G6.1

The G4S_2.0 (Galileo for Science) project is a new proposal funded by the Italian Space Agency (ASI) and aims to perform a set of measurements in the field of Fundamental Physics with the two Galileo satellites DORESA and MILENA. Indeed, the accurate analysis of the orbits of these satellites — characterized by a relatively high eccentricity of about 0.16 — and of their clocks — the most accurate orbiting the Earth — allows to test relativistic gravity by comparing the predictions of Einstein's theory of General Relativity with those of other theories of gravitation. After a general introduction to the project objectives, we will present the preliminary activities of G4S_2.0 which are being developed by IAPS-INAF in Rome. The results of G4S_2.0 will be particularly useful for the applications of the Galileo FOC satellites in the fields of space geodesy and geophysics as some of these activities will concern the improvement of the precise orbit determination of the satellites through an enhancement of the dynamic model of their orbits, analyzing, in particular, the modelling of non-conservative forces.

How to cite: Lucchesi, D., Fiorenza, E., Lefevre, C., Lucente, M., Magnafico, C., Peron, R., Santoli, F., Sapio, F., and Visco, M.: The Galileo satellites DORESA and MILENA and their goals in the field of Fundamental Physics within the Galileo for Science (G4S_2.0) project, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7873, https://doi.org/10.5194/egusphere-egu21-7873, 2021.

Since mid-2020, various Very Long Baseline Interferometry (VLBI) observation programs organized by the International VLBI Service for Geodesy and Astrometry (IVS) are scheduled using a new algorithm inspired by evolutionary processes based on selection, crossover and mutation. It mimics the biological concept "survival of the fittest" to iteratively explore the scheduling parameter space looking for the best solution.

In this work, we will present the general workflow of the algorithm as well as discuss its strengths and potential weaknesses. Moreover, we will highlight how the improved scheduling affects the precision of geodetic parameters. In the case of difficult-to-schedule OHG sessions, an improvement in the precision of the geodetic parameters of up to 15% could be identified based on Monte-Carlo simulations, as well as an increase in the number of observations of up to 10% compared to classical scheduling approaches.

How to cite: Schartner, M., Plötz, C., and Soja, B.: Improved VLBI scheduling through evolutionary strategies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1250, https://doi.org/10.5194/egusphere-egu21-1250, 2021.

One of he most common problems of the daily surveying/geodetic/cartographic practice is reliable transformation between two geodetic reference systems. There are plenty of well-known transformation’s models (e.g. 3D Helmert transdformation, 2D silimilarity transformaton, etc) applied for this purpose. Transformation scope is to optimally absorb the systematic inconsistencies of geodetic reference systems. In many cases, the pure deterministic approach is not sufficient, as the geodetic reference systems contain systematic effects, that are not successfully eliminated or reduced. Hence, a more sophiscticated methodology should be implemented, in order to enhance transformation’s accuracy. In this paper a case study is presented, including testing of  different deterministic transformation models between the old Greek Datum and the new official 1987 Hellenic Geodetic Reference System The estimated residuals do not fullfill the present needs of accuracy, thus we further implement some specified prediction models. The final outcome reads an improvement of the transformation between these two geodetic reference systems.

How to cite: Perperidou, D.-G., Moschopoulos, G., Ampatzidis, D., Mouratidis, A., and Tsimerikas, A.: The role of the prediction stategies for the reliable transformation beetween two geodetic reference systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11095, https://doi.org/10.5194/egusphere-egu21-11095, 2021.

The new data release of the Gaia satellite operated by the European Space Agency recently published its 3rd data release (Early Data Release 3, EDR3). The dataset contains astrometric data of about 1.8 billion objects detected at optical frequencies and therefore it outperforms any catalog of astrometric information up to date. The reference frame defined by Gaia EDR3 is aligned to the International Celestial Reference System by referring to counterparts in its realization, the third International Celestial Reference Frame (ICRF3), which is calculated from very long baseline interferometry (VLBI) observations of extragalactic objects at radio frequencies.
The Gaia dataset is known to be magnitude-dependent in terms of astrometric calibration. As the objects in ICRF3, although bright at radio frequencies, are mostly faint at optical frequencies, the optically bright Gaia frame has to be linked to ICRF3 by additional counterparts besides objects in ICRF3. The non-rotation of the optically bright Gaia frame is especially important as optically bright objects can, besides astrophysical studies, be used for navigation in space, where other geodetic systems like global navigation satellite systems are out of reach. Suitable additional counterparts are radio stars which are observed by VLBI relative to extragalactic objects in ICRF3. We discuss the orientation and spin differences between the optically bright Gaia EDR3 and VLBI data of radio stars and their impact on the Gaia data usage.

How to cite: Lunz, S., Anderson, J., Xu, M. H., Heinkelmann, R., Titov, O., Johnson, M., and Schuh, H.: Linking the optically bright Gaia frame to the third International Celestial Reference Frame, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8604, https://doi.org/10.5194/egusphere-egu21-8604, 2021.

The Gravity Recovery Object Oriented Programming System (GROOPS) is a software package written in C++ that enables the user to perform core geodetic tasks. The software features gravity field recovery from satellite and terrestrial data, the determination of low-earth-orbiting satellite orbits from global navigation satellite system (GNSS) measurements, and the computation of GNSS constellations and ground station networks. For an easy and intuitive setup of complex workflows, GROOPS contains a graphical user interface to create and edit configuration files. The source code of GROOPS is released under the GPL v3 license and is available on GitHub (https://github.com/groops-devs/groops) together with documentation, a cookbook with guided examples, and installation instructions for different platforms. In this contribution we give a software overview and present results of different applications and data sets computed with GROOPS.

How to cite: Kvas, A., Behzadpour, S., Eicker, A., Ellmer, M., Koch, B., Krauss, S., Pock, C., Rieser, D., Strasser, S., Suesser-Rechberger, B., Zehentner, N., and Mayer-Guerr, T.: GROOPS: An open-source software package for GNSS processing and gravity field recovery, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10574, https://doi.org/10.5194/egusphere-egu21-10574, 2021.

In modelling of atmospheric loading effects in terrestrial gravimetry by numerical weather models, often the Inverse Barometer (IB) hypothesis is applied over oceans. This simple assumption implies an isostatic compensation of the oceans to atmospheric pressure changes, causing no net deformation of the seafloor. However, the IB hypothesis is in general not valid for periods shorter than a few weeks and, consequently, the ocean dynamics cannot be neglected. In particular, for the correction of high precision gravity time series as e.g. obtained from superconducting gravimeters, it is essential to model even small contributions in order to separate different effects. When including non-tidal ocean loading effects from ocean circulation models into atmospheric models, special care has to be taken of the interface between the atmosphere and the oceans in order not to account contributions twice.

The established approach for modelling non-tidal ocean loading effects is revised in this study. When combining it with the modelling of atmospheric effects for terrestrial gravimetry, it is shown that Newtonian attraction contributions from the atmosphere may be accounted twice. To solve this problem, an alternative is proposed and tested which further reduces the variability of the gravity residuals, as shown for a set of four superconducting gravity meters globally distributed.

The improvement is achieved by a different treatment of the Newtonian attraction component related to the IB effect. Discrepancies up to the μGal level are demonstrated, depending on the location of the station. With several simplifications, the approach can be made operational and included in existing services, further improving the compatibility of terrestrial gravity time series with satellite gravity observations.

How to cite: Antokoletz, E. D., Wziontek, H., Dobslaw, H., and Tocho, C. N.: Revisiting non-tidal ocean loading corrections for high precision terrestrial gravimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5308, https://doi.org/10.5194/egusphere-egu21-5308, 2021.

Surface mass anomalies on Earth modify the external gravity field via both Newtonian attraction and elastic deformation of the underlying crust. Time-variable mass transport divergence leading to quickly changing surface mass distributions induces additional horizontal pressure gradients that feed back into the dynamics of the transport process. In view of the present-day accuracy of geodetic observations, this feedback is well known to be important for global ocean tide modelling (Ray, 1998). The same feedback, however, is also affecting the barotropic response of the global oceans to surface wind stress and atmospheric pressure loading. It is typically termed as "Self Attraction and Loading" and can be seen as one contribution to sea-level variability induced by "Gravity, Rotation, and Deformation (GRD)" as defined by Gregory et al. (2019).

In this presentation, we will specifically discuss the contribution to sea-level variability induced by surface pressure variations over the continents, which are by now often ignored in numerical ocean modelling. Induced ocean bottom pressure signals are specifically prominent at the shortest periods between hours and days, and frequently exceed 1 hPa in coastal regions. The signals are found to be relevant for the satellite gravimetry missions GRACE and GRACE-FO, and the process will be therefore included in the next release of the AOD1B non-tidal de-aliasing product.  

How to cite: Dobslaw, H., Shihora, L., and Sulzbach, R.: High-Frequency Sea-Level Variations driven by Self-Attraction and Loading from Atmospheric Surface Pressure at the Continents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12970, https://doi.org/10.5194/egusphere-egu21-12970, 2021.

Satellite altimetry provides measurements of sea surface height of centimeter-level accuracy over open oceans. However, its accuracy reduces when approaching the coastal areas and over land regions. Despite this downside, altimetric measurements are still applied successfully in these areas through altimeter retracking processes. This study aims to calibrate and validate retracted sea level data of Envisat, ERS-2, Topex/Poseidon, Jason-1, 2, SARAL/AltiKa, Cryosat-2 altimetric missions near the Indian coastline. We assessed the reliability, quality, and performance of these missions by comparing eight tide gauge (TG) stations along the Indian coast. These are Okha, Mumbai, Karwar, and Cochin stations in the Arabian Sea, and Nagapattinam, Chennai, Visakhapatnam, and Paradip in the Bay of Bengal. To compare the satellite altimetry and TG sea level time series, both datasets are transformed to the same reference datum. Before the calculation of the bias between the altimetry and TG sea level time series, TG data are corrected for Inverted Barometer (IB) and Dynamic Atmospheric Correction (DAC). Since there are no prior VLM measurements in our study area, VLM is calculated from TG records using the same procedure as in the Technical Report NOS organization CO-OPS 065. 

Keywords— Tide gauge, Sea level, North Indian ocean, satellite altimetry, Vertical land motion

How to cite: Murshan, M., Devaraju, B., Balasubramanian, N., and Dikshit, O.: Validation of sea surface heights from satellite altimetry along the Indian coast, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4862, https://doi.org/10.5194/egusphere-egu21-4862, 2021.

An algorithm for distinguishing closed multicore circulations from digital maps of dynamic topography (DT) is described. The algorithm is based on the expansion of circulations over the area from their cores (local maxima/minima of the DT) until the DT thresholds corresponding to these cores are reached. The algorithm is performed in several iterations until the points belonging to the closed circulations are completely exhausted. The algorithm is an exact numerical solution of the problem of determining the value of the DT for a closed loop, the most distant from the core of circulation. The algorithm takes into account the problems of nesting circulations of different signs into each other, the possible intersecting of circulations with different signs on the numerical grid, and the possible existence of islands or floating ice inside the circulations. A method is described for merging smaller DT maps to larger maps with the circulations distinguished from the smaller maps.

How to cite: Tarakanov, R.: Distinguishing Closed Circulations from the Satellite Maps of the Dynamic Topography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-622, https://doi.org/10.5194/egusphere-egu21-622, 2021.