to ground-based instruments. Two comprehensive datasets are used to detect ionospheric irregularities, i.e., electron density from the Langmuir probe and GPS TEC from the GPS receiver onboard Swarm satellites. We derive several parameters to describe ionospheric irregularities, e.g., the rate of change of density index (RODI) and electron density gradients (\nabla_Ne) from the electron density data, rate of change of TEC index (ROTI) from GPS data (these parameters are included in the Swarm IPIR product). The statistics show that electron density and plasma irregularities are modulated by the Earth’s magnetic field in both hemispheres. The climatology maps in magnetic coordinate show predominant plasma irregularities near the dayside cusp, polar cap, and nightside auroral oval. The spatial distribution of plasma irregularities are also modulated by the interplanetary magnetic field (IMF), e.g., when considering the IMF By component, the irregularity distribution in the cusp and polar cap shows asymmetry between the Northern (NH) and Southern hemispheres (SH), i.e., for negative IMF By, the irregularities are stronger in the dusk (dawn) sector in the NH (SH) and vice versa. This feature is consistent with high-latitude ionospheric convection pattern that is regulated by the IMF By. Plasma irregularities are also strongly controlled by solar activity within the current declining solar cycle. Ionospheric irregularities in polar caps show clear seasonal variation with higher values near equinoxes and lower values near solstices for both hemispheres.

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ESA‘s low earth orbit three-satellite mission Swarm is equipped with geodetic type dual-frequency GPS receivers and provides data from near-polar orbits at initial altitudes of about 460km (Swarm A/C) and 530km (Swarm B). In addition, since 15 July 2014, it provides dual-frequency GPS data with a 1Hz instead of 0.1Hz sampling which in turn leads to a relatively dense spatial sampling.

Because of the low earth orbit of Swarm most of the topside ionosphere is contained in the line of sight between the Swarm GPS receivers and the GPS satellites. This setting allows a precise estimation of the relative topside slant TEC which in turn may be used for topside ionospheric tomography or validation of ionospheric models. The GPS data quality of Swarm is, however, affected by fast changes of slant TEC. Systematic biases were observed in orbit processing and gravity field estimation. Because tomographic approaches are sensitive to such artifacts in the GPS-data, weighting or screening approaches need to be adopted. Besides the biases in the data, ionospheric irregularities need to be identified for model training and validation. Artifacts may be caused by ionospheric plasma bubbles, which reduce the local electron density and are thus well known to scatter GPS signals.

We investigate the dynamics of the geometry-free linear combination of the GPS carrier phase data and its application to ionosphere reconstruction techniques. We also investigate the sensitivity of our tomographic and reconstruction approaches to GPS data which is suspected to be biased and to ionospheric plasma bubbles. Eventually, we evaluate methods to reduce the impact of suspicious data and derive a criterion to identify equatorial plasma bubbles in GPS phase observables. The results will be validated with respect to the official Swarm Ionospheric Bubble (IB) Index product. In addition to the insitu IB-Index a GPS derived bubble Index can provide additional information about the location or shape of the bubble. Also, the GPS bubble index might be useful for screening or weighting in orbit processing.

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The LEO (Low Earth Orbit) satellite-based TEC (Total Electron Content) and ROTI (Rate Of change of TEC Index) are highly relevant for users in navigation and communications in view of strong plasma gradients causing GPS signal degradation and can lead to the losses of the signal. ESA’s Swarm constellation mission, consisting of three identical satellites, was launched on 22 November 2013. Among other instruments, each Swarm satellite carries a dual frequency GPS (Global Positioning System) Receiver (GPSR), which can be used to derive TEC and ROTI data. Swarm ROTI is defined as a standard deviation of Rate Of change of TEC (ROT) and it describes the small-scale variability of the line of sight electron content resulting from the ionosphere and plasmasphere. We present the climatology of TEC and ROTI using 5 years of Swarm constellation mission observations and investigate the seasonal and local time dependence at high and low latitudes. Also, the newly derived TEC and ROTI products for the ESA’s GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) mission, which flew from 2009 to 2013 at approximately 270 km altitude, are presented. We compare GOCE results to the Swarm observations for the same local times and seasons, taking into account similar solar conditions. Contrary to the Swarm satellite data, GOCE TEC observations contain a considerable part of the F region electron density. The comparison between both data sets enables the estimation of the electron content variation in the upper atmosphere.

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Ionospheric irregularities can cause scintillation of L-band and lower frequency radio signals. Observed scintillation effects include fading or loss of signal power and rapid phase fluctuations, which can degrade data quality of GNSS and spaceborne radar systems. Examples of the degradation include increased error in GNSS-based precise positioning and L-band Earth remote sensing imagery. Space weather and geomagnetic storms can cause dramatic variations in ionospheric, thermospheric, and magnetospheric (ITM) coupling processes. These variations can change ionospheric state and foster or suppress development of ionospheric irregularities in polar, middle-latitude, and low latitude regions.

Using ground-based dual-frequency GNSS data, the rate of TEC index (ROTI) can be derived that measures ionospheric irregularities at scale sizes ranging from a few hundred meters to a few tens of kilometers. ROTI data from thousands of stations have been used to produce global map of ionospheric irregularities (GMII, Figure 1). Such GMII can be generated at 5-minute cadence to monitor and study global activities of irregularities under nominal geophysical conditions and during space weather evets. The same GNSS data can also be assimilated into the 4-dimentional global assimilative ionospheric model (GAIM) and global ionospheric map (GIM) to reproduce global-scale state and variations of the ionosphere.

This presentation will show examples of ROTI measurements, GMIIs, and effects of ionospheric irregularities on GNSS-based precise positioning and spaceborne remote sensing radar systems. Ionospheric irregularities at various latitudes and in different longitude sectors during major space weather events will be investigated using ROTI and GMII. Global features of small-scale ionospheric irregularities are compared with large-scale ambient state modeled using GAIM and GIM. The space weather, geomagnetic and ionospheric conditions under which irregularities develop in different regions are analyzed, including auroral electrojet, auroral oval expansion and electric field gradient and fluctuations in the subauroral region, as well as electro- and thermospheric dynamics at low latitudes. A path to prediction of ionospheric irregularities will be discussed, which can benefit from (1) studies using GMII, GAIM, and GIM to identify ITM coupling processes that foster or suppress irregularity development, and (2) data-driven prediction techniques based on massive data sources.

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The VARION (Variometric Approach for Real-Time Ionosphere Observation) algorithm, able to estimate sTEC (slant total electron content) in real time, was demonstrated to be an effective tool to detect Travelling Ionospheric Disturbances [3]. Hence, one of most significant challenges is currently to understand if VARION can detect ionospheric scintillation, namely the fluctuation in phase and amplitude of the GNSS signal. Typically, scintillation occurs in the equatorial and polar regions and, in extreme cases, can result in loss of signal tracking, leading to the impossibility of positioning.
Specifically, observations coming from Low-Latitude Ionospheric Sensor Network (LISN) were analysed, in order to investigate the zone of ionospheric equatorial anomaly. LISN is characterized by a permanent array of geophysical instruments, including GNSS stations, magnetometers and ionosondes, which allow to monitor low, middle and high atmosphere in the equatorial region [2].
The VARION algorithm was thus modified in order to be able to identify the scintillation phenomena without losing its fundamental feature: the capability to work in real time.
The results were then compared with amplitude and phase scintillation indices in order to find a correlation between VARION outcomes and scintillation phenomena.
Furthermore, the statistical analysis proposed in [1] was here modified and applied for the determination of the duration of the scintillation itself. This procedure could reveal useful to better understand this kind of phenomena.
In conclusion, the contribution of VARION could be fundamental: the study of ionospheric scintillations is indeed essential for systems styles forecasts capable of guaranteeing the robustness and reliability of satellite communication systems (GNSS).

References
[1] Fratarcangeli, F., G. Savastano, M. C. D'Achille, A. Mazzoni, M. Crespi, F. Riguzzi, R. Devoti, and G. Pietrantonio (2018b), Vadase reliability and accuracy of real-time displacement estimation: Application to the central italy 2016 earthquakes, Remote Sensing, 10 (8), doi:10.3390/rs10081201.
[2] http://lisn.igp.gob.pe/.
[3] Savastano, G., Komjathy, A., Verkhoglyadova, O., Mazzoni, A., Crespi, M., Wei, Y., and Mannucci, A. J. (2017). Real-time detection of tsunami ionospheric disturbances with a stand-alone GNSS receiver: A preliminary feasibility demonstration. Scientific reports, 7, 46607. doi: 10.1038/srep46607.

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Space weather means today an own, very up-to-date and interdisciplinary field of research. It de-scribes physical processes in space mainly caused by the Sun’s radiation of energy. The manifestations of space weather are multiple, for instance, the variations of the Earth’s magnetic field or the changing states of the upper atmosphere, in particular the ionosphere and the thermosphere.
The most extreme known space weather event happened at September 1, 1859 – the Carrington storm. Other prominent recent, but much weaker events have been the Halloween storm at October 28 – 30, 2003, or the St. Patrick’s storm at March 17, 2015.
The strength of these events, their impacts on modern society and the possibility of much stronger future events have brought several countries such as US, UK, Japan, Canada and China to recognize the necessity of studying these impacts scientifically, of developing protection strategies and procedures and to establish space weather data centres and space weather services. As a consequence of these activities the Focus Area on Geodetic Space Weather Research (FA GSWR) was implemented under the GGOS umbrella of IAG. The main objectives of the FA GSWR are (1) the development of improved ionosphere models, (2) the development of improved thermosphere models and (3) the study of the coupled processes between the thermosphere and the ionosphere. Consequently, the objectives of the FA GSWR are including both fundamental research and practical applications.

From the geodetic point of view space weather research must be based on the use and the combination of all space-geodetic observation techniques, on geodetic methods for real-time modelling and forecast approaches. Assimilation strategies must be developed to consider additional information such as Sun observations. Consequently, geodetic space weather research comprises the basic ideas of GGOS.

In this contribution we focus on the theoretical investigations to approach the objectives of the FA GSWR and present the current status.

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As part of the Space Situational Awareness (SSA) ESA program, we developed a space weather data repository (http://ssa.spacescience.ro/) for solar and geomagnetic indices. All 40 nowcast and forecast indices can be requested in different formats, by accessing the repository directly or via the ESA SSA Space Weather Service Network (http://swe.ssa.esa.int/). Some of the indices are used, e. g., by Met Office UK, for atmospheric drag calculation.
Currently, we work towards extending the data repository into a flexible configuration, able to incorporate both ground and in-situ data. As a first step, we plan to test the extension by storing also the sheet current intensity of the large scale field-aligned current (FAC), ∆B_y/μ_0 [A/m], as derived from magnetic field observations of the Swarm spacecraft. The large-scale FAC intensity was shown to correlate well with magnetic activity and could be used as an alternative proxy for space weather effects. Moreover, computing this quantity does not require extensive calculations, which makes it suitable for near real time applications.
The extended repository will be accessible directly and will be easily scaled to include additional data, if required. Together with the web interface, we plan to provide also a REST interface, that will facilitate the future integration in the ESA SSA Space Weather Service Network. This will take advantage of the next Space Weather portal version, where ESA plans to include an interface suitable for the easy integration of products stored on non-ESA servers.

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Since the ionosphere is impacting various communication and navigation applications relying on radio signal transmission, accurate monitoring and forecasting of the ionosphere is of great importance. For this purpose, physics based modelling of the coupled thermosphere ionosphere system is rather important, because good forecasts of ionospheric variability especially during storms need to consider the various physical driving processes in the thermosphere and ionosphere. This includes the energy transmission from the solar wind to the magnetosphere and ionosphere-thermosphere, as well as the electric field modifications, enhancement of ionosphere currents and thermosphere circulation and composition disturbances.
One of the state of the art numerical models is the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model. The implementation of the model has been enhanced step by step over decades. Recent developments improved the capabilities of reproducing thermosphere ionosphere conditions during storms. These capabilities will be demonstrated based on comparison of CTIPe results during the St. Patrick’s Day storm on 17 March 2015 with ground and space based observations. We will make use of Swarm measurements, ionosondes and GNSS based TEC estimations.
The validation results show a rather good reproduction of thermosphere conditions with CTIPe. Especially radiative cooling has improved significantly. The deviations between model and observations are larger for the ionosphere. Based on the validation results the limitations of the model are discussed and next steps for implementation are proposed.

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The European Space Agency's (ESA) Space Situational Awareness (SSA) Programme aims to develop space weather (SWE) products that will help predicting the severe SWE events, which can have devastating effects on critical infrastructures in the EU, such as communication and navigation systems, power grids, aviation, etc. Currently, a comprehensive system to monitor, predict and disseminate SWE information and alerts is in development, which will help reducing costs and improving the reliability of these infrastructures. The fundament of the ESA SWE Network are five Expert Service Centres (ESCs), each focusing on different topics. The Ionosphere ESC comprises the expertise on the ionosphere and thermosphere monitoring, modeling, nowcasting and forecasting. It provides products and services that are tailored to the needs of communication and navigation system users, which rely on the transmission of radio signals and operators in the space surveillance and tracking domain who need space weather information, e.g. for proper satellite drag calculation.
Four Swarm derived ionosphere products are the first LEO satellite measurements provided in different services on the SSA SWE Portal (http://swe.ssa.esa.int/). These are in-situ electron densities, Total Electron Content (TEC), Rate Of change of TEC (ROT) and the Ionosphere Bubble Index (IBI). These products are excellent for research and validation of ground-based products. But, the applicability for end-users in the transionospheric radiolink and space surveillance and tracking domains is rather low. Limitations in the simultaneous spatial-temporal coverage of the given region and the timeliness of the data were identified as critical points (lessons learned for the future missions). This initiated discussions on the usage of Swarm data in the SSA SWE Network, how to best use space-based monitoring of thermosphere and ionosphere conditions in the Ionosphere ESC. Perspectives and potential use of Swarm products and other LEO satellite missions are going to be described in this presentation.

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