S8

The GNSS State Space Representation (SSR) technology is widely accepted to be the most versatile approach for real-time GNSS corrections. It is employed in several commercial and scientific PPP and PPP-RTK services. Its main advantage over observation space representation (OSR) techniques (e.g., RTK or network RTK) is the intrinsic support for broadcast applications disseminating corrections to an unlimited number of users.
A complete set of SSR corrections consists of the five basic components: clock, orbit, bias, ionosphere, and troposphere corrections for the different GNSS, frequencies, and signals. In a classical OSR service, the lump-sum of these five basic components is computed by the service provider for the user position and sent to the user. This implies that a user does not need to know the underlying models used by the server. In contrast to OSR, an SSR user must compute the influences of the five SSR components itself. For that reason, SSR models are part of an SSR format documentation. The models chosen in different SSR formats are a compromise between target accuracy, complexity, required bandwidth, and computational workload of the rover.
In this conference contribution, we give an overview of different ionosphere and troposphere models used in different open SSR formats. The focus is on SSR formats supporting the high resolution atmospheric corrections (Compact SSR, SPARTN, SSRZ, 3GPP-LPP), but also formats with reduced message sets are addressed (IGS-SSR, RTCM-SSR). We motivate the frequently used multi-stage approach to separate atmospheric corrections into functional (spherical harmonics, polynomials) and residual parts. For the ionosphere, we compare different types of polynomials, vertical and slant TEC, and interpolation heights as well as the advantage of a sun-fixed coordinate frame. For the troposphere, we discuss the advantages and disadvantages of metric vs. relative and slant vs. zenith delay corrections, respectively, and This overview of different ionosphere and troposphere models in SSR formats is intended to help an SSR user to choose a suitable SSR service.

How to cite: Wübbena, G., Wübbena, J., Wübbena, T., Perschke, C., and Schmitz, M.: Comparison of different ionosphere and troposphere models in open SSR correction formats in terms of accuracy, complexity, and bandwidth, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-15, https://doi.org/10.5194/iag-comm4-2022-15, 2022.

Presently, the high and increasing number of worldwide permanent Global Navigation Satellite System (GNSS) receivers providing the raw measurements in real-time (RT) and openly from a large fleet of GNSS transmitters, has opened the door to an accurate real-time determination of the distribution of the ionospheric electron content, either at global scale.

The motivations are diverse for such a scientific and technological challenging target, for instance: (1) to facilitate the fast carrier phase ambiguity resolution of roving users and corresponding fast decimeter-error level navigation at large distances (hundreds of kilometers) from supporting permanent GNSS receivers (see Hernández-Pajares et al. 2000, Juan et al. 2012, Olivares-Pulido et al. 2021); and (2) the possibility of generating in real-time realistic global maps of ionospheric storm state and Vertical Total Electron Content (VTEC) gradients like it can be done with a latency of 1 day (Liu et al. 2021a, 2022 respectively) based on UQRG GIMs, one of the best behaving ones in the International GNSS Service (IGS), see Roma-Dollase et al. (2018). This might be feasible thanks the recent advances in individual and collaborative real-time global VTEC mapping (see Yang et al. 2021 and Liu et al. 2021b, respectively) and in global VTEC forecasting (Monte-Moreno et al. 2022). These potential new RT products might be useful as ionospheric storm semaphores and quantifiers for example for contributing to the integrity of single-frequency GNSS based navigation in civil aviation.

One possible evolution in this field might include the consistent 4D combination of the already existing RT GNSS measurements with the potential RT additional geodetic measurements sensitive to the ionospheric delay, like Doppler measurements (DORIS), and like dual-frequency vertical delay measurements over the oceans provided by altimeters, such as JASON-3 among others (Hernández-Pajares et al. 2021). This would significantly extend the coverage and accuracy at global scale, improving as well the vertical resolution.

References

Hernández‐Pajares, M., Juan, J. M., Sanz, J., & Colombo, O. L. (2000). Application of ionospheric tomography to real‐time GPS carrier‐phase ambiguities resolution, at scales of 400–1000 km and with high geomagnetic activity. Geophysical Research Letters, 27(13), 2009-2012.

Juan, J. M., Hernández-Pajares, M., Sanz, J., Ramos-Bosch, P., Aragon-Angel, A., Orus, R., ... & Tossaint, M. (2012). Enhanced precise point positioning for GNSS users. IEEE transactions on geoscience and remote sensing, 50(10), 4213-4222.

Liu, Q., Hernández‐Pajares, M., Lyu, H., Nishioka, M., Yang, H., Monte‐Moreno, E., ... & Orús‐Pérez, R. (2021a). Ionospheric storm scale index based on high time resolution UPC‐IonSAT global ionospheric maps (IsUG). Space Weather, 19(11), e2021SW002853.

Liu, Q., Hernández-Pajares, M., Yang, H., Monte-Moreno, E., Roma-Dollase, D., García-Rigo, A., ... & Ghoddousi-Fard, R. (2021b). The cooperative IGS RT-GIMs: A reliable estimation of the global ionospheric electron content distribution in real time. Earth System Science Data, 13(9), 4567-4582.

Liu, Q., Hernández‐Pajares, M., Yang, H., Monte‐Moreno, E., García‐Rigo, A., Lyu, H., ... & Orús‐Pérez, R. (2022). A New Way of Estimating the Spatial and Temporal Components of the Vertical Total Electron Content Gradient Based on UPC‐IonSAT Global Ionosphere Maps. Space Weather, 20(2), e2021SW002926.

Monte-Moreno, E., Yang, H., & Hernández-Pajares, M. (2022). Forecast of the Global TEC by Nearest neighbour technique. Remote Sensing, 14(6), 1361.

Olivares-Pulido, G., Hernández-Pajares, M., Lyu, H., Gu, S., García-Rigo, A., Graffigna, V., ... & Orús-Pérez, R. (2021). Ionospheric tomographic common clock model of undifferenced uncombined GNSS measurements. Journal of Geodesy, 95(11), 1-13.

Roma-Dollase, D., Hernández-Pajares, M., Krankowski, A., Kotulak, K., Ghoddousi-Fard, R., Yuan, Y., ... & Gómez-Cama, J. M. (2018). Consistency of seven different GNSS global ionospheric mapping techniques during one solar cycle. Journal of Geodesy, 92(6), 691-706.

Yang, H., Monte-Moreno, E., Hernández-Pajares, M., & Roma-Dollase, D. (2021). Real-time interpolation of global ionospheric maps by means of sparse representation. Journal of Geodesy, 95(6), 1-20.

How to cite: Hernández-Pajares, M., Liu, Q., Yang, H., Monte-Moreno, E., Roma-Dollase, D., and García-Rigo, A.: Real-time determination of the free electrons distribution in the ionosphere: Some motivations, collaborative activities and potential evolution, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-51, https://doi.org/10.5194/iag-comm4-2022-51, 2022.

For many decades the International GNSS Service (IGS) is delivering high-precision ionospheric products for various scientific applications. The Ionosphere Associated Analysis Centers (IAAC), for instance, provide routinely maps of the Vertical Total Electron Content (VTEC), i.e. the integral of the electron density along the height to correct measurements for ionospheric influences, usually disseminated with latencies of days to weeks and based on post-processed observations. Precise GNSS applications, however, such as autonomous driving or precision farming, require the use of high-precision and high-resolution ionospheric correction models in real-time. Hence, the Joint Working Group 4.3.1 “Real-time Ionosphere Monitoring and Modeling” was launched together with IGS and Global Geodetic Observing System (GGOS) within IAG sub-commission 4.3.

The structure and objectives of IAG JWG 4.3.1 during 2019-2023 are described first. Progress of the JWG during 2019-2022, are then presented in detail, which can be summarized as follows:

• Discussion of different time scales for designing / delivering different types of IGS ionospheric State Space Representation (SSR) messages.
• Finalization of global ionosphere corrections in IGS RT-WG proposed SSR data format, which had been released in early October 2020 (Phase 1).
• Continue working on the IGS experimental RT-combination of CAS, CNES, UPC and WHU RT-GIMs, and routine generation of IGS RT-GIMs from two ACs.
• Discussion on delivering ionospheric corrections using alternative base functions (e.g. B-Splines) together with accuracy information or quality indicator.
• Using DORIS near-real-time data to validate the quality real-time GNSS generated ionospheric models.
• Working with IRI-WG on the assimilation of IGS RT-GIM into IRI model.

Finally, the next-step work of JWG 4.3.1 in the generation, comparison, combination and dissemination of experimental two- and/or three-dimensional ionospheric information in support of real-time ionospheric monitoring and associated scientific applications are given.

How to cite: Li, Z. and the IAG 4.3.1 Working Group Member: IAG JWG 4.3.1 Real-time Ionosphere Monitoring and Modeling: Status during 2019-2022, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-12, https://doi.org/10.5194/iag-comm4-2022-12, 2022.

Benefiting from the global multi-frequency and multi-constellation GNSS measurements provided by the International GNSS Real-Time Service (IGS-RTS), real-time global ionospheric maps (RT-GIMs) have been provided by several ionospheric analysis centers of the International GNSS Service (IGS) since 2017. Considering the potential unstable ionospheric streams from individual analysis centers in real applications, we propose a sliding window based differential slant total electron content (dSTEC) weighting technique for the combination of Real-Time Global Ionospheric Maps (RT-GIMs). The combined RT-GIMs are generated using real-time ionospheric streams from the Chinese Academy of Sciences (CAS), Centre National d’Etudes Spatiales (CNES), Polytechnic University of Catalonia (UPC) and Wuhan University (WHU). The performance of combined RT-GIMs is validated in both ionospheric correction and positioning domains. The evaluation in the ionospheric correction domain is performed by comparison with the IGS final combined GIM, i.e. the IGS-GIM, as well as the independent ionospheric TEC observables derived from the Jason-3 altimetry and near-real-time DORIS observation data. The positioning performance of combined RT-GIMs is also evaluated in ionospheric corrected single-frequency standard point positioning (SF-SPP) and ionospheric constricted single-frequency precise point positioning (SF-PPP), by analyzing the 95% quantile of positioning residuals. Compared to IGS-GIM corrected results, the positioning accuracy of our combined RT-GIMs decreases by 5.1% and 6.8% in SF-SPP and SF-PPP analysis, respectively. When compared to BDGIM (the BDS-3 global broadcast ionospheric model) corrected results, the positioning accuracy of combined RT-GIMs increases by 17.2% in SF-SPP and 9.8% in SF-PPP, respectively. CAS combined RT-GIM streams are transmitted in both RTCM-SSR (IONO01IGS0) and IGS-SSR (IONO01IGS1) standards, which are accessible from the IGS (products.igs-ip.net:2101) casters since January 2022.

How to cite: Wang, N., Li, Z., Li, A., and Liu, A.: The Combined Real-Time Global Ionospheric Maps for Precise GNSS Applications, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-11, https://doi.org/10.5194/iag-comm4-2022-11, 2022.

A new ionospheric model called NTCM G (Neustrelitz TEC Model) is now available on the European GNSS Service Centre (GSC) website (https://www.gsc-europa.eu/news/) for correcting ionospheric delay of single-frequency Galileo signals. Investigation shows that NTCM G requires less computing resources compared to the existing reference NeQuick G model while providing a good performance to single-frequency Galileo users. NTCM G does not replace the NeQuick G, but offers an alternative with reduced computational load and complexity for certain users. The NTCM G model was developed by the German Aerospace Centre (DLR) and validated by the Joint Research Centre (JRC) of the European Commission with the support of the European Space Agency (ESA). The description of the source code and its implementation were carried out jointly between DLR and JRC, with the European Union Agency for the Space Programme (EUSPA) and ESA supporting the review and the publication of the model description. The reference implementation of the core algorithm and the auxiliary functions are available for download on the GSC website. The software package provides a portable and validated C/C++ implementation (Matlab and Simulink implementations are also available), including testing functions and testing vectors. NTCM G uses the same Az coefficients broadcast for NeQuick G by the Galileo space segment. Since the Az coefficients are computed in the Galileo ground segment a few hours before the NTCM G works as an ionosphere prediction model for its users.

How to cite: Hoque, M., Sgammini, M., Menzione, F., Perez, R. O., and Chatre, E.: Good performance, less computation: A new ionospheric model for the Galileo Open Service, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-29, https://doi.org/10.5194/iag-comm4-2022-29, 2022.

Ionosphere plays an important role in radio communication, positioning and navigation as well as in various Earth observation techniques based on electromagnetic wave propagation. Thus, modeling, monitoring and forecasting of the ionosphere has been a rather active up-to-date research topic and various models have been proposed by different researchers. The availability of globally distributed dual frequency GNSS observations and ionosphere products delivered by the IGS and other institutions provided an unprecedented time series data source for developing ionosphere forecasting models. In addition, the availability of software tools for massively parallel numerical algorithms programmable into Graphical Processing Unit hardware have delivered a boosted computation power available to researchers.  In parallel, the application of machine learning and especially deep learning methods not only into the Ionosphere research but also to various research on Earth sciences have increased. In this work, an overview of recent developments in ionosphere forecasting research is presented with a spot on those which use especially machine learning and deep learning techniques. The opportunities and challenges are listed with a classification of different approaches in the literature. An outlook is provided for further research directions in the use of learning techniques for long and short term forecasting of Ionosphere. And finally, a potential interoperability in dissemination and the use of recently developed forecasting models are discussed.

How to cite: Durmaz, M.: Recent Developments in Ionosphere Forecasting: a Machine Learning Perspective, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-50, https://doi.org/10.5194/iag-comm4-2022-50, 2022.

Ionospheric irregularities can be caused by from sun activity variations, which may cause irregularities in electron density within the ionospheric layer and, subsequently, plasma perturbations. Typical examples of these irregularities are ionospheric scintillations. The ionospheric irregularities can cause fluctuations in the signal intensity transmitted from the satellite by reducing the signal-to-noise ratio. In addition, scintillation can lead to extreme fluctuations in the phase of a signal transmitted. Ionospheric irregularities originate destructive effects on radio signals transmitted from global navigation satellite systems (GNSS). This phenomenon can generate fluctuations in the signal intensity transmitted from the satellite by decreasing the signal-to-noise ratio of the transmitted wave. The primary purpose of this research will be to detect, model, and predict ionospheric irregularities using a hybrid machine learning algorithm. In addition, using prediction values obtained from the proposed Hybrid models allow measuring the effect of ionospheric perturbations on GNSS ground-based precise positioning accuracy. This modeling and prediction algorithm can contribute to reducing the error of the ionospheric irregularities for satellite-based communication and navigation systems performance. For this purpose, near the equatorial ionization anomaly (EIA), GNSS ground-based stations in South America, are recommended since ionospheric disturbances most impact these regions. The proposed method can play a precaution role in alerting GNSS users that the observation epoch will be disturbed by ionospheric perturbations, and GNSS users can eliminate error-infected observations from the dataset.

How to cite: Atabati, A., Jazireeyan, I., Alizadeh, M., Flury, J., Pirooznia, M., and Schuh, H.: Prediction of Ionospheric Irregularities using a Combination of Machine Learning Algorithms, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-42, https://doi.org/10.5194/iag-comm4-2022-42, 2022.

In this study, a multi-station and multi-instrument system, developed for ionospheric scintillation and equatorial spread-F (ESF) specification in the Taiwan-Philippines sector, is outlined. The issues related to the scintillation and ESF event observed on Oct. 26, 2021, and at magnetic quiet conditions are presented and discussed. We first indicate the existence of a plasma bubble in the Taiwan-Philippines sector using the FormoSat-7 / Constellation Observing System for Meteorology, Ionosphere and Climate-2 (FS7/COSMIC2) GPS/GLONASS radio occultation (RO) observations. We verify the latitudinal extent of the tracked plasma bubble using the recorded ionograms from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) located at Hualien (23.89°N, 121.55°E, dip latitude 17°N), Taiwan. We further discuss the spatial and temporal variabilities of two-dimensional vertical scintillation index VS₄ maps based on the simultaneous GPS L1-band signal measurements from 133 ground-based receivers located in Taiwan and the surrounding islands. We also operate two high-sampling software-defined GPS receivers and characterize the targeted plasma irregularities by carrying out spectrum analyses of the received signal. As a result, the derived plasma irregularities moved eastward and northward, and smaller irregularity scale higher the spectral index and stronger scintillation intensity at lower latitudes on the aimed irregularity feature.

How to cite: Tsai, L.-C., Su, S.-Y., Lv, J.-X., and Liu, C.-H.: GNSS and HF radar measurements for detecting F-region irregularities in the Taiwan-Philippines sector, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-18, https://doi.org/10.5194/iag-comm4-2022-18, 2022.

Trans-ionospheric radio signals of global navigation satellite systems (GNSS) like GPS, GLONASS, GALILEO and BeiDou may suffer from rapid and intensive fluctuations of their amplitude and phase caused by small-scale irregularities of the ionospheric plasma. Such disturbances occur frequently in the equatorial region during the evening hours due to plasma flow inversion or during geomagnetic storms in the polar region. This phenomenon is called radio scintillation and can strongly disturb or even disrupt the signal transmission. We like to present an overview of the latest results as well as present and future work with respect to monitoring, modelling and forecasting of small-scale ionospheric irregularities. We will also discuss the scintillation impact on GNSS and possible mitigation measures. The work presented are results from the Working Group 4.3.5 of IAG Sub-Commission 4.3.

How to cite: Berdermann, J.: Small-scale ionospheric irregularities and their effects on GNSS - A status Report, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-19, https://doi.org/10.5194/iag-comm4-2022-19, 2022.

Natural hazards, that occur in the lower part of the Earth's atmosphere, the troposphere, are affecting higher atmosphere layers. Many of these phenomena, such as thunderstorms, heavy rainfalls, tsunamis, or volcanic eruptions create acoustic waves (AWs), and internal gravity waves (IGWs) in the atmosphere. These waves considerably affect the propagation of radio waves traveling through the ionosphere via transferring energy to this layer, causing significant variations in its parameters.

In this paper, IGWs generated by thunderstorms and tsunamis are investigated. For this task, we used double-frequency measurements from the Global Navigation Satellite Systems (GNSS) along with radio occultation (RO) measurements from Low Earth Orbiting (LEO) satellites, such as Formosat3/ COSMIC (F3/C) and SWARM.

The average increase of ionospheric irregularity amplitudes under severe thunderstorm conditions was 30% compared to calm conditions (non-lightning days). The Rate of TEC Index (ROTI) on high thunderstorm days showed an average increase of 25% within the lower ionosphere. In addition, a significant increase in amplitude and activity of IGWs and IAWs during the thunderstorms was observed.

In the case of tsunamis, the critical frequency of the F2 layer (foF2) showed clear disturbances at the GNSS ground stations due to the tsunami-generated IGWs. During the tsunami, the Ionospheric Electron Density (IED) decreased in altitudes below approximately 300km by 27%. However, above this height, the IEDs increased by 64% up to about 750 km altitude, with a maximum amount of 3.77 × 105 elec/cm3 at 355 km altitude. The average increase of ROTI during the arrival of the tsunami to the stations was 8%.

Keywords: Natural hazards, ionospheric parameters, thunderstorm, tsunami, internal gravity waves, troposphere-ionosphere vertical coupling.

How to cite: Alizadeh, M., Foroodi, Z., Rahmani, Y., and Schuh, H.: Natural hazard effects on ionospheric parameters detected by various space geodetic techniques, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-35, https://doi.org/10.5194/iag-comm4-2022-35, 2022.

The characteristics of equatorial plasma bubbles, which occur during the nighttime and along the course of the geomagnetic equator in the F-region of the ionosphere, are investigated using space-based GPS radio occultation measurements from the FormoSat-3/COSMIC mission. These observations provide unique altitudinal resolution on a global scale. On the other hand, the influence of pre-reversal enhancement (PRE) and gravity wave activity on the plasma bubble characteristics are studied on a long-term basis using the International Reference Ionosphere (IRI) model and TIMED/SABER measurements, respectively.

The occurrence and altitudinal characteristics of plasma bubbles are investigated in four regions, i.e., America, Africa, Asia, and the Pacific. Both the attributes of plasma bubbles are compared with the PRE and gravity waves to understand the preconditioning as well as the uplift of plasma bubbles. A brief statistical study indicated a strong positive correlation between the occurrence rate of plasma bubbles and PRE, mainly in the American sector, followed by the African, Asian, and Pacific sectors. In addition, a positive correlation was detected between the PRE and plasma bubble altitudes during the solar maximum period. However, a negative correlation was observed during the solar minimum period. Furthermore, gravity wave activity was investigated to understand the influence on plasma bubble altitudes during the solar minimum period. Interestingly, the gravity wave activity seemed to have a significant influence on plasma bubble altitudes during the solar minimum years, while, the PRE controlled them during solar maximum years across different regions during different seasons.

How to cite: Kepkar, A., Arras, C., Wickert, J., Schmidt, T., Alizadeh, M., and Schuh, H.: Effects of pre-reversal enhancements and gravity wave activity on the characteristics of equatorial plasma bubbles, 2nd Symposium of IAG Commission 4 “Positioning and Applications”, Potsdam, Germany, 5–8 Sep 2022, iag-comm4-2022-22, https://doi.org/10.5194/iag-comm4-2022-22, 2022.