The Copernicus Precise Orbit Determination (CPOD) Service generates routinely precise orbital products of Sentinel-1, -2, -3 and -6 for its operational use by ESA and EUMETSAT. The accuracy of these products depends on the quality of the inputs used, particularly the gravity field and the GNSS orbits and clocks. This abstract focusses on the impact of the gravity field using the COST-G geopotential models.

Currently, the CPOD Service is using the EIGEN-GRGS-RL04 mean gravity field model, which dates back in 2019. In the framework of COST-G (Combination Service for Time-variable Gravity Fields) a new series of gravity fields is being generated quarterly by AIUB (Astronomical Institute, University of Bern), as a fit to a weighted combination of monthly gravity fields generated by different analysis centres. The monthly gravity fields models are generated based on GRACE-FO data, so they are capable of accurately catching up the recent changes in the gravity field. These models improve the operational orbit results significantly. Since these fitted signal models (FSM) only cover the GRACE-FO period they are not suitable for consistent reprocessing of entire mission times beyond 2018. In order to support such reprocessing activities an additional COST-G FSM has been generated covering the GRACE and GRACE-FO period. 

The use of the COST-G products has been proposed to substitute the current EIGEN-GRGS-RL04 mean gravity field model within the Copernicus Precise Orbit Determination (CPOD) Service. Based on limited reprocessing campaigns (from 2019 onwards) of Sentinel-3 A&B using both geopotentials (EIGEN and COST-G), it has shown that the COST-G FSMs represent improvements on the orbital accuracy and yields smaller dispersion of empirical accelerations.

Thanks to the recent availability of a new full COST-G FSM covering the GRACE and GRACE-FO period, the present work will extend this analysis to the Sentinel-1, -2 and -6 missions as well, including the full mission (e.g., from 2014 in the case of Sentinel-1A), to confirm the potential benefits of COST-G FSMs.

It is worth mentioning that this analysis will be performed with FocusPOD, a new GMV state-of-the-art POD SW.

How to cite: Berzosa, J., Fernández Martín, C., Matuszak, M., Fernández Usón, M., Fernández Sánchez, J., Nogueira Lodd, C., Femenias, P., Peter, H., and Meyer, U.: Reprocessing of Copernicus Sentinel POD solutions with COST-G geopotential models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16137, https://doi.org/10.5194/egusphere-egu23-16137, 2023.

The European Space Agency (ESA) Swarm mission was launched in November 2013 and consists of three identical satellites flying in near-polar low Earth orbits to study the dynamics of the Earth’s magnetic field. In the Swarm Data, Innovation, and Science Cluster framework, precise science orbits (PSO) are computed for the Swarm satellites from onboard GPS observations. These PSO consist of a reduced-dynamic orbit to precisely geotag the magnetic and electric field observations, and a kinematic solution with covariance information, which can be used for determining large-scale time variable changes of Earth’s gravity field. In addition, high-resolution thermospheric densities are computed from onboard accelerometer data. Due to accelerometer instrument issues, these data are currently only available for Swarm-C and the early mission phase of Swarm-A. Therefore, also GPS-derived thermospheric densities are computed, which have a lower temporal resolution but are available for all Swarm satellites during the entire mission. The Swarm density data can be used to study the influence of solar and geomagnetic activity on the thermosphere.

We will present the current status of the processing strategy used to derive the Swarm PSO and thermospheric densities and show recent results. For the PSO, our processing strategy includes a realistic satellite panel model for solar and Earth radiation pressure modelling, integer ambiguity fixing, and a screening procedure to reduce the impact of ionospheric scintillation-induced errors. Validation by independent Satellite Laser Ranging data shows the Swarm PSO have high accuracy, with an RMS of the laser residuals of about 1 cm for the reduced-dynamic orbits, and slightly higher values for the kinematic orbits. For the thermospheric densities, our processing strategy includes a high-fidelity satellite geometry model and the SPARTA gas-dynamics simulator for gas-surface interaction modelling. Comparisons between Swarm densities and NRLMSIS model densities show noticeable scaling differences, indicating the Swarm densities' potential to contribute to thermosphere model improvement. The accuracy of the Swarm densities is dependent on the aerodynamic signal size. For low solar activity, the error in the radiation pressure modelling becomes significant, especially for the higher-flying Swarm-B satellite.

The Swarm precise orbit and thermospheric density products are available for users at the dedicated ESA Swarm website (ftp://swarm-diss.eo.esa.int). The Swarm densities are also available at our thermospheric density database (http://thermosphere.tudelft.nl). This database also includes thermospheric densities for the CHAMP, GRACE, GOCE, and GRACE-FO satellites. For future work, it is planned to further improve the Swarm densities, especially for low solar activity conditions, by including a more sophisticated radiation pressure modelling of the Swarm satellites.

How to cite: van den IJssel, J., Siemes, C., and Visser, P.: Precise science orbits and thermospheric densities for the Swarm mission, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5432, https://doi.org/10.5194/egusphere-egu23-5432, 2023.

Along with other space geodetics, the Global Navigation Satellite Systems (GNSS) technique has a profound impact on the realization of terrestrial reference frames and the provision of geodetic and geophysical parameters such as Earth Rotation Parameters (ERP) and geocenter (GCC) coordinates. In GNSS data processing ambiguity resolution (AR) is critical to achieve high-precision estimates. Due to the uncalibrated phase delay (UPD), currently most International GNSS Service (IGS) Analysis Centers (ACs) adopt the double-differenced (DD) ambiguity resolution strategy, which eliminates UPD by between-satellite and between-station differencing. Undifferenced (UD) ambiguity resolution, which recovers the integer characteristics of the UD ambiguities by directly estimating the UPD, was originally developed for Precise Point Positioning and nowadays is also applied to network processing, for example for Precise Orbit Determination (POD). In this study, we conducted a comparative study on the UD and DD ambiguity resolution for GNSS POD data processing. We demonstrated that the UD strategy significantly improves the solution compared to the DD solutions. The GPS orbit precision is improved by 20%, 12%, and 5% in the along, cross, and radial component, respectively. Moreover, the ERP are significantly improved, and the agreement with the IGS final products is improved by 28% and 23% for the offsets of x-pole and y-pole, respectively, and by 23%, and 17% for the corresponding rates. Additionally, the repeatability of station coordinates and GCC coordinates is also slightly improved. More important, we confirmed that the two strategies are theoretically equivalent and the outperformance of UD strategy could be explained by its strong robustness to wrong-fixings and its capability to fix the most UD ambiguities, while wrong-fixings and missing fixings are hardly avoidable in the DD strategy due to the requirement of forming DD-ambiguities.

How to cite: Tang, L., Wang, J., Nie, L., Zhu, H., Ge, M., and Schuh, H.: Improving GNSS data analysis with undifferenced integer ambiguity resolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3589, https://doi.org/10.5194/egusphere-egu23-3589, 2023.

A global Terrestrial Reference Frame (TRF) is realized today by four space geodetic techniques, i.e., Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). The current goals for the TRF realization are specified with an accuracy of 1 mm and long-term stability of 0.1 mm/year according to the Global Geodetic Observing System requirements.

Differences in the characteristics of each space geodetic technique might introduce systematic effects into TRF realization. Among the crucial systematics, we can distinguish modeling and calibration deficiencies, i.e., direct solar radiation pressure (SRP) modeling for GNSS and DORIS, GNSS antenna calibration uncertainties, South Atlantic Anomaly handling for DORIS, and time and range bias handling for SLR. The current realization of the TRF is based on independent solutions for each of the four contributing space geodetic techniques that are just connected on the ground via local ties and the Earth Rotation Parameters. A new opportunity to challenge the systematic errors is to co-locate space techniques onboard one satellite, as it is planned for the Genesis-1 mission. The Genesis-1 satellite is the first ever satellite co-locating sensors related to GNSS, SLR, DORIS, and VLBI.

In this study, we present preliminary results for orbit reconstruction using simulated GNSS observations for the Genesis-1 satellite. We use the Bernese GNSS Software for different orbital altitudes, including the currently planned altitude of 6000 km. We assess the quality of the orbit reconstruction using simulated GNSS data from observations of the zenith-, nadir-, and both zenith and nadir-looking antennas. Additionally, we assess the visibility from the ground tracking network of VLBI, SLR, and DORIS, for the different satellite altitudes.

How to cite: Bury, G., Arnold, D., Dach, R., and Jäggi, A.: Simulation of GNSS observations for Genesis-1, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7927, https://doi.org/10.5194/egusphere-egu23-7927, 2023.

We discuss a concept of probabilistic orbit. If correctly constructed, an ensemble of equally plausible orbits is a proper representation of the underlying true and unknown probability distribution of orbits. Hence, the probabilistic orbit is determined by the entire sample and its statistical properties rather than any individual ensemble member. We explore the concept by starting from an ensemble of atmospheric state estimates generated at ECMWF (N=51). The ensemble mean closely corresponds to the most likely atmospheric state estimate, and the ensemble spread to its inherent uncertainty. We compute separately a GNSS orbit solution for each unique atmospheric state (N=51). Thus, we essentially sample one uncertain information source the GNSS orbits are known to be sensitive for. Thereby, uncertainty in the atmospheric state estimate is explicitly propagated to the ensemble of orbit solutions. The concept leads to interesting corollaries. Precise point positioning using the probabilistic orbit, for instance, becomes probabilistic, too.

How to cite: Järvinen, H., Navarro Trastoy, A., Tuppi, L., and Mayer-Gürr, T.: Probabilistic orbit solutions in GNSS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12696, https://doi.org/10.5194/egusphere-egu23-12696, 2023.

The Copernicus Precise Orbit Determination (CPOD) Service is a consortium led by GMV, responsible for providing precise orbital products and auxiliary data files from the Copernicus Sentinel-1, -2, -3, and -6 missions to the corresponding Payload Data Ground Segment (PDGS) processing chains at ESA and EUMETSAT.

Since April 2014, the CPOD Service has been supporting the Copernicus program as soon as the different Sentinel satellites were launched. During the last 8 years, the CPOD Service has been using the ESA/ESOC SW NAPEOS, to compute the precise orbits. During these years, new algorithms and standards were implemented in NAPEOS, to improve the accuracy of the products. Currently, the accuracy of the orbital solutions computed by the CPOD Service is state-of the art, and similar to the solutions computed by other entities including AIUB, CNES, DLR, ESA, GFZ, JPL, TU Delft, and TUM, all of them members of the CPOD Quality Working Group (QWG).

Starting on January 2021, GMV has been developing a new POD SW called FocusPOD, as an internal R&D activity. Following current trends (e.g., GIPSY-X, GODOT), FocusPOD has been written from scratch in C++ & Python, with a completely new design, with the goal of supporting future CPOD Services evolutions, among other projects.

In terms of architecture, the core layers of the FocusPOD SW have been designed as a library, to support a flexible development of applications. For example, constructing multiple distributed programs is possible, as well as building a single binary that decodes the GNSS L0 data, reads the different input files (e.g., GNSS orbits and clocks), pre-processes the observations, propagates the initial state vector, performs the least-square adjustment, fixes ambiguities, and constructs the final product; all with a single execution that allows reducing the processing time and minimizes the usage of HW resources. The functionality of each binary may be constructed as needed.

One of the key differences with respect to the previous SW is the decision to separate data and algorithms in the design of FocusPOD. This has triggered the development of a data model to keep in the processing memory all the data, organized following its physical meaning, and relating different elements. This will allow developing new algorithms independently of the data. Another relevant aspect is the use of advanced mechanisms to keep and search large amounts of data, a key element to exploit the SW in future use cases.

The architecture of FocusPOD will be presented, as well as its performance. In terms of architecture, the benefits of the chosen design will be highlighted together with the lessons learnt from the implementation. In terms of performance, it will be shown that the new SW presents improvements in runtime with respect to the legacy system, reducing the product generation timeliness, while reaching the same levels of accuracy as other state-of-the-art SW packages. The achievable accuracy in LEO POD will be presented by showing the differences against external reference solutions and SLR residuals analysis of the Sentinel satellites.

How to cite: Fernández Martín, C., Berzosa Molina, J., Bao Cheng, L., Muñoz de la Torre, M. Á., Fernández Usón, M., Lara Espinosa, S., Terradillos Estévez, E., Fernández Sánchez, J., Peter, H., Féménias, P., and Nogueira Loddo, C.: FocusPOD, the new POD SW used at CPOD Service, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1908, https://doi.org/10.5194/egusphere-egu23-1908, 2023.

The Gravity Recovery Object Oriented Programming System (GROOPS) is meanwhile a highly sophisticated and well accepted tool in the scientific community which enables the user to perform core geodetic tasks. The key features of the software include gravity field recovery from satellite and terrestrial data, the processing of global navigation satellite system (GNSS) constellations and ground station networks and the determination of satellite orbits from GNSS measurements. In the course of the continuous development of GROOPS, we will extend the software by satellite laser ranging (SLR) as an additional feature. Satellite laser ranging offers the possibility to measure distances from low earth orbiting (LEO) satellites with an accuracy in the mm and cm range. A major advantage of this observation technique is that only a passive space technology, called laser retroreflectors are required onboard. Satellite laser ranging can be used as a tool for independent validation of precise determined LEO satellite orbits. Thus, through the extension of GROOPS by SLR we will also be able to perform a validation of our own determined satellite orbits, which we publish regularly. In this work we will give an overview of the integration of the SLR processing, and how corrections regarding the satellites centre of mass, antenna offsets, laser retroreflector offsets and satellite/station specific range biases are handled. Additionally, we will show the orbit validation results of several satellite missions with laser retroreflectors onboard.

How to cite: Suesser- Rechberger, B., Mayer-Guerr, T., Dumitraschkewitz, P., Tieber-Hubmann, C., and Krauss, S.: Extension of the software toolkit GROOPS by satellite laser ranging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5244, https://doi.org/10.5194/egusphere-egu23-5244, 2023.

The absolute positional accuracy of altimetry satellites is one key aspect for subsequent analyses in Earth observation. To determine the position of a satellite with highest accuracy, various observation techniques are obtainable. Recent satellites like the Copernicus Sentinel-3A/3B and 6A Michael Freilich (MF) satellites are equipped with multiple observation techniques onboard. We use the techniques Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), Global Positioning System (GPS) and Satellite Laser Ranging (SLR) to generate orbits based on DORIS-only, on the combination of DORIS and SLR, and on GPS-only. To examine the accuracy of the orbits, we compare them inter-technique-wise and with an external combined orbit solution, which is assumed to have superior absolute accuracy and minimal residual systematic errors. Subsequently, we strive to generate reference frame realizations for the DORIS technique in order to be able to contribute an additional solution to the International DORIS Service (IDS) in the future. Therefore, we use the DORIS only-orbit solutions, and generate weekly local reference frames for each of the three satellites and in combination. We evaluate these based on the reference frame defining parameters, i.e. origin, scale and orientation.

How to cite: Schreiner, P., Reinhold, A., Neumayer, K. H., and Koenig, R.: Sentinel POD based on DORIS, GPS and SLR and subsequent reference frame determination based on DORIS-only, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6489, https://doi.org/10.5194/egusphere-egu23-6489, 2023.

Numerous non-gravitational accelerations (NGAs) – due to radiation or gas-surface interactions – act on a typical satellite in low Earth orbit. The modeling of these accelerations is a key aspect of (reduced-)dynamic satellite precise orbit determination. Opposed to a purely empirical estimation of NGAs, especially orbit solutions with a high dynamical stiffness require an explicit NGA forward modelling. This is most important, e.g., for altimetry satellites which require utmost orbit accuracy especially in radial direction.

For explicit NGA modelling, a commonly employed strategy is to describe the satellite body in terms of a relatively simple macro model, i.e., a geometry composed of a number of elementary shapes like flat plates, spheres or cylinders, each of which with a specific size, orientation and surface property. During orbit integration, for each individual elementary surface NGAs are then computed based on analytical expressions and the sum over all surfaces yields the total NGA of the satellite at integration epoch. Often each of the elementary surface of the satellite macro model is treated fully independent from the other surfaces and interactions, in particular the (partial) occultation or shadowing of one surface by other surfaces is neglected. In case of satellites with significant non-convex shapes (like the altimetry satellite Sentinel-6A) this can lead to a marked degradation of NGA modeling. On the other hand, very detailed satellite geometry models together with techniques like ray tracing can provide highly accurate NGAs, however, at the price of significantly increased processing times.

In this contribution we assess the performance of a relatively simple self-shadowing algorithm for satellites described by a collection of flat convex polygons. Based on the relative plate locations, for each plate we analytically determine the amount of shadow cast by the other plates. We quantify the cost in terms of processing time and the impact on dynamic orbit solutions for the Copernicus Sentinel satellites in terms of empirical orbit parameters, observation residuals as well as other orbit quality metrics. We compare the so-derived NGAs and orbit solutions to results obtained with different other macro models and alternative approaches to take self-shadowing into account.

How to cite: Arnold, D., Peter, H., and Jäggi, A.: Simple self-shadowing in precise orbit determination of Copernicus Sentinel satellites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13569, https://doi.org/10.5194/egusphere-egu23-13569, 2023.

Ocean tides are small, periodic disturbances of the ocean state variables with frequencies related to the solar and lunar ephemerides. Most prominently, they are observed at the coasts by tide gauge measurements, where variations of the free sea surface height are visible even to the bare eye. On the other hand, tide gauges are just one geodetic technique that can identify ocean tide signatures. Especially satellite altimetry is considered the critical technology for constructing highly accurate data-constrained ocean tide atlases. However, the number of frequencies available from such atlases is limited by the signal-to-noise ratio of those observations, so not all tidal lines are readily available. To account for unresolved minor ocean tides, linear admittance approaches are utilized. On the other hand, unconstrained hydrodynamic ocean tide modeling with TiME (Sulzbach et al., 2021, 2022) allows us to calculate the oceanic response for any given tidal line explicitly.

Due to the long-wavelength character of ocean tidal loading, its geophysical fingerprint is not restricted to the oceans but extends horizontally (causing deformations of the crust to which instruments are attached) and vertically (causing periodic changes in gravity at orbital heights of in particular low-Earth orbiting satellites). Thus, tidal loading systematically disturbs both satellite orbits and also geodetic observations from ground stations with SLR, GNSS, and DORIS.

In this contribution, we compare tidal predictions from different ocean tide atlases. In addition to available Stokes coefficients of the gravity potential, we calculate vertical and horizontal displacements for 34 partial tide solutions from FES14, 17 from EOT20, and 57 from the data-unconstrained model TiME based on a spherical harmonic decomposition of maximum degree 4999. We focus on the influence of minor ocean tide solutions on the tidal predictions either drawn from the broadband TiME22-catalog or derived with the help of linear admittance from EOT20/FES14 major tides. The results are validated with time series of selected geodetic measurement systems to quantify the influence of the minor tide treatment on their residuals to motivate further efforts to reduce ocean tide residuals in precise orbit determination.

How to cite: Sulzbach, R., Balidakis, K., Öhlinger, F., Mayer-Gürr, T., Dobslaw, H., and Thomas, M.: Impact of Minor Ocean Tides on Surface Deformations and the Earth’s Gravity Field, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15862, https://doi.org/10.5194/egusphere-egu23-15862, 2023.

In the framework of the European Space Agency’s Moonlight program, a satellite navigation system is planned for positioning, navigation, timing, and communication on the Moon.  The constellation will consist of three or four lunar orbiters in eccentric orbits – with periselene above the northern hemisphere and aposelene above the southern hemisphere; the latter is of special interest in terms of future lunar missions. The eccentric orbits introduce a challenge for precise orbit determination, because large gravity perturbations due to the lunar gravity field occur in the periselene passes, whereas the aposelene passes are associated with the smallest gravity gradients and thus the largest errors in orbit determination.

We evaluate the non-gravitational and gravitational perturbing forces acting on lunar orbiters in eccentric orbits. The simulated orbits consider lunar gravity field based on the GRAIL mission, gravity perturbations from the Sun, Earth, and planets considering Earth’s oblateness, tidal deformations, direct solar radiation pressure with lunar and Earth eclipses, albedo, antenna thrust, and relativistic effects. We discuss three methods of proposed representation of the broadcast orbits: based on Keplerian parameters and a set of one-per-revolution corrections (GPS-like or Galileo-like), based on Chebyshev polynomials with a variable number of coefficients and arc-length representation, and based on a series of positions and velocities (GLONASS-like). We discuss the advantages and limitations of all three representations and accuracies provided by different approaches depending on the number of assumed coefficients and arc lengths. Finally, we discuss the impact of inconsistent treatment of the origin, scale, and orientation of the lunar reference frame on the determined positions on the Moon. 

How to cite: Sośnica, K., Zajdel, R., Bury, G., Di Benedetto, M., Durante, D., Sesta, A., Fienga, A., Linty, N., Belfi, J., and Iess, L.: Precise orbits for the lunar navigation system: challenges in the modeling of perturbing forces and broadcast orbit representation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5575, https://doi.org/10.5194/egusphere-egu23-5575, 2023.

DORIS (Doppler Orbitography Radio positioning Integrated by Satellite) is a space-based piggyback satellite system that when used as a terrestrial measurement system can provide an accuracy of 10 cm in the absolute position measurement and in the order of sub-cm accuracy in relative measurements. When it rides piggyback on any satellite system can provide an orbital accuracy up to cm level. Precise Orbit Determination (POD) is the process of accurately tracking the position and velocity of a satellite in orbit. DORIS is an excellent satellite tracking system supporting the precise orbit determination of satellites. The accuracy of the POD does not affect only the quality of the estimated orbit ephemerides, but also the quantities derived from a free-network solution, mainly the station coordinates and the Earth rotation parameters (ERP). The precisely determined orbit can be used in many altimetric applications including estimating the mean sea surface, marine geoid, and vertical land motion, to derive vertical deflection and mean dynamic topography. Apart from that, the gravity field variations can also be determined more precisely.  GNSS data observations alone have been used so far in India to study the plate tectonics movements and earthquake predictions in the Indian subcontinent. Since DORIS and GNSS both have homogeneous network distributions and are complementary to each other, the DORIS data observations can be combined with GNSS data observations to study plate tectonics movements and earthquake predictions more effectively. Not only that since in DORIS the signal is transmitted from the ground, but processing DORIS measurement residue helps us to better understand and model what occurs in between the layers of the atmosphere which can be used for precise tropospheric modeling.

This space-based geodetic measurement technique has been used for the past three decades the world over, for precise measurement on the ground as well as on all oceanographic and altimetric satellite missions, it has not been introduced in India so far. Since our Space Research Organization has been launching several satellite missions and DORIS is being used for computation of the precise orbits, this technique has come to stay for a long time. The National Centre for Geodesy was set up by the Department of Science and Technology at the IIT Kanpur has also plans to set up a Geodesy village, where the DORIS station is going to be established and our DORIS data will be used in defining ITRF.

How to cite: Kumar, V., Dikshit, O., and Balasubramanian, N.: DORIS-based Precise Orbit Determination and its geodetic applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13557, https://doi.org/10.5194/egusphere-egu23-13557, 2023.