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© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. In this article, we show the capability of a global navigation satellite system (GNSS) precise orbit determination (POD) low Earth orbit (LEO) data to detect anomalous ionospheric disturbances in the spectral range of the signals associated with earthquakes and tsunamis, applied to two of these events in Papua New Guinea (PNG) and the Solomon Islands during 2016. This is achieved thanks to the new PIES approach (POD-GNSS LEO Detrended Ionospheric Electron Content Significant Deviations). The significance of such ionospheric signals above the swarm LEOs is confirmed with different types of independent data: in situ electron density measurements provided by the Langmuir Probe (LP) onboard swarm LEOs, DORIS, and ground-based GNSS colocated measurements, as it is described in this article. In this way, we conclude the possible detection of the tsunami-related ionospheric gravity wave in PNG 2016 event, consistent with the most-recent theory, which shows that a tsunami (which is localized in space and time) excites a spectrum of gravity waves, some of which have faster horizontal phase speeds than the tsunami. We believe that this work shows as well the feasibility of a future potential monitoring system of ionospheric disturbances, to be made possible by hundreds of CubeSats with POD GNSS receivers among other appropriate sensors, and supported for real-time or near real-time confirmation and characterization by thousands of worldwide existing ground GNSS receivers. This work has been partially supported by the COSTO ESA-funded project (ESA ITT AO/1-9514/18/NL/IA). Also it was partially supported by the 2017 SGR-0851 grant of the Generalitat de Catalunya and by the EU project 101007599 - PITHIA-NRF. The data for this paper are available and they can be requested from H. Yang (h.yang@upc.edu) and M. Hernandez-Pajares (manuel.hernandez@upc.edu).

In the last decades, advances in satellite technologies, data analysis techniques, and models, and a growing number of analysis centers allow modeling the ionospheric electron content with unprecedented accuracy. International GNSS Service (IGS) Ionosphere Associated Analysis Centers (IAAC) continuously provide global ionospheric maps (GIMs) based on processing GNSS data from the ground IGS network. It is a great advantage that these GIMs are often based on very different modeling techniques and thus, are also characterized by different accuracy levels. Due to the dynamic nature of the ionosphere, there is a permanent need to improve the modeling techniques of ionospheric key parameters such as the vertical total electron content (vTEC).In this presentation, we evaluate a new ionosphere model from DGFI-TUM denoted OTHG, which is a candidate for a new IGS IAAC product. The OTHG model is based on tensor products of trigonometric B-spline functions in longitude and polynomial B-spline functions in latitude for a global representation (Goss et al. 2019). Here, we complement our earlier investigations of the seven analysis center models (Wielgosz et al. 2021) with new results for the OTHG GIMs. For these investigations, we use our own validation methodology presented in Krypiak-Gregorczyk et al. (2017), which is based on GIM-derived slant TEC (sTEC) comparison with carrier phase geometry-free combination of GNSS signals. In the presented study, we use one year of GNSS data collected by 25 globally distributed stations. The overall yearly RMS value is calculated for each product based on all 365 days of continuous observations from all stations. The results show that the overall RMS of the tested GIMs ranges from 0.93 TECU to 1.29 TECU. The OTHG GIMs performed as one of the best. In addition, GIM vTEC comparisons to Jason-2 and Jason - 3 altimetry data are studied. In these analyses, the OTHG GIMs also showed a good performance. Therefore, it can be concluded that DGFI-TUM is a valuable ionospheric product for the research community. Goss A., Schmidt M., Erdogan E., Görres B., Seitz F. (2019) High-resolution vertical total electron content maps based on multi-scale B-spline representations. Annales Geophysicae, 37(4), 10.5194/angeo-37-699-2019Krypiak-Gregorczyk A., Wielgosz P., Borkowski A. (2017) Ionosphere Model for European Region Based on Multi-GNSS Data and TPS Interpolation, Remote Sensing, 9(12), 1221, DOI:10.3390/rs9121221Wielgosz P., Milanowska B., Krypiak-Gregorczyk A., Jarmołowski W. (2021) Validation of GNSS‑derived global ionosphere maps for different solar activity levels: case studies for years 2014 and 2018. GPS Solutions 25, 103. https://doi.org/10.1007/s10291-021-01142-x

The adverse effects of ionospheric delays limit the positioning accuracy of single-frequency GNSS users. To mitigate these effects, GNSS system providers make several ionospheric delays models available for their global users. For example, the GPS has offered the Klobuchar model from the beginning. More recently, Galileo users can use the NeQuick G model. In the meantime, several independent models available for real-time navigation have emerged. Recent examples are the NTCM (Neustrelitz Total Electron Content Model) correction model provided by the German Aerospace Center (DLR) and real-time global ionosphere maps (RT-GIMs) provided by the National Centre for Space Studies (CNES).In this contribution, we evaluate the performance of several global ionospheric delay correction models in SPP mode. We used single-frequency pseudorange data from 12 GNSS stations distributed globally, covering different latitudes for the evaluation. The test data includes GNSS observations from DOY 93/2020 to DOY 80/2021, covering almost one full year of increasing solar activity. We validated the performance of the NTCM-G model driven by the Galileo Az parameters against the Klobuchar, NeQuick 2, NeQuick G, and CNES RT GIMs models. Finally, we compared the results to reference solutions obtained with CODE GIM and also using the ionosphere-free linear combination. We showed that NTCM-G corrections presented accuracy comparable with the NeQuick G model and better than the Klobuchar one.

In this manuscript, we introduce the Ionospheric Tomographic Common Clock (ITCC) model of undifferenced uncombined GNSS measurements. It is intended for improving the Wide Area precise positioning in a consistent and simple way in the multi-GNSS context, and without the need of external precise real-time products. This is the case, in particular, of the satellite clocks, which are estimated at the Wide Area GNSS network Central Processing Facility (CPF) referred to the reference receiver one; and the precise realtime ionospheric corrections, simultaneously computed under a voxel-based tomographic model with satellite clocks and other geodetic unknowns, from the uncombined and undifferenced pseudoranges and carrier phase measurements at the CPF from the Wide Area GNSS network area. The model, without fixing the carrier phase ambiguities for the time being (just constraining them by the simultaneous solution of both ionospheric and geometric components of the uncombined GNSS model), has been successfully applied and assessed against previous precise positioning techniques. This has been done by emulating real-time conditions for Wide Area GPS users during 2018 in Poland., Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. The research has been performed under the support of ATOMIC ESA funded project and the manuscript has been written under PYTHIA EC project support. The ground GNSS network data was provided by Leica in Poland and the position of the GPS transmitters from IGS.

Ionosphere Associate Analysis Centers (IAACs) of the International GNSS Service (IGS) independently produce global ionosphere maps (GIMs) of the total electron content (TEC). The GIMs are based on different modeling techniques, resulting in different TEC levels and accuracies. In this study, we evaluated the accuracy and consistency of the IAAC GIMs during high (2014) and low (2018) solar activity periods of the 24th solar cycle. In our study, we applied two different evaluation methods. First, we carried out a comparison of the GIM-derived slant TEC (STEC) with carrier phase geometry-free combination of GNSS signals obtained from 25 globally distributed stations. Second, vertical TEC (VTEC) from GIMs was compared to altimetry-derived VTEC obtained from the Jason-2 and Jason-3 satellites and complemented for plasmaspheric TEC. The analyzed GIMs obtained STEC RMS values reaching from 1.98 to 3.00 TECU and from 0.96 to 1.29 TECU during 2014 and 2018, respectively. The comparison to altimetry data resulted in VTEC STD values that varied from 3.61 to 5.97 TECU and from 1.92 to 2.78 TECU during 2014 and 2018, respectively. The results show that among the IAACs, the Center for Orbit Determination in Europe global maps performed best in low and high solar activity periods. However, the highest accuracy was obtained by a non-IGS product—UQRG GIMs provided by Universitat Politècnica de Catalunya. It was also shown that the best results were obtained using a modified single layer model mapping function and that the map time interval has a relatively small influence on the resulting map accuracy.

The study investigates Swarm data including in-situ electron density (ED) measured by Langmuir Probes (LP) and total electron content (TEC) from precise orbit determination (POD) GNSS receivers in time of Chile-Illapel earthquake (EQ) and tsunami in 2015. The research is based on the symbiosis of Swarm data, ground GNSS data and seismic records combined with the information on EQs and tsunamis. The FFT-based filtering and short-term Fourier transform (STFT) analysis are used in detection of seismic ionospheric disturbances (SID) in ED from LP and POD TEC data. The classification of the spectral characteristics of disturbing along-track signals is supported by their simultaneous search in ground GNSS observations, which gives an opportunity for the validation of the spectral recognition. Ground GNSS data, due to several tens of satellites and thousands of stations, provide the only full spatiotemporal view on SIDs and enable the inspection of their spatial shapes, spatial relations and speeds. The location of dense ground GNSS networks is however limited to selected places. Swarm and other LEO satellite data, in turn, are globally distributed, but they are dense only along the orbital tracks. Therefore, 1D nature of Swarm along-track observations, fast satellite movement and limited chance for spatiotemporal correlation due to the non-repeating orbits, strongly require spectral analysis for better recognition of the signals. The detection of SIDs from along-track Swarm data is also complicated due to the variety of disturbing signals occurring in the ionosphere, and the spectral analysis is also crucial there. STFT spectral approach to along-track Swarm data gives an opportunity for distinguishing the signals of different origin. The analyses of Swarm data provide interesting observations of ionospheric disturbances not only directly related with the largest EQ events and tsunami, but also occurring during entire periods of enhanced seismic activity and at larger distances from EQ epicenter. The disturbing signals triggered by the largest EQs and tsunami were also observed. However their amplitude in the ionosphere is not always such dominating as the amplitude of some other, associated disturbances on the neighboring days. This difference in scale can suggest that the electron disturbances in the ionosphere are rather more generally related to the crustal motion and seismic activity, than solely correlated with large EQs.

The study intercompares three stochastic interpolation methods originating from the same geostatistical family: least-squares collocation (LSC) known from geodesy, as well as ordinary kriging (OKR) and universal kriging (UKR) known from geology and other geosciences. The objective of this work is to assess advantages and drawbacks of fundamental differences in modeling between these methods in imperfect data conditions. These differences primarily refer to the treatment of the reference field, commonly called ‘mean value’ or ‘trend’ in geostatistical language. The trend in LSC is determined globally before the interpolation, whereas OKR and UKR detrend the observations during the modeling process. The approach to detrending leads to the evident differences between LSC, OKR and UKR, especially in severe conditions such as far from the optimal data distribution. The theoretical comparisons of LSC, OKR and UKR often miss the numerical proof, while numerical prediction examples do not apply cross-validation of the estimates, which is proven to be a reliable measure of the prediction precision and a validation of empirical covariances. Our study completes the investigations with precise parametrization of all these methods by leave-one-out validation. It finds the key importance of the detrending schemes and shows the advantage of LSC prior global detrending scheme in unfavorable conditions of sparse data, data gaps and outlier occurrence. The test case is the modeling of vertical total electron content (VTEC) derived from GNSS station data. This kind of data is a challenge for precise covariance modeling due to weak signal at higher frequencies and existing outliers. The computation of daily set of VTEC maps using the three techniques reveals the weakness of UKR solutions with a local detrending type in imperfect data conditions.

Since 1998 Ionosphere Associate Analysis Centers (IAAC) of the International GNSS Service (IGS) routinely provide global ionosphere maps (GIMs). They are used for a wide range of geophysical applications, including supporting precise positioning and improving space weather analysis. These GIMs are generated by different analysis centers with the use of different modelling techniques. Therefore they have different accuracy levels, which has already been evaluated in several studies. Until 2014 all GIMs were provided with 2-hour temporal resolution, and since 2015 some of the IAACs have started to provide their products with higher resolutions, up to 30 - 60 minutes. Since GIMs have different temporal resolutions, we investigated whether map interval affected their accuracies.In this study we carried out IAAC GIM accuracy analysis for years 2014 and 2018, corresponding to high and low solar activity periods, respectively. Since in 2014 IAAC GIMs had 2-hour resolution, we also evaluated UQRG maps supplied with 15-minute interval. For low solar activity period (2018) we evaluated 4 models: CASG, CODG, EMRG and UQRG. In addition, we studied ionosphere map performance during two selected geomagnetic storms: on 19 February 2014 and 17 March 2015. Our accuracy evaluation was based on GIM-TEC comparisons to differential STEC derived from GNSS data and VTEC derived from altimetry measurements.The results show that temporal interval has no significant impact on the overall, annual map RMS during both high and low solar activity periods. However, during geomagnetic storms, when reducing map interval, the map accuracy improves by almost 25%.

Three Swarm satellites are equipped with Langmuir Probes (LP) measuring in-situ electron density of Earth electric field and POD GNSS receivers determining topside total electron content (TEC) in the upper ionosphere. It is proved that different events on the Earth and in its atmopshere have their own impact on Earth electric field, and the earthquakes are in this group. Many strong earthquakes induce tsunamis, which are also suspected as contributing to the gravity waves having an impact on the ionospheric TEC. These reasons encourage to the study on the sensitivity of Swarm LP and POD GNSS data to the abovementioned phenomena. Referring to the sensitivity of TEC data derived from GNSS stations to Earthquakes, sensitivity of GNSS and LP data at around 500 km high orbit is analyzed here. A similar orbital height can be found in case of many LEO missions equipped at least with GNSS POD receivers, which makes Swarm especially interesting data acquisition platforms.The investigation of Swarm data in view of Tsunamis and earthquakes is difficult due to several factors. There are only three satellites, the two of which fly almost together, which gives in fact only two points of the survey. The orbital repetition period is long, which seriously limits the number of comparable observations in terms of the location and time of the day. Finally, the number of large earthquakes and tsunami events in time of Swarm science mission is low, and many Earthquakes do not coincide sufficiently with Swarm passes in time and space. All these factors, however, doesn’t exclude an opportunity of analyzing of Swarm data passes above the earthquakes of magnitude nearby 8, linked with the tsunamis reaching several decimeters.Swarm LP data is detrended and analyzed before the earthquakes and also during the earthquakes and resulting tsunami events. The GNSS POD topside TEC from Swarm is analyzed together as a background for LP data. In-situ electron density disturbances occurring during a pass close to the earthquake is compared to selected STEC measurements between LEO and GNSS satellites. Additionally absolute STEC values from selected nearby ground stations are analyzed in order to find existing correlations for detected disturbances in the electric and magnetic fields. All the observations are sparse in time and space, and therefore, leave some unanswered questions and uncertainties. However, several interesting perturbations over earthquake/tsunami events are observable in both Swarm LP data and GNSS TEC data.

Recent Solar Cycle 24 is characterized by the occurrence of three strong disturbances near an equinox. In this contribution, the responses of the ionosphere to the equinox storms in 2012, 2013 and 2015 driven by coronal mass ejections were analyzed. Due to the dynamic nature of changes in the ionosphere, the accuracy and resolution of existing global ionosphere models are often insufficient to reflect the storm-time effects in detail. Bearing this in mind, a new highly accurate and high-resolution regional ionosphere model was applied to study the response of this layer to severe geomagnetic storms over Europe. New regional total electron content (TEC) maps were derived exclusively from precise global navigation satellite systems (GNSS) carrier phase data. Although this study is based on space-geodetic technique, it was also carried out in relation to the observations provided from European ionosondes. In addition, the regional maps were compared to the final IGS Global Ionosphere Maps, where the new solution showed a better detail level. The results of storm-time temporal TEC changes provided by GNSS data were confirmed by NmF2 changes derived from ionosondes. Both data sources confirmed their high compatibility for studying the disturbed ionosphere. The magnetic storms that occurred on 7, 9, 12 and 15 March 2012 were different in nature. The largest change in the total electron content was observed during the storm of 9 March. This storm was associated with an interplanetary coronal mass ejection on 7 March that arrived on Earth 2 days later. The other analyzed events in 2013 and 2015 occurred on the same day of year—17 March. They were triggered by coronal mass ejections, which also hit the Earth magnetosphere at the same time of day. However, again the observed response of the ionosphere to these events was different.

High precision Global Navigation Satellite Systems (GNSS) positioning and time transfer require correcting signal delays, in particular higher-order ionospheric (I2+) terms. We present a consolidated model to correct second- and third-order terms, geometric bending and differential STEC bending effects in GNSS data. The model has been implemented in an online service correcting observations from submitted RINEX files for I2+ effects. We performed GNSS data processing with and without including I2+ corrections, in order to investigate the impact of I2+ corrections on GNSS products. We selected three time periods representing different ionospheric conditions. We used GPS and GLONASS observations from a global network and two regional networks in Poland and Brazil. We estimated satellite orbits, satellite clock corrections, Earth rotation parameters, troposphere delays, horizontal gradients, and receiver positions using a global GNSS solution, Real-Time Kinematic (RTK) and Precise Point Positioning (PPP) techniques. The satellite-related products captured most of the impact of I2+ corrections, with the magnitude up to 2 cm for clock corrections, 1 cm for the along- and cross-track orbit components, and below 5 mm for the radial component. The impact of I2+ on troposphere products turned out to be insignificant in general. I2+ corrections had limited influence on the performance of ambiguity resolution and the reliability of RTK positioning. Finally, we found that I2+ corrections caused a systematic shift in the coordinate domain that was time- and region-dependent, and reached up to -11 mm for the North component of the Brazilian stations during the most active ionospheric conditions. .

In this paper, the authors summarize a simple and efficient approach developed to mitigate the problem in precise Global Navigation Satellite Systems (GNSS) positioning originated by the most frequent ionospheric wave signatures: the medium-scale traveling ionospheric disturbances (MSTIDs). The direct GNSS Ionospheric Interferometry technique (hereinafter dGII), presented in this paper, is applied for correcting MSTID effects on precise Real Time Kinematic (RTK) and tropospheric determination. It consists of the evolution of the former climatic Differential Delay Mitigation Model for MSTIDs (DMTID), for real-time conditions, using ionospheric data from a single permanent receiver only. The performance is demonstrated with networks of GNSS receivers in Poland, treated as users under real-time conditions, during two representative days in winter and summer seasons (days 353 and 168 of year 2013). In range domain, dGII typically reduces the ionospheric delay error up to 10–90% of the value when the MSTID mitigation model is not applied. The main dGII impact on precise positioning is that we can obtain reliable RTK position faster. In particular, the ambiguity success rate parameter increases, from 74% to 83%, with respect to the original uncorrected observations. The average of time to first fix is shortened from 30 s to 13 s. The improvement in troposphere estimaton, due to any potential impact of the MSTID mitigation model, was most difficult to demonstrate.

The ionosphere can be modeled and studied using multi-frequency GNSS signals and their geometry-free linear combination. Therefore, a number of GNSS-derived ionospheric models have been developed and applied in a broad range of applications. However, due to the complexity of estimating the carrier phase ambiguities, most of these models are based on low-accuracy carrier phase smoothed pseudorange data. This, in turn, critically limits their accuracy and applicability. Therefore, we present a new methodology of estimating the phase bias of the scaled L1 and L2 carrier phase difference which is a function of the ambiguities, the ionospheric delay, and hardware delays. This methodology is suitable for ionospheric modeling at regional and continental scales. In addition, we present its evaluation under varying ionospheric conditions. The test results show that the carrier phase bias of geometry-free linear combination can be estimated with a very high accuracy, which consequently allows for calculating ionospheric TEC with the uncertainty lower than 1.0 TECU. This high accuracy makes the resulting ionosphere model suitable for improving GNSS positioning for high-precision applications in geosciences.

GNSS signals have become a valuable tool in studying the Earth's ionosphere as their dual-frequency observables allow for calculation ionospheric total electron content (TEC). Recent studies estimated relative error of popular GNSS-TEC maps at the level of 20-30 %. This motivates research community to develop new modelling and interpolation methods. In this paper, we demonstrate new approach to GNSS-TEC estimation. In the new approach we use solely carrier phase multi-GNSS observables and thin plate splines (TPS) for accurate ionosphere modeling. The model allows for providing TEC maps for Europe with high spatial and temporal resolutions - 0.2x0.2 degrees and 2.5 minutes, respectively. In addition, we present the performance of our approach during one the most intense ionospheric storms of 24th Solar cycle that took place in March 2015. The results showed that the accuracy of our maps was better with residuals lower by at least 50 % comparing the widely used product.

The aim of this study was to assess the performance of several approximation techniques for ionospheric total electron content (TEC) mapping. Approximation techniques based on data-fitting with local or general two-dimensional polynomials, local planes or distance-dependent interpolation were applied and tested. For the ionosphere modeling, dual-frequency GPS data from Polish GBAS system (ASG-EUPOS) were used, and TEC was estimated together with hardware delays from phasesmoothed pseudoranges. Next, grids of vertical TEC values with spatial resolution of 0.25 degrees in both latitude and longitude were generated using the evaluated approximation techniques. Subsequently the grids were used to create regional TEC maps with 5-minute temporal resolution, and also to create ionospheric delay corrections for GPS positioning. The quality of the resulting ionospheric maps was tested twofold, firstly by comparison to high-quality CODE global ionosphere maps (GIM), which were generated using data from about 150 GPS sites of the International GNSS Service (IGS). Secondly, by creating double-differenced ionospheric delay corrections and comparing them to reference values derived from the reference network data processing. For the correction tests, two perpendicular baselines directed North-South (N-S) and West-East (W-E) and reaching up to 100 km were selected. The approximation methods were analyzed with a special emphasis on the diverse ionospheric conditions. For the testing, a quiet ionosphere day of 20 March 2012 and an active ionosphere day of 9 March 2012 were selected. The results show that the regional models properly represent the changing ionosphere, with the best results provided by data-fitting into local functions.

This research is motivated by the recent IGS Ionosphere Working Group recommendation issued at the IGS 2010 Workshop held in Newcastle, UK. This recommendation encourages studies on the evaluation of the application of COSMIC radio occultation profiles for additional IGS global ionosphere map (GIM) validation. This is because the reliability of GIMs is crucial to many geodetic applications. On the other hand, radio occultation using GPS signals has been proven to be a promising technique to retrieve accurate profiles of the ionospheric electron density with high vertical resolution on a global scale. However, systematic validation work is still needed before using this powerful technique for sounding the ionosphere on a routine basis. In this paper, we analyze the properties of the ionospheric electron density profiling retrieved from COSMIC radio occultation measurements. A comparison of radio occultation data with ground-based measurements indicates that COSMIC profiles are usually in good agreement with ionosonde profiles, both in the F2 layer peak electron density and the bottom side of the profiles. For this comparison, ionograms recorded by European ionospheric stations (DIAS network) in 2008 were used.

Simultaneous GPS observations from about 150 stations of European Permanent Network (EPN) have been used for studying dynamics of latitudinal profiles and structure of mid-latitude ionospheric trough (MIT). For the analyses, the TEC maps over Europe were created with high spatial and temporal resolution. The latitudinal profiles were produced from TEC maps with one-hour interval for geographic latitude range from 35N to 75N. The structure of latitudinal profiles relates to the occurrence of the ionospheric trough. The location of the trough depends on season, local time, and both geophysical and geomagnetic conditions. The trough structure in GPS-TEC demonstrates a smooth shape. The trough occurrence as a distinguished structure is more distinct during winter. The relation of TEC in the trough minimum to the equator and polar walls amounted to a factor of 2–4. The diurnal, seasonal as well as storm-time dynamics of the latitudinal profiles and the trough-like structure during different geomagnetic conditions are also presented. Day by day snapshots demonstrate great variability of TEC profiles and MIT, essential changes of the ionosphere structure took place during a storm. Similarly to the F2 region trough, the position of the TEC trough shifts towards lower latitudes during disturbances. In the storm-time, the TEC trough was recognized during day-time at latitudes lower than 70N. This paper presents the statistics describing the dynamics of the trough location.

The paper presents the structure, performance and preliminary results of new ionosphere monitoring service over the ASG-EUPOS network of GNSS reference stations in Poland. This dense permanent network consisting of ~110 stations is a part of the European Position Determination System (EUPOS) project involving 18 countries of Central and Eastern Europe. The service was developed at the University of Warmia and Mazury in Olsztyn and as one of few uses the undifferenced carrier phase measurements from dual-frequency receivers for modeling of the ionosphere. It provides ionosphere maps (TEC) with spatial resolution of 0.25×0.25 degrees and with temporal resolution of 5 minutes. In addition, the service provides daily and monthly differential code bias (DCB) values for each of the GNSS receivers of the ASG-EUPOS system. The DCBs are estimated based on phase-smoothed pseudoranges.

This research presents accuracy analysis of ionospheric corrections embedded in VRS data stream provided by ASG-EUPOS reference network. The analysis are provided for disturbed ionospheric conditions that occurred in November 2013. The research are based on processing VRS station data and comparing the results to processing of real station data located in the same place. In particular, double-differenced ionospheric delay residuals are evaluated and compared. As a reference, double-differenced ionospheric delay residuals under quiet ionospheric conditions were also calculated. The results show that the quality of the ionospheric information embedded in VRS data stream deteriorates during ionospheric disturbances, however, it is still sufficient to support precise GNSS positioning.

The ionosphere plays a crucial role in space weather that affects satellite navigation as the ionospheric delay is one of the major errors in GNSS. On the other hand, GNSS observations are widely used to determine the amount of ionospheric total electron content (TEC). An important aspect in the electron content estimation at regional and global scale is adopting the appropriate interpolation strategy. In this paper we propose and validate a new method for regional TEC modeling based on least squares collocation (LSC) with noise variance estimation. This method allows for providing accurate TEC maps with high spatial and temporal resolution. Such maps may be used to support precise GNSS positioning and navigation, e.g. in RTK mode and also in the ionosphere studies. To test applicability of new TEC maps to positioning, double-difference ionospheric corrections were derived from the maps and their accuracy was analyzed. In addition, the corrections were applied to GNSS positioning and validated in ambiguity resolution domain. The tests were carried out during a strong ionospheric storm when the ionosphere is particularly difficult to model. The performance of the new approach was compared to IGS and UPC global, and CODE regional TEC maps. The results showed an advantage of our solution with resulting accuracy of the relative ionospheric corrections usually better than 10 cm, even during the ionospheric disturbances. This proves suitability of our regional TEC maps for, e.g. supporting fast ambiguity resolution in kinematic GNSS positioning.

In order to enhance our understanding of the rich plasma physical processes that drives the solar-terrestrial space environment we need to dramatically increase our ability to perform multi point measurements with sensors of different types. The magnetosphere-ionosphere-thermosphere system is strongly affected by electric and magnetic fields, particle precipitation, heat flows and small scale interactions. The changes of the near Earth plasma conditions are produced mainly by natural perturbations, but some of them also have anthropogenic origin. The diagnostics of the ionospheric plasma property as electron and ion density, temperature and velocity can provide essential inputs for modeling the Space Weather conditions. The aim of this presentation is to show global distribution of main plasma parameters during different geomagnetic conditions and seasons diagnosed by various measuring techniques as: in situ wave and plasma diagnostics registered on board of DEMETER satellite, GPS observations collected at IGS/EPN network, GPS observation carried out at the Antarctic and Arctic IGS (International GNSS Service) stations used and the data retrieved from FORMOSAT-3/COSMIC radio occultation measurements. We are willing to present and validate the properties of the ionospheric electron density profiling retrieved from FORMOSAT-3/COSMIC radio occultation measurements. The comparison of radio occultation data with ground-based measurements indicates that usually COSMIC profiles are in a good agreement with ionosonde profiles both in the F2 layer peak electron density (NmF2) and the bottom side of the profiles. For this comparison ionograms recorded by European ionospheric stations (DIAS network) during 2008 year were used. We would like also to discuss the limitation of presented diagnose techniques with respect to different geomagnetic condition and localisation in space.

Different radio sounding techniques are used to the study of the ionosphere state. In the given paper analysis of ionospheric electron density and contribution of different parts of the ionosphere to the total electron content (TEC) was carried out on the base of measurements provided by GPS transionospheric sounding, vertical ionospheric sounding and radio occultation (RO). It was considered the region of the European mid-latitudes with 2 closely located ionosonde stations — Pruhonice (50.0 N, 14.6 E) and Juliusruh (54.6 N, 13.4 E). Ionosonde data viz., ionograms, critical frequency of F2 layer (foF2) values and electron density profiles, were provided by European Digital Upper Atmosphere Server (DIAS). To obtain the bottomside ionospheric electron content (IECb) we integrate the bottomside Ne profile derived from digisonde. Topside ionosonde profile, obtained by fitting a model to the peak electron density value, was used to estimate the topside ionospheric electron content (IECt). Also the electron density profiles, derived from FormoSat-3/COSMIC RO measurements, were involved into analysis. The numerical integration was done in order to obtain COSMIC-derived IECb and IECt estimates. Therefore, for the present study the upper limit of the ionosphere has been taken to be at 750–800 km (altitude of COSMIC satellites). The total electron content values were calculated using the observations of the ground-based GPS stations located close to ionosondes. The vertical GPS TEC estimates can be split into two contributions, one part due to the bottomside ionosphere and other part due to the topside ionosphere. The topside part of TEC contains IECt and PEC (plasmaspheric electron content). So, comparison of GPS TEC, F3/C IEC and ionosonde IEC was carried out for different seasonal conditions during period of low solar activity. Special attention was focused on the differences in topside and bottomside parts contribution to TEC for night and daytime hours.

This paper presents accuracy analyses of two near real-time tropospheric delay estimation models, IGGHZG and IGGHZM, in fast-static precise positioning. The first model is based on the processing of GPS data collected from ground-based reference stations, and the second one uses metrological data fed into the Saastamoinen model. These two methods were developed within the ASG+ project by researchers from Wroclaw University of Environmental and Life Sciences. The accuracy of these models was evaluated by comparison to tropospheric zenith total delays (ZTDs) obtained from the ASG-EUPOS network final solution and the UNB3m model. The applicability of these models to precise positioning was studied by their application to ultra-fast-static positioning, in which the resulting coordinates' accuracy and ambiguity resolution performance were analysed. Baselines from 65 to 72 km, with height differences from 39 to 379 m, were processed. The obtained results show that the GPS-based model can be successfully used in precise positioning, and the meteo-based model still requires improvements, mostly related to the interpolation technique.

The electron density profiles retrieved from the COSMIC radio occultation measurements were examined in order to estimate the possibility of its use as additional data source to study changes in electron density distribution occurred during ionospheric storms. The ionosphere behaviour during moderate geomagnetic storm which occurred on October 11, 2008 was analysed. The short-duration positive effect was revealed distinctly in GPS TEC and ionosonde measurements. For the European mid-latitude region it reached the factor of 2 or more relative to the undisturbed conditions. COSMIC data were analyzed and their validity was tested by comparison with ground-based measurements. It was shown the good agreement between independent measurements both in quiet and disturbed conditions. Analysis of COSMIC-derived electron density profiles revealed changes of the bottom-side and topside parts of the ionosphere.