Sub-Commission 4.3

Atmospheric Remote Sensing

Chair: Michael Schmidt (DGFI-TUM, Munich, Germany,
Vice-Chair: Ehsan Forootan (Cardiff University, Wales)
Secretary: TBD

Terms of Reference

The Earth’s atmosphere can be structured into various layers depending on physical parameters such as temperature or charge state. From the geodetic point of view the atmosphere is nowadays not only seen as a disturbing quantity which has to be corrected but also as a target quantity, since almost all geodetic measurement techniques, such as GNSS, satellite altimetry, VLBI, SLR, DORIS and radio occultations, provide valuable information about the state of and the dynamics within the atmosphere. One up-to-date challenge is to combine all these data efficiently to extract as much information as possible. Space weather events, gravity waves, natural hazards, climate change and autonomous driving are a few modern catchwords in this context.
For many decades the International GNSS Service (IGS) is delivering high-precision tropospheric and ionospheric, i.e. atmospheric products. 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 and final orbits to the user. Precise GNSS applications, however, such as autonomous driving or precision farming, require the use of high-precision and high-resolution atmospheric correction models in real-time. Thus, real-time modelling of the atmosphere is one of the key tasks of the SC 4.3.

Space weather and especially its impacts and risk are gaining more and more importance in politics and sciences, since our modern society is highly depending on space-borne techniques, e.g., for communication, navigation and positioning. Coupling processes between different atmospheric layers and interrelations with climate change are other up-to-date topics. Besides sounding the atmosphere and studying space weather effects by modern evaluation methods, the promising GNSS reflectometry technique (GNSS-R) is another research topic within the SC 4.3.
The SC 4.3 focuses on a coordinated research in understanding processes within and between the different atmospheric layers using space-geodetic measurements and observations from other branches such as astrophysics or heliophysics. Furthermore, it is concentrated on developing new strategies, e.g., for prediction and detection of small and medium atmospheric structures.


• Studying and solving problems of atmosphere research for up-to date applications such as autonomous driving.
• Bridging the gaps between modern geodetic observation techniques such as radio occultations or GNSS reflectometry, measurements from other scientific branches such as astrophysics and geophysics with the geodetic community
• Exploration of the synergies between geodesy and other scientific branches such as astrophysics and geophysics
• Investigation of ionosphere phenomena such currents or scintillations
• Support of atmosphere prediction models based on the combination of data from different observation techniques, e.g. by developing sophisticated estimation procedures
• Improvement of precise positioning and navigation on the basis of new atmosphere models
• Development of real- and near real-time techniques for atmosphere monitoring

Program of activities

• To promote research collaboration among groups from geodesy and other branches worldwide dealing with atmosphere research and applications
• To organize and/or participate in scientific and professional meetings (workshops, conference sessions, etc.)
• To maintain a web page concatenating the Sub-Commission activities and reports
• To encourage special issues, e.g. of Journal of Geodesy, on research, applications, and activities related to the topics of this Sub-Commission
• Close co-operations with other elements of the IAG structure, such as IAG Commission 1, the ICCT, the ICCC and GGOS.

Overview about Joint Study Groups (JSG), Working Groups (WG) and Joint Working Groups (JWG) of or related to the SC 4.3

• JWG 4.3.1 Real-time ionosphere monitoring and modelling (joint with IGS and GGOS)
• WG 4.3.2 Prediction of ionospheric state and dynamics
• WG 4.3.3 Ionosphere scintillations
• JWG 4.3.4 Validation of VTEC models for high-precision and high resolution applications (joint with IGS, GGOS and Comm. 1)
• WG 4.3.5 Sensing small-scale structures in the lower atmosphere with tomographic principles
• WG 4.3.6 Real-time troposphere monitoring
• WG 4.3.7 Geodetic GNSS-R

• JSG from other leading commissions joint with Commission 4, SC 4.3:

o JSG 1: Coupling processes between magnetosphere, thermosphere and ionosphere (IAG ICCT, joint with GGOS Focus Area on Geodetic Space Weather Research)

• JWGs from other leading commissions joint with Commission 4, SC 4.3:

o JWG 1: Electron density modelling (GGOS Focus Area on Geodetic Space Weather Research) o JWG 2: Improvement of thermosphere models (GGOS Focus Area on Geodetic Space Weather Research, joint with ICCC)
o JWG 3: Improved understanding of space weather events and their monitoring by satellite missions (GGOS Focus Area on Geodetic Space Weather Research);
o JWG 1.x.x: Intra- and inter-technique atmospheric ties (Comm. 1)
o JWG x: Zenith Total Delays and gradients quality control for climate (ICCC, joint with Comm. 4)

JWG 4.3.1 Real-time ionosphere monitoring and modelling
(Joint with IGS and GGOS)

Chair: Zishen Li (China)
Vice-Chair: Ningbo Wang (China)

Terms of Reference (ToR)/Description:
The WG focuses on the methodology development for real-time ionosphere monitoring and modelling with the use of multiple space-geodetic observations, in particular the inclusion of ionospheric observables from the new GNSS constellations (e.g. Galileo and BeiDou), altimetry, DORIS and radio occultation (RO) techniques aside from the ground-based GPS/ionosonde measurements. The approaches for the retrieval of precise ionospheric parameters (e.g. total electron content), generation of two- and/or three-dimensional ionosphere maps on regional and global scales, independent validation and potential combination of ionospheric information from different providers in real-time will be developed and analysed. The dissemination of real-time ionospheric information following RTCM, 5G or other standards will be discussed in support of both scientific researches and technological applications. The connections to IGS, iGMAS, IDS, IRI, NeQuick and other communities will also be involved for the possible establishment of joint WGs or experimental campaigns on real-time ionosphere monitoring.
Within the next four years we will focus on:
• close connections to different scientific communities (e.g. IGS, iGMAS and IDS) for the coordination of RT/NRT ground- and space-based measurements in support of real-time ionosphere monitoring.
• comparison of different approaches for real-time ionosphere modelling by the combination of multi-constellation observations with distinct characteristics (e.g. biases and time latencies).
• development of methods for the independent validation and potential combination of real-time ionospheric information from different providers.
• discussions on the distribution of real-time ionospheric information within scientific and technological communities (e.g. target parameter, data format and time latency).
• generation and dissemination of experimental two- and/or three-dimensional ionospheric information in support of real-time ionosphere monitoring and associated scientific applications.
• possible joint sub working groups or experimental campaigns on real-time ionosphere monitoring in close scientific collaboration with IGS and IDS WGs, COSPAR IRI and NeQuick WGs, among others.

WG 4.3.2 Prediction of ionospheric state and dynamics
Chair: Mainul Hoque (Germany,
Vice-Chair: Eren Erdogan (Germany,

Terms of Reference (ToR)/Description:
Ionospheric disturbances can affect technologies in space and on Earth disrupting satellite and airline operations, communications networks, navigation systems. As the world becomes ever more dependent on these technologies, ionospheric disturbances as part of space weather pose an increasing risk to the economic vitality and national security. Having the knowledge of ionospheric state in advance during space weather events is becoming more and more important.
With the modernization of Global Navigation Satellite Systems (GNSS), the use of multi-constellation, multi-frequency observations including new signals enables continuous monitoring of the Earth’s ionosphere using worldwide distributed sensor stations. Other ground based techniques such as vertical sounding (VS), Incoherent Scatter Radar (ISR), Very Low Frequency (VLF) or Radio Beacon (RB) measurements provide complementary ionospheric observations. The radio occultation (RO) technique provides one of the most effective space-based methods for exploring planetary atmospheres. The availability of numerous medium Earth orbit satellites deployed by GPS, GLONASS, Galileo, BeiDou navigation systems allows continuous monitoring of the Earth’s ionosphere and neutral atmosphere by tracking GNSS signals from low Earth orbiting (LEO) satellites. Other space-based techniques include ionosphere estimation using dual-frequency altimeter data (e.g. TOPEX-Poseidon, Jason 2 & 3 missions), using radio beacon measurements from DORIS (geodetic orbit determination and positioning system) receivers onboard LEO satellites and GNSS reflectometry.
The availability of ionospheric data from different sensors has increased in many folds during the last decades. In one hand the ionosphere has been sounded by a large number of sensors providing a vast database. On the other hand, the accuracy of the measuring techniques has been improved significantly. As an example, the IGS is routinely generating ionospheric total electron content (TEC) maps from GNSS data since 1998. The inter-dependency of different space weather parameters (e.g., TEC, peak electron density and height, solar flux, geomagnetic indices, interplanetary magnetic field components etc.) paves the way for determining ionospheric prediction algorithm. With the availability of fast computing machines as well as the advancement of the machine learning techniques and Big Data algorithms, the prediction of ionospheric state and dynamics is possible in near real time.
Within the next four years we will focus on:
• study the inter-dependency of different space weather parameters (e.g., TEC, solar flux, geomagnetic indices, interplanetary magnetic field components etc.) during quiet and perturbed conditions, trend analysis and algorithm development for predicting space weather parameters
• develop global as well as regional prediction approaches considering that the high latitude phenomena/processes are different from the low latitude phenomena/processes and hemispherical asymmetry (e.g., South Atlantic Anomaly)
• modelling the phenomena which are closely connected to nighttime filling of the ionosphere such as the Nighttime Winter Anomaly (NWA), Weddell Sea Anomaly (WSA) and the Okhotsk Sea Anomaly (OSA). It is assumed that the midsummer nighttime anomaly (MSNA) and related special anomalies such as the WSA and the OSA are closely related to the NWA via enhanced wind-induced uplifting of the ionosphere.

WG 4.3.3 Ionosphere scintillations
Chair: Jens Berdermann Germany;
Vice-Chair: Lung-Chih Tsai (Taiwan;

Terms of Reference (ToR)/Description:
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. The main effects of scintillation on trans-ionospheric radio system are signal loss and phase cycle slips, causing difficulties in the signal lock of receivers. All GNSS signals are affected, but the influence of the small scale irregularities is expected to differ since the signals are transmitted by different carrier frequencies and are constructed in different ways. Furthermore, the sensitivity of receivers in respect to scintillation events differ between various GNSS receiver types and an advanced analysis using “bitgrabber” systems are needed to rate their vulnerability.
In spite of the importance of irregular density variations for the science of the ionosphere and for space weather operations, no fully sufficient global model for such disturbances is available.
Within the next four years we will focus on:
• understanding the climatology of ionospheric scintillations, namely, its variation with latitude, season, local time, magnetic activity and solar cycle,
• investigation of the GNSS signal frequency and receiver impact on signal loss and phase cycle slips during scintillation events
• Global modelling and forecasting of scintillations taking into account temporal and regional (Polar and Equatorial region) differences.

JWG 4.3.4 Validation of VTEC models for high-precision and high resolution applications
(Joint with IGS)

Chair: Anna Krypiak-Gregorczyk (Poland,
Vice-Chair: TBD

Terms of Reference (ToR)/Description:
Global and regional VTEC models are routinely used in Space Weather studies, but also in high-precision applications like e.g. GNSS positioning. There are currently many analysis centers and research groups providing operational and test VTEC maps. Indeed, the global ionospheric maps (GIMs) are being systematically produced and openly provided by the IGS Ionosphere Working Group (IIWG) since 1 June 1998. IGS GIMs are developed as an official product of IIWG by performing a weighted mean of the various Analysis Centers (AC) VTEC maps. There are also important empirical models like the International Reference Ionosphere (IRI) or NeQuick that are based on statistical analysis of the results of measurements.
However, IGS ACs and other groups use different mathematical models and estimation techniques resulting different resolutions, accuracies and time delays of their products. Therefore, there is a need to compare and validate existing VTEC models in order to better understand their performance and quality, and hence to better understand the ionosphere and foster VTEC models usage in geosciences community.
Within the next four years we will focus on:
• close connections to different scientific communities – primarily to IGS and GGOS
• comparison of GNSS-derived VTEC maps and empirical models
• VTEC validation with external data, such as altimetry, DORIS, Swarm, radio occultation (RO) and ground-based ionosonde measurements
• VTEC validation is precise GNSS positioning
• development of new validation techniques

WG 4.3.5 Real-time Troposphere Monitoring
Chair: Cuixian Lu (Wuhan University,
Vice-Chair: Galina Dick (Germany,

Terms of Reference (ToR)/Description:
The main objective of this WG is to develop, optimize and assess new real-time or ultra-fast GNSS tropospheric products, and exploit the full potential of multi-GNSS observations in weather forecasting. Tropospheric zenith total delays, tropospheric linear horizontal gradients, slant delays, integrated water vapour (IWV) maps or other derived products in sub-hourly fashion are foreseen for future exploitation in numerical and non-numerical weather nowcasting or severe weather event monitoring.
The use of Precise Point Positioning (PPP) processing strategy will play a key role in developing new products because it is an efficient and autonomous method, it is sensitive to absolute tropospheric path delays, it can effectively support real-time or ultra-fast production, it may optimally exploit data from all GNSS multi-constellations, it can easily produce a full variety of parameters such as zenith total delays, horizontal gradients or slant path delays and it may also support as reasonable as high temporal resolution of all the parameters. Last, but not least, the PPP is supported with the global orbit and clock products provided by the real-time service of the International GNSS Service (IGS).

Within the next four years we will focus on:
• development of real-time multi-GNSS processing algorithms and strategies for high-resolution, rapid-update NWP and nowcasting applications.
• development of new/enhanced GNSS tropospheric products and exploit the full potential of multi-GNSS (GPS, GLONASS, Galileo and BeiDou) observations for use in the forecasting of severe weather.
• development and validate methods for initialization of NWP models using new/enhanced operational multi-GNSS tropospheric products and for use in nowcasting.
• assessing the benefit of new/enhanced GNSS products (real-time, gradients, slants…) for numerical and non-numerical nowcasting.
• stimulate the development of application software for supporting routine production.
• demonstrate real-time/ultra-fast production, assess applied methods, software and precise orbit and clock products.
• setup a link to the potential users, review product format and requirements.

WG 4.3.6 Sensing small-scale structures in the lower atmosphere with tomographic principles
Chair: Gregor Moeller (Switzerland, ETH Zurich, Mathematical and Physical
Vice-Chair: Chi Ao (USA, NASA,

Terms of Reference (ToR)/Description:
The working group on troposphere tomography intends to bring together researchers and professionals working on tomography-based concepts for sensing the neutral atmosphere with space-geodetic and complementary observation techniques, sensitive to the water vapour distribution in the lower atmosphere.
While geodetic GNSS networks are nowadays the backbone for troposphere tomography studies, further local densifications, e.g. at airports or cities are necessary to achieve very fine spatial and temporal resolution. Besides, InSAR interferograms, GNSS radio occultation or microwave radiometer profiles are a valuable asset, which can provide the necessary complementary information for stabilizing the tomography system. Furthermore, in the next decade CubeSat missions are expected, which are designed for tomography processing. These constellations can operate independently from ground-based networks and due to a more favourable observation geometry, will allow for sensing globally the water vapour distribution in the neutral atmosphere with increased spatial resolution.
Within the next study period (2019-2023), the working group on troposphere tomography intends to address current challenges in tropospheric tomography with focus on space-based measurements using tomography principles. Hereby, the main objectives are:
• Evaluating approaches for the densification of existing dual-frequency geodetic networks;
• Working towards a dynamical tomography model – adaptable to varying input data (continuous-time image reconstruction, trade-off between model resolution and variance size);
• Setting up a benchmark campaign for the combination of ground-based GNSS with radio occultation and other observation techniques like InSAR;
• Assessing existing ray-tracing approaches for the reconstruction of space-based observations;
• Working on standards for data exchange (SINEX TRO 2.0 or other formats).

WG 4.3.7 Geodetic GNSS-R
Chair: Sajad Tabibi (Luxembourg,
Vice-Chair: Felipe Nievinski (Brazil,

Terms of Reference (ToR)/Description:
The radio waves broadcast by Global Navigation Satellite Systems (GNSS) satellites have been used for unanticipated purposes, such as remote sensing of the environment. The most prominent example for a novel application from recent years is the usage of reflected GNSS signals as a new tool for remote sensing. GNSS Reflectometry (GNSS-R) has been used to exploit signals of opportunity at L-band for ground-based sea and lake level studies at several locations in the last few years. Although geodetic-quality antennas are designed to boost the direct transmission from the satellite and to suppress indirect surface reflections, the delay of reflections with respect to the line-of-sight propagation can be used to estimate the water-surface level in a stable terrestrial reference frame. GNSS-R has started to make an impact in the disciplines of geodesy and remote sensing, with diverse applications such as sea-level, snow depth, and soil moisture monitoring, observations are highly relevant to the goals of the Global Geodetic Observing System (GGOS). Thus, the overall aim of this working group is to further demonstrate and consolidate the value of GNSS-R for the geodesy, oceanography, cryosphere, and hydrology communities.
Within the next four years we will focus on:
• identification of GNSS-R products which have a strong relation to IAG services and goals.
• maintain interactions with neighboring societies (such as the IEEE Geoscience and Remote Sensing Society, GRSS) and cooperate with technological, engineering, and operational entities related to GNSS (e.g., the International GNSS Service, IGS), identifying common goals and detecting potential synergies.
• organization of working meetings with GNSS-R experts, while also inviting stakeholders from the geodetic community to participate in such events.
• maintain the GNSS-R site guidelines for installing multi-purpose GNSS stations
• maintain the inventory of GNSS stations used for reflectometry purposes, currently available for sea-level applications, possibly extending it to other applications as well.
• organization of a near-operational demonstration project on GNSS-R for coastal sea level monitoring.
• organization of algorithm inter-comparison exercises. These can be based on either synthetic data or field measured GNSS data. Validation will be based, respectively, on the simulation configuration or independent in situ data (e.g., tide gauges for sea level applications). The treatment of external corrections, such as atmospheric effects, should be considered. Challenging conditions should also be addressed, such as large tidal range (~ 4-5 m) and the impact of multi-GNSS revisit time on tidal constituent

Overview about Joint Study Groups (JSG), Working Groups (WG) and Joint Working Groups (JWG) led by other IAG components (Commissions, ICCT, ICCC, GGOS)

JWG 1.x.x: Intra- and Inter-Technique Atmospheric Ties
(Led by Commission 1 Reference Frames, joint with Commission 4, Sub-commission 4.3)
Chair: Kyriakos Balidakis (Germany,
Vice-Chair: Daniela Taller (Germany)

Terms of Reference (ToR)/Description:
The differences between atmospheric parameters (mainly zenith delays and gradients) at co-located stations that observe nearly simultaneously, and stem from external systems (e.g., meteorological sensors or weather models) are understood as atmospheric ties. Atmospheric ties mainly exist because of differences in (i) the observing frequency, (ii) the relative position, and (iii) the observing system set-up.
The acquisition of accurate atmospheric delay corrections is of paramount importance for mm-level positioning employing space geodetic techniques. Atmospheric delay corrections may stem from dedicated instruments such as water vapor radiometers, meteorological sensors, numerical weather models, or from the geodetic data itself. While the latter is fairly common for modern GNSS and VLBI, observation geometry and accuracy limitations inherent to other systems such as SLR and DORIS impede the accurate atmospheric parameter estimation, thus hindering among else positioning. To this end, it might be useful to compare and combine atmospheric parameters at co-located sites, in a manner similar to the combination of station and satellite coordinates, as well as Earth rotation parameters (via local, space, and global ties, respectively). The multi-technique combination is indispensable to the distinction between real signals and undesired technique-specific artefacts. Nowadays, the multi- technique combination is facilitated by the increasing investments in state-of-the-art geodetic infrastructure at co-located sites. However, a host of systematic and random errors render the combination via atmospheric ties a difficult task. Moreover, since atmospheric delays are dependent upon essential climate variables (pressure, temperature, and water vapor), differences in long-term atmospheric delay time derivatives at co-located stations might offer an insight into local climate change.
The purpose of this working group is to answer the questions:
• How can one relate atmospheric parameter estimates and the time derivatives thereof that refer to different place, time, and observing system? What are the limits in distance, time lag, and observing system?
• What is the optimal way to combine atmospheric parameters?
• Which are the risks from including atmospheric ties in a multi-technique terrestrial reference frame combination?
Specific program activities
Comparison of atmospheric delay estimates from single-technique geodetic analysis (GNSS, SLR, VLBI, and DORIS) Comparison of atmospheric delays from state-of-the-art meso-beta scale weather models (e.g., ERA5 and MERRA2), and high-resolution WRF runs Assessment of spatial and temporal correlation between atmospheric parameters Assessment of multi-technique combination employing atmospheric ties on the single site and global TRF level.

JWG x: Zenith Total Delays and gradients quality control for climate

(Led by ICCC, joint with Commission 4, Sub-Commission 4.3)
Chair: Rosa Pacione
Vice-Chair: Marcelo Santos

Terms of Reference (ToR)/Description:

JSG 1: Coupling processes between magnetosphere, thermosphere and ionosphere
(Led by ICCT; joint with GGOS, Focus Area on Geodetic Space Weather Research and Commission 4, Sub-Commission 4.3)

Chair: Andres Calabia Aibar (China,
Vice-Chair: TBC

Terms of Reference (ToR)/Description:
Consequences of upper-atmosphere conditions on human activity underscore the necessity to better understand and predict the effects of Magnetosphere-Ionosphere-Thermosphere (MIT) processes and coupling, and prevent from potential detrimental effects on orbiting, aerial, and ground-based technologies. For instance, major concerns include the perturbation of electromagnetic signals passing through the ionosphere for accurate and secure use of Global Navigation Systems (GNSS), and the lack of accurate aerodynamic-drag models required for accurate tracking, decay, and re-entry calculations of Low Earth Orbit (LEO) objects, including manned and unmanned artificial satellites. In addition, ground power grids and electronics of satellites could be influenced, e.g., by the magnetic field generated by sudden changes in the current system due to solar storms. Figure1 illustrates the proposed new structure of the Focus Area on Geodetic Space Weather Research (FA-GSWR) as a double tetrahedron.
Monitoring and predicting the Earth’s upper atmosphere processes driven by solar activity is highly relevant to science, industry and defence. These communities emphasize the need to increment the research efforts for better understanding of the MIT responses to highly variable solar conditions, as well as detrimental space weather effects on our life and society. On the one hand, the electron-density variation produces the perturbation in speed and direction of electromagnetic signals propagated through the ionosphere, and reflects as a time-delay in the arrival of the modulated components from which pseudo-range measurements of Global Navigation Satellite Systems are made, and an advance in the phases of the signal’s carrier waves which affects carrier-phase measurements. On the other hand, aerodynamic-drag associated with neutral-density fluctuations resulting from upper atmospheric expansion/contraction in response to variable solar and geomagnetic activity, increases drag and decelerates Low Earth Orbits, dwindling lifespan of space-assets, and making tracking difficult.
Through the interrelations, dependencies, and coupling patterns between ionosphere, thermosphere, and magnetosphere variability, the JSG 1 aims to improve the understanding of the coupled processes in the MIT system, and considerations of the solar contribution. In addition, tides from the lower atmosphere forcing can feed into the electrodynamics, and have a composition effect leading to changes in the MIT system. In this scheme, our tasks are addressed to exploit the knowledge of the tight MIT coupling by investigating multiple types of magnetosphere, ionosphere, and thermosphere observations. The final outcome will help to enhance the predictive capability of empirical and physics-based models through interrelations, dependencies, and coupled patterns of variability between the essential geodetic variables.
• Characterize and parameterize the global modes of MIT variations associated with diurnal, seasonal, and space weather drivers, as well as the lower atmosphere forcing.
• Determine and parameterize the mechanisms responsible for discrepancies between observables and the present models.
• Detect and investigate coupled processes in the MIT system for the deciphering of physical laws and principles such as continuity, energy and momentum equations and solving partial differential equations.

JWG 1: Electron density modelling
(Led by GGOS; joint with Commission 4, Sub-Commission 4.3)

Chair: Fabricio dos Santos Prol (Germany,
Vice-Chair: Alberto Garcia-Rigo (Spain,

Terms of Reference (ToR)/Description:
The main goal of this group is to disseminate and evaluate established methods of 3D electron density estimation in terms of electron density, peak height, Total Electron Content (TEC), or other derived products that can be effectively used for GNSS positioning or for analyzing perturbed conditions due to representative space weather events. It is planned to generate products, showing the general error given by such 3D electron density estimations and, also, distribute information regarding to space weather conditions. To achieve this main goal, the following objectives are defined.
• Develop a database, where the methods from the group members will be able to be evaluated in terms of GNSS, radio-occultation, DORIS, in-situ data, altimeters, among other electron density and TEC measurements.
• Evaluate established methods for 3D electron density estimation in order to define their accuracy related to specific parameters of great importance for Space Weather and Geodesy.
• Generate products indicating the space weather conditions and expected errors of the methods.
• Carry out surveys in order to detect if the products are linked to the user’s specific needs. Based on an analysis of the user needs, re-adaptations will be identified in order to improve the products in an iterative process. It is planned to define which parameters are of interest for the users and to detect additional information that may be required.

JWG 2: Improvement of thermosphere models
(Led by GGOS; joint with Commission 4, Sub-Commission 4.3)

Chair: Christian Siemes (The Netherlands,
Vice-Chair: Kristin Vielberg (Germany,

Terms of Reference (ToR)/Description:
Mass density, temperature, composition and winds are important state parameters of the thermosphere that affect drag and lift forces on satellites. Since these significantly influence the orbits of space objects flying at altitudes below 700 km, accurate knowledge of the state of the thermosphere is important for applications such as orbit prediction, collision avoidance, evolution of space debris, and mission lifetime predictions. Drag and lift forces can be inferred from space geodetic observations of accelerometers, which complement other positioning techniques such as GNSS, satellite laser ranging or radar tracking of space objects. The objective of the working group is to improve thermosphere models through providing relevant space geodetic observations and increasing consistency between datasets by advancing processing methods. Broadening the observational data basis with geodetic space observations, which are available now for a time span of 20 years, will also benefit climatological studies of the thermosphere.
• Review space geodetic observations and state-of-the-art processing methods
• Advance processing methods to increase consistency between observational datasets
• Improve thermosphere models through providing accurate and consistent space geodetic observations
• Study the impact of improved observational datasets and advanced processing methods on orbit determination and prediction
1. Use of improved thermosphere models and observational data sets to forward the investigation of thermosphere variations in the context of climate change.

JWG 3: Improved understanding of space weather events and their monitoring by satellite missions
Led by GGOS Joint with IAG Commission 4, Sub-Commission 4.3
Chair: Alberto Garcia-Rigo (Spain,
Vice-Chair: Benedikt Soja (USA,
Terms of Reference (ToR)/Description:
Space weather events cause ionospheric disturbances that can be detected and monitored thanks to estimates of the vertical total electron content (VTEC) and the electron density (Ne) of the ionosphere. Various space geodetic observation techniques, in particular GNSS, satellite altimetry, DORIS, radio occultations (RO) and VLBI are capable of determining such ionospheric key parameters. For the monitoring of space weather events, low latency data availability is of great importance, ideally real time, to enable triggering alerts. At present, however, only GNSS is suited for this task. The use of the other techniques is still limited due latencies of hours (altimetry) or even days (RO, DORIS, VLBI).
The JWG 3 will investigate different approaches to monitor space weather events using the data from different space geodetic techniques and, in particular, combinations thereof. Simulations will be beneficial to identify the contribution of different techniques and prepare for the analysis of real data. Different strategies for the combination of data will be investigated.
Furthermore, the geodetic measurements of the ionospheric parameters will be complemented by direct observations of the solar corona, where solar storms originate, as well as of the interplanetary medium. Spacecrafts like SOHO or ACE have monitored the solar corona and the solar wind for decades and will be beneficial, together with data from other spacecrafts like SDO, in assessing the performance of geodetic observations of space weather events. Data from Parker Solar Probe, which will allow even greater insights, has just recently been made publicly available. Geodetic VLBI is also capable of measuring the electron density of the solar corona when observing targets angularly close to the Sun and will be useful for comparisons. Other solar-related satellite missions such as Stereo, DSCOVR, GOES, etc. provide valuable information such as solar radiation, particle precipitation and magnetic field variations. Other indications for solar activity – such as the F10.7 index on solar radio flux, SOLERA as EUV proxy or rate of Global Electron Content (dGEC), will also be investigated. The combination and joint evaluation of these data sets with the measurements of space geodetic observation techniques is still a great challenge. Through these investigations, we will gain a better understanding of space weather events and their effect on Earth’s atmosphere and near-Earth environment.
• Selection of a set of historical representative space weather events to be analysed.
• Determination of key parameters and products affected by the selected space weather events.
• Identification of the main parameters to improve real time determination and the prediction of ionospheric/plasmaspheric VTEC and Ne estimates as well as ionospheric perturbations in case of extreme solar weather conditions.
• Improving the (near) real time determination of the electron density within the ionosphere and plasmasphere to detect space weather events.
• Combination of measurements and estimates derived from space geodetic observation techniques by conducting extensive simulations, combining different data sets and testing different algorithms.
• Comparison and validation using external data, in particular data from spacecraft dedicated to monitoring the solar corona.
• Interpretation of the results. Correlate acquired data/products with space weather events’ impact on geodetic applications (e.g. GNSS positioning, EGNOS performance degradation).