The International GNSS Service (IGS) publishes operationally GPS and GLONASS orbit and clock products with the highest accuracy. These final products result from a combination using as input products determined by the IGS Analysis Centers (ACs). The method to perform the combination was developed in the early nineties by Springer and Beutler and is used until nowadays despite some updates made over the years mainly to improve the clock combination and the alignment with the current ITRF. Over the past years, towards the Multi-GNSS Experiment and Pilot Project (MGEX) the IGS has been putting efforts into extending its service. Several MGEX ACs contribute by providing solutions containing not only GPS and GLONASS but also Galileo, BeiDou, and QZSS. For MGEX an orbit and clock combination is still not consolidated inside the IGS and requires studies in order to provide a consistent solution.

We will present a least-squares framework for a multi-GNSS orbit combination, where the weights used to combine the ACs' orbits are determined by least-squares variance component estimation.  In this contribution, we will introduce and compare two weighting strategies, where either AC specific weights or AC plus constellation specific weights are used. Both strategies are tested using MGEX orbit solutions for a period of two and a half years. They yield similar results where the agreement between combined and individual products is around one centimeter for GPS and up to a few centimeters for the other constellations. The agreement is generally slightly better using the AC plus constellation weighting. A comparison of our combination approach with the official combined IGS final solution using three years of GPS, and GLONASS orbits from the regular IGS processing show an agreement of better than 5 mm and 12 mm for GPS and GLONASS, respectively. An external validation using Satellite Laser Ranging is performed for our combined MGEX orbit solutions with both weighting schemes and shows offsets values in the millimeter level for all constellations except to QZSS where the values reach a few centimeters.

How to cite: Mansur, G., Sakic, P., Brack, A., Männel, B., and Schuh, H.: Combination of GNSS orbits using variance component estimation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1102, https://doi.org/10.5194/egusphere-egu21-1102, 2021.

The determination of GNSS orbits is generally based on the processing of pseudorange and carrier phase measurements from a station network, with an Orbit Determination and Time Synchronization (ODTS) process. This process involves the satellite and ground station clocks as part of the GNSS measurement reconstruction. The clocks are generally estimated as a snapshot parameter, without assuming any correlation between epochs. However, the stability of satellite and some station clocks, based on technologies of hydrogen, cesium or rubidium, allows for a significant predictability. Taking advantage of this predictability the ODTS process can be improved, especially in those cases where the station network is limited or does not provide a good coverage for certain areas.

The clock modelling can be directly done by estimating additional parameters in the filter. A quadratic model is generally estimated for each clock, keeping a small snapshot contribution to account for the stochastic part and for potential deviations with respect to the theoretical behavior of the clock. The detection of this kind of deviations in the satellite and station clocks becomes a major factor for achieving a good performance with these techniques. In case the clock experiences feared events like phase or frequency jumps, the estimated clock model stops being valid and the estimation of model parameters needs to be reset.

In case a composite clock algorithm is used to provide the reference timescale for the ODTS, the estimation of clock models can rely on this algorithm. Algorithms of composite clock are generally based on a Kalman filter that estimates as part of the state vector the differences between each contributing clock and the composite timescale. These differences can be used not only to define the reference timescale of the ODTS, but also to remove the deterministic part of the clocks in the measurement reconstruction. As for the case of clock modelling, for algorithms of composite clock the detection and correction of anomalies in the contributing clocks becomes a critical point.

In this work, the integration of orbit determination, clock modelling and composite clock algorithms will be described. The impact of clock modeling techniques on the GNSS orbit determination accuracy will be presented, both considering a direct estimation of clock models in the ODTS and the estimation provided by the composite clock algorithm. These analyses will be based on NEODIS, the orbit determination software developed by Thales Alenia Space, which integrates with a Kalman filter approach GNSS orbit determination and composite clock algorithms.

How to cite: Roldan, P., Guerin, P., Anton, J., Laurenti, M., and Trilles, S.: Clock modelling techniques for an enhanced GNSS orbit determination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1331, https://doi.org/10.5194/egusphere-egu21-1331, 2021.

The normal equations are widely used to combine elementary least squares solutions, to solve very large problems which are not possible to handle directly. The principle is to reduce each problem to a minimal set of parameters present in the global problem, without removing the corresponding information, and connect them. For instance, one important application is the combination over years of daily network solutions, as performed for the ITRF (Altamimi et al., 2016) [1].

The approach can also be used in orbit determination to connect arcs solutions in order to construct the solution of a global arc. This was applied for example for GPS constellation solutions as in the article written by Beutler et al. (1996) [2]. Due to the size of the problems, it is interesting to divide for example a three days solution into three one day solutions. Another advantage is that the one day solutions are usually efficiently processed by the orbit determination software. For rapid or ultra-rapid GNSS products this is also very interesting, as the solutions are needed very often for small shifts of the global arc (for example 24 hours arcs, shifted every 6 hours in the case of ultra-rapid products). A further extension is to construct recursive solutions from these elementary arcs, leading to a filter similar to a Kalman filter.

We propose a unified methodology, associated with an efficient implementation compatible with our least squares software GINS, allowing us to solve the various problems ranging from arc connection to sequential filtering. The final objective is to construct efficient GNSS ultra-rapid products.

The application on a simple problem consisting in connecting different SLR arcs is shown, as a test case to develop and implement the methodology. In this case, the global solution can also be directly constructed for validation purposes. This study includes the construction of the solution at the end points of the elementary arcs, and also the recovery of the global solution state vectors at every epoch.

The next step will be to implement more complex parameterizations (including measurement parameters, which are not present in the SLR test case), and to apply this for GNSS constellation solutions.

How to cite: Mercier, F., Bhattacharjee, S., Perosanz, F., and Lemoine, J.-M.: Combining multiple arcs for orbit determination using normal equations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3129, https://doi.org/10.5194/egusphere-egu21-3129, 2021.

The Non-Gravitational Perturbations (NGP), out of which the Solar Radiation Pressure (SRP) is the largest, have a significant impact on GNSS satellite orbits. In addition to the SRP, other relevant perturbations should also be taken into account, as this may result in substantial modelling errors if underestimated. Particularly, the force model should also consider Earth’s albedo in terms of the emitted and reflected radiation, as well as a physical satellite model (box-wing) with its optical and thermal properties.
GNSS satellite orbit modelling may suffer from deficiencies for various reasons (simplification of the complexity of the used model or uncertainty of the input information). The impact of such model errors on global GNSS data analyses is assessed in an error propagation study based on simulated observations. The influence of artificially introduced orbit errors on estimated parameters, e.g. Earth rotation parameters, orbit parameters (initial conditions and dynamical orbit parameters), station coordinates, station-wise troposphere parameters, as well as receiver and satellite clock corrections is investigated. In this study a dedicated simulation environment is used to analyse the relation between results and certain individual shortcomings in the NGP models. In addition, apart from a commonly used epoch-wise clock estimation, the analytical models for satellite clock corrections are introduced in order to exploit the high stability of the passive H-masers on-board the Galileo satellites. The simulation environment also allows to assess how the impact of float- versus fixed-ambiguities.
Finally, simulation-based analyses offer an excellent framework for more detailed validations and further refinements of the physical satellite models, which will consequently stabilize the global solution.

How to cite: Kalarus, M., Dach, R., Villiger, A., and Jaeggi, A.: Propagation of satellite orbit modelling deficiencies into the global GNSS solutions – simulation-based study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6341, https://doi.org/10.5194/egusphere-egu21-6341, 2021.

Three orbital effects emerging from general relativity are typically considered for Earth-orbiting satellites: the Schwarzschild effect, Lense-Thirring effect or frame-dragging, and the de Sitter or geodetic precession effect. For circular orbits and short satellite orbital arcs, the dominating Schwarzschild effect is difficult to determine, because it causes a constant radial acceleration which can be absorbed by a small modification in the gravitational constant GM term or a constant offset in the estimated semi-major axis of a satellite orbit. To separate the effects caused by the Schwarzschild effect from other orbital effects, especially those emerging from orbit modeling issues of non-gravitational accelerations, eccentric satellite orbits should be employed.

The first pair of satellites belonging to the Galileo satellite system was accidentally launched into non-circular orbits with height variations between from 17,180 km for the perigee to 26,020 km for the apogee. The eccentric orbits introduced new opportunities for the verification of the effects emerging from general relativity when employing the Galileo constellation. Galileo satellites are equipped with two techniques for precise orbit determination: microwave GNSS antennas and SLR retroreflectors which allow for deriving their orbits of superior quality.

In this study, we discuss effects in GNSS orbits emerging from general relativity. We concentrate on those effects that exceed the value of 1 mm over 1 day, thus are of fundamental importance for precise orbit determination in satellite geodesy and precise high-quality products of the International GNSS Service. We show that the semi-major axis of Galileo satellites in eccentric orbits varies between -29 mm in perigee to -9 mm in apogee due to the Schwarzschild term. For GNSS geostationary satellites with the inclination angle close to zero, the omission of the de Sitter effect may cause an error of the determination of the right ascension of ascending node exceeding the value of 1 meter after 1 day. Finally, we discuss the suitability of using GPS, GLONASS, and Galileo satellite orbits to determine the values of the Post-Newtonian Parameters γ and β and all limitations related to the observability of these parameters at GNSS heights and systematic errors emerging from non-gravitation orbit perturbations.

How to cite: Sośnica, K., Bury, G., Zajdel, R., Kaźmierski, K., Ventura-Traveset, J., Prieto Cerdeira, R., and Mendes, L.: General Relativistic effects acting on GNSS orbits with a focus on Galileo satellites launched into incorrect orbital planes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7127, https://doi.org/10.5194/egusphere-egu21-7127, 2021.

With the successful launch of the last Geostationary Earth Orbit (GEO) satellite in June 2020, China has completed the construction of the third generation BeiDou navigation satellites system (BDS-3). BDS-3 global services have been initiated in July 2020 with the constellation of 3 GEO, 3 Inclined Geosynchronous Orbit (IGSO) and 24 Medium Earth Orbit (MEO) satellites. In order to further improve the performance of BDS-3 services, the quality of BDS-3 precise orbit product needs further enhancements.

       The solar radiation pressure (SRP) is the main non-conservative orbit perturbation for GNSS satellites and is the key to improve BDS-3 precise orbit determination. In this study, we focus on the SRP models for BDS-3 satellites. Firstly, the widely used Extended CODE Orbit Model with five parameters (ECOM-5) is assessed. With one-year observations of 2020 from both iGMAS and MGEX networks, the five parameters of ECOM model (D0, Y0, B0, Bc and Bs) are estimated for each BDS-3 satellite. The D0 estimates show an obvious dependency on the elevation angle of the Sun above the satellite orbital plane (denoted as β). In addition, large variations can be noticed in eclipse seasons, which indicate the dramatic changes of SRP. The Y0 estimates vary from -0.6 nm/s² to 0.6 nm/s² for MEO, -1.0 to 1.0 nm/s² for IGSO and -1.0 to 1.5 nm/s² for GEO satellites. The B0 estimates of several satellites exhibit a clear dependency on the β angle. The largest variation of B0 appears at C45 and C46, changing from 1.0 nm/s² at 15 deg to 8.3 nm/s² at 64 deg, which implies that the solar panels of these two satellites may have an obvious rotation lag. To compensate the deficiencies of BDS-3 SRP modeling, we introduce several additional parameters into ECOM-5 model (e.g. introducing higher harmonic terms). The POD performances can be improved by about 10% and 40% for BDS-3 MEO/IGSO and GEO satellites, respectively.

       Except for the empirical model, we also study the semi-empirical SRP model such as the a priori box-wing model. Since the geometrical and optical properties from BDS-3 metadata are general and rough, we apply more detailed geometrical and optical coefficients for BDS-3 satellites. The POD performance can be improved by about 10% compared to empirical SRP models. Furthermore, considering Earth radiation pressure will have an impact of about 1.3 cm in radial component for MEO satellites.

How to cite: Li, J., Yuan, Y., Huang, S., Liu, C., Lou, J., and Li, X.: Examination and Enhancement of solar radiation pressure model for BDS-3 satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8307, https://doi.org/10.5194/egusphere-egu21-8307, 2021.

A physical a priori box-wing solar radiation pressure (SRP) model is widely used by most analysis centers for Galileo and QZSS (Quasi-Zenith Satellite System) satellites, complemented by an ECOM or ECOM2 (Empirical CODE Orbit Model) model. For the other constellations, for instance GPS and GLONASS satellites, optical properties of satellite surfaces are not publicly available, especially for GPS Block IIF and GLONASS satellites. By fixing satellite surface areas and total mass to the values from some unpublished documents, we estimate satellite surface optical properties based on true GNSS measurements covering long time periods (typically this should be longer than a full beta angle time range to reduce correlations between parameters). Meanwhile, various physical effects are considered, such as yaw bias, radiator emission and thermal radiation of solar panels. We find that yaw bias of GPS Block IIA and IIR satellites does not dominate the Y-bias, it is likely that heat generated in the satellite is radiated from louvers or heat pipes on the Y side of the satellite. It is also noted that the ECOM Y0 estimates of both GPS and GLONASS satellites show clear anomaly during eclipse seasons. This indicates that the radiator emission is present when the satellite crosses shadows. Since satellite attitude during eclipse seasons could be different from the nominal yaw, potential radiator effect in the –X surface could be wrongly absorbed by the ECOM Y0 as well. By considering all the estimated parameters in an a priori model we observe clear improvement in satellite orbits, especially for GLONASS satellites. China’s Beidou-3 satellites are now providing PNT (positioning, navigation and timing) service globally. Satellite attitude, dimensions and total mass are publicly available. Also, the absorption optical properties of each satellite surface are given. With all this information, we estimate the other optical properties of Beidou satellites considering similar yaw bias, radiator and thermal radiation effects as those in GPS and GLONASS satellites.

How to cite: Duan, B., Hugentobler, U., Selmke, I., and Marz, S.: Physical a priori solar radiation pressure models for GNSS satellites with the focus on BDS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12358, https://doi.org/10.5194/egusphere-egu21-12358, 2021.

We present the status of our work on producing a new high-precision, physics-based radiation force model for the GPS IIF spacecraft. The details of the spacecraft model, i.e. geometry and surface material properties, are given. The methods used to build the spacecraft model from various information sources are described. Overall, the radiation force model accounts for the direct solar force, the recoil force due to reflected (diffuse and specular) radiation, and also thermal forces (re-radiation and solar panel thermal gradient). The bus component of the radiation force model is computed using ray-tracing techniques. The performance of the new model is compared against one that uses a box-wing spacecraft model. The assumptions and limitations of the modelling are discussed.

How to cite: Li, G. and Bhattarai, S.: Towards a high-precision analytical radiation force model for the GPS IIF spacecraft, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12544, https://doi.org/10.5194/egusphere-egu21-12544, 2021.

For low Earth orbit satellites, the atmospheric drag is the largest non-gravitational force. With increasing satellite altitude, the acceleration due to the Earth radiation pressure (ERP) decreases, whereas the effect of the Solar radiation pressure (SRP) becomes prevalent. Models of these non-gravitational forces are applied in satellite gravimetry, thermospheric density estimation, and in precise orbit estimation of the spherical satellite laser ranging (SLR) satellites.

In earlier investigations, we found that estimating systematic errors in radiation pressure force models appears possible based on an inverse procedure using GRACE data. Our preliminary results show that the outgoing radiation from CERES SYN data is too small for both longwave and shortwave data.

Here, we want to test the suitability of another approach to obtain estimates for correcting existing radiation data. In this method, we focus on the estimation of scale factors for radiation pressure accelerations of different SLR satellites with the long-term aim to obtain corrections for existing radiation datasets. During the precise orbit determination procedure of the spherical SLR satellites, the scale factors are commonly estimated with a variety of other parameters. Here, we test different parametrizations of the scale factors of both ERP and SRP accelerations and their behaviour for different SLR satellites such as Stella and Ajisai during varying solar conditions. Besides the separate estimation of scale factors for ERP and SRP accelerations, we will estimate global (monthly) scale factors for ERP accelerations from a variety of SLR satellites. Finally, we will investigate the potential of the resulting scale factors to correct existing radiation datasets in the future. At this stage, a comparison to our preliminary estimates from our previous investigations turns out as helpful.

How to cite: Vielberg, K., Löcher, A., and Kusche, J.: Investigating scale factors of radiation pressure accelerations of SLR satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4965, https://doi.org/10.5194/egusphere-egu21-4965, 2021.

The Astronomical Institute of the University of Bern (AIUB) collaborates with the Federal Agency for Cartography and Geodesy (BKG) in Germany to develop new procedures to generate products for the International Laser Ranging Service (ILRS). In this framework the SLR processing of the standard ILRS weekly solutions of spherical geodetic satellites at AIUB, where the orbits are determined in 7-day arcs together with station coordinates and other geodetic parameters, is extended from LAGEOS-1/2 and the Etalon-1/2 satellites to also include the LARES satellite orbiting the Earth at much lower altitude. Since a lower orbit experiences a more variable enviroment, e.g. it is more sensitive to time-variable Earth's gravity field, the orbit parametrization has to be adapted and also the low degree spherical harmonic coefficients of Earth's gravity field have to be co-estimated. The impact of the gravity field estimation is studied by validating the quality of other geodetic parameters such as geocenter coordinates, Earth Rotation Parameters (ERPs) and station coordinates. The analysis of the influence of LARES on the SLR solution shows that a good datum definition is important.

How to cite: Geisser, L., Meyer, U., Arnold, D., Jäggi, A., and Thaller, D.: Impact of low-degree gravity field estimation in the SLR data processing of spherical satellites at AIUB, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4305, https://doi.org/10.5194/egusphere-egu21-4305, 2021.

The processing configuration for our IDS contribution to the International Terrestrial Reference Frame (ITRF2020) realization was defined. We adopted the last standards and models recommended by IERS. We took into account the IDS recommendations to mitigate the non-conservative force model error on satellites, to mitigate the effect of the South Atlantic Anomaly on the DORIS receivers and to improve the stability of the DORIS scale.

A Precise Orbit Determination (POD) status for DORIS satellites by taking into account all these improvements will be presented for the processed time span. We will give statistical results such as one per revolution empirical acceleration amplitudes and orbit residuals. We will also give some comparisons to some external precise orbits used for altimetry. Some external validations of our orbits will be done, such as with independent SLR measurements processing as well as through the use of altimeter crossovers when available. We will also look at the impact of our new ITRF2020 configuration on the DORIS geocenter and scale.

How to cite: Capdeville, H., Mezerette, A., and Lemoine, J.-M.: DORIS results on Precise Orbit Determination and on geocenter and scale solutions from CNES/CLS IDS Analysis Center contribution to the ITRF2020 , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5384, https://doi.org/10.5194/egusphere-egu21-5384, 2021.

Launched in 1992, the TOPEX/Poseidon (T/P) mission is one of the first major altimetry missions. It is the predecessor of the Jason satellites which orbit the Earth on a very similar orbit. The geodetic space technique SLR (Satellite Laser Ranging) provides observations of this mission by targeting the Laser Retroreflector Array (LRA) mounted on the spacecraft. The T/P LRA is extremely large and not optimally designed. It thus causes big variations in the LRA phase center. These variations are a significant limiting factor of the orbit accuracy which makes it essential to apply a measurement correction for precise orbit determination. Up to now, only tabulated LRA corrections are available which require an interpolation.

In this contribution, we present a new approach to determine station-dependent LRA corrections to improve the phase center variations. The approach is based on a continuous analytical correction function which only uses the observation azimuth and zenith angle in combination with four parameters. These parameters are computed within an estimation process for each observing SLR station. Therefore, uncorrected SLR residuals based on raw SLR normal point observations are used. The correction value is added to the SLR measurement and counteracts the LRA phase center variations.

The advantages of this method are the continuous functional, which is easy to implement in existing software packages, as well as the avoidance of an interpolation between tabulated values. Furthermore, the differences between orbits determined with and without the LRA correction will be presented. Station coordinate time series and orbit comparisons with external T/P orbits are investigated in order to prove the high quality of the obtained LRA corrections.

How to cite: Zeitlhöfler, J., Bloßfeld, M., Rudenko, S., and Seitz, F.: Estimation of station-dependent LRA correction parameters for the TOPEX/Poseidon mission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12361, https://doi.org/10.5194/egusphere-egu21-12361, 2021.

Precise orbits of altimetry satellites are a prerequisite for the investigation of global, regional, and coastal sea levels together with their changes, since accurate orbit information is required for the reliable determination of the water surface height (distance between the altimeter position in space and the water surface). Orbits of altimetry satellites are nowadays usually computed using DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), SLR (Satellite Laser Ranging), and, of some satellites, GPS (Global Positioning System) observations of a global network of tracking stations. Significant progress in the improvement of altimetry satellite orbit quality has been achieved in the last 30 years. However, the differences of the sea level and its trend computed using up-to-date orbit solutions derived at various institutions using different software packages, types of observations (DORIS+SLR as compared to GPS+DORIS) and different up-to-date models still exceed the requirements of the Global Climate Observing System for the uncertainties of the regional sea level (< 1 cm) and its trend (< 1 mm/year).

In this study, we evaluate the current accuracy of orbits of altimetry satellites derived by various institutions in the state-of-the-art reference frames using up-to-date background models for precise orbit determination by using various observation types. We present some results of our analysis of geographically correlated errors and radial orbit differences for various orbit solutions. We also discuss possible reasons causing the orbit differences and potential ways to reduce them.

How to cite: Rudenko, S., Dettmering, D., Bloßfeld, M., Zeitlhöfler, J., and Alkahal, R.: On the current accuracy of altimetry satellite orbits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12148, https://doi.org/10.5194/egusphere-egu21-12148, 2021.

The Copernicus Sentinel-1 SAR (Synthetic Aperture Radar) mission consists of two satellites A and B launched in April 2014 and April 2016, respectively. The Copernicus POD (Precise Orbit Determination) Service is responsible for the generation of precise orbital products of the mission requiring a high orbit accuracy of 5 cm in 3D RMS in the comparison to external processing facilities.

The operational POD setup at the Copernicus POD Service has passed through several updates during the last years. For instance the ITRF update from ITRF08 to ITRF14 at the end of January 2017, the fundamental background model update in May 2020, and the switch to updated GPS antenna reference point coordinates together with the introduction of carrier phase ambiguity fixing at the end of July 2020 have been done to mention just the major changes in the processing. To provide a homogeneous and up-to-date orbit time series for the two satellites a reprocessing of the full mission period is done.  

The quality control of the reprocessed Copernicus Sentinel-1 orbits is done by analysing processing metrics and by comparing the results to orbits, which were independently reprocessed by members of the Copernicus POD Quality Working Group (QWG).

Results from the full Copernicus Sentinel-1 POD reprocessing campaign are presented together with the accuracy and quality assessment of the orbits.

How to cite: Peter, H., Fernández, M., Arnold, D., Duan, B., Simons, W., Gini, F., Wermuth, M., Hackel, S., Fernández, J., Jäggi, A., Hugentobler, U., Visser, P., Zandbergen, R., and Féménias, P.: Copernicus POD Service - Orbit reprocessing for Copernicus Sentinel-1 satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5296, https://doi.org/10.5194/egusphere-egu21-5296, 2021.

A classical reduced-dynamic GPS-based Precise Orbit Determination (POD) strategy for Low Earth Orbit (LEO) satellites is often based on a limited explicit modelling of satellite dynamics and modelling deficiencies are compensated by numerous empirical parameters. With better gravitational models and the advances in satellite surface force modeling, uncertainties in the satellite dynamics are significantly reduced. Furthermore, single-receiver ambiguity resolution allows for more robust POD as well. Therefore, a dynamic POD strategy using  significantly fewer estimated empirical parameters can be implemented to generate dynamic orbits, which allow for force modeling sensitivity analyses and evaluating potential errors in the adopted GPS antenna reference points or phase center offsets, etc.

This presentation outlines the recent dynamic POD methodology developments at the Astronomical Institute of the University of Bern (AIUB) and investigates the POD performances for a few dedicated space geodesy satellite missions (Swarm, GRACE-FO, Sentinel-1, Sentinel-2, Sentinel-3 and Jason-3) that are operated at altitudes ranging from 430 to 1350 km. The focuses will be on satellite gravitational and non-gravitational force modeling, satellite dynamics parametrization, and orbit validations for different types of satellites. Results reveal that the dynamic POD strategy is flexible and robust to generate high-quality orbits for those satellites, showing reliable agreements with the independent ambiguity-fixed kinematic orbits and the external Satellite Laser Ranging (SLR) measurements.

How to cite: Mao, X., Arnold, D., Kobel, C., Villiger, A., and Jäggi, A.: GPS-based dynamic orbit determination for low Earth orbit satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6644, https://doi.org/10.5194/egusphere-egu21-6644, 2021.

The Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2) mission was launched on June 25, 2019 into six evenly spaced circular orbital planes of 24° inclination with initial altitudes of 725 km. By February 2021 the COSMIC-2 satellites will be lowered to an operational altitude of about 520 km. The satellites carry an advanced Tri‐GNSS (Global Navigation Satellite System) Radio-occultation System (TGRS) instrument to provide high vertical resolution profiles of atmospheric bending angle and refractivity, as well as measurements of ionospheric total electron content, electron density, and scintillation. The TGRS payload tracks GPS and GLONASS signals on two upward looking antennas used for precise orbit determination (POD). We compute one- and two-antenna GPS and GPS+GLONASS POD solutions at both orbit altitudes and assess the orbit quality and systematic orbit errors using different metrics. In particular, we also use different POD setups to compute kinematic solutions employing single-receiver ambiguity fixing and test their contribution to selected months of gravity field recovery based on Swarm GPS data.

How to cite: Jaeggi, A., Arnold, D., Weiss, J., and Hunt, D.: Assessment of COSMIC-2 reduced-dynamic and kinematic orbit determination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12027, https://doi.org/10.5194/egusphere-egu21-12027, 2021.

Global Navigation Satellite Systems such as the Global Positioning System (GPS) are a unique tool for deriving very precise orbits of Low Earth orbiting (LEO) satellites equipped with onboard GPS receivers. LEO precise orbit determination (POD) requires the proper modeling of antenna phase center variations (PCVs) for both the GPS transmitter and the LEO receiver antennas. While for the GPS antennas the nadir-dependent values from the official absolute antenna phase center model igs14.atx of the International GNSS Service (IGS), consistent with the underlying GPS orbit and clock products, are used, official PCV maps are usually not available for the LEO receiver antennas. If these variations are not considered, however, this may result in systematic errors in the derived LEO orbits. LEO PCV maps can be determined and exploited in different ways. One possibility is to use the PCV maps from ground calibrations provided by the manufacturer, which usually do not reflect, however, the influence of error sources which are additionally encountered in the actual spacecraft environment, e.g., near-field multipath. Alternatively, one can make use of GPS measurements and POD results to estimate the PCV map empirically, as it is done in this study.

In this study, the influence of different attitude modes on Jason-3 POD using GPS observations and PCV map estimation is investigated. As Jason-3 in an altimetry satellite, its main objective is to measure global sea-level rise. Therefore, it is of particular importance to precisely determine the radial component of the orbit and proper PCV modeling is of high importance. As Jason-3 is experiencing different attitude modes, yaw-steering and fixed-yaw attitude with either the positive or negative x-axis pointing in the direction of flight, PCV maps are expected to be better disentangled from other error sources. In this study, we are analyzing PCV maps determined from residual stacking using GPS data from the different attitude modes and from different orbit parametrizations. First results indicate that PCV maps estimated from time spans of different attitude modes differ and systematic orbit differences are occurring in a reduced-dynamic POD.

How to cite: Kobel, C., Arnold, D., and Jäggi, A.: Impact of different attitude modes on Jason-3 precise orbit determination and antenna phase center modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4831, https://doi.org/10.5194/egusphere-egu21-4831, 2021.

In December 2018 and April 2019, two 3-unit cube satellites of the company Astrocast were launched into orbit. Both satellites are equipped with our low-cost single-frequency multi-GNSS payload board, which provides almost continuous on-board receiver solutions containing the position from GNSS code observations and the velocity from Doppler measurements. We make use of these independent observation types (positions and velocities) to identify and analyse systematic biases in the receiver solution. Therefore, we estimate the parameters of a dynamic orbit model using three different approaches: fitting the orbit model (1) to the positions only, (2) to the velocities only and (3) to both, positions and velocities.

After removing outliers, the position residuals from the position-only approach are at a level of about 5 m, the velocity residuals from the velocity-only approach at about 15 cm/s. When computing the positions with the velocity-only approach, however, the residuals are much larger and show a once-per revolution periodicity with amplitudes of up to 40 m. Besides that, we identify two offsets in the residuals which are independent of the observation type: a radial position bias of -3 m and an along-track velocity bias of -1.2 cm/s. Additionally, we observe two offsets which are dependent on the observation type: an along-track offset of 13 m in the position residuals when using the velocity-only approach and a radial offset of 1.3 cm/s in radial velocities when using the position-only approach.

The periodicity in radial and along-track direction is related to the orbit eccentricity and may be due to a general deficiency, when using velocities to estimate geometric orbit parameters. When comparing the orbits from the position-only and the velocity-only approach, we find an offset in the right ascension of the ascending node, which corresponds to a maximum cross-track position difference of 40 m at the equator. We show that this effect is caused by a periodic bias in the velocity solutions with a maximum at the poles. A possible cause for such a periodicity in the velocity solutions may be dynamic effects in the receiver tracking loops related to the LEO satellite velocity relative to the GNSS constellation, which can vary strongly within one revolution.

Our results show that both, the radial position offset and the along-track velocity offset are dependent on the altitude of the satellite and are likely to be caused by ionospheric refraction. The explanation for the along-track position offset and the along-track velocity offset, however, is not that obvious. We found that these two offsets are geometrically related and, thus, must have the same physical cause. Based on the combined position-and-velocity approach we demonstrate that they originate from a velocity bias rather than from a position bias. To explain the physical cause of such a radial velocity offset, we will study the ionospheric effects on GNSS code and Doppler measurements in more detail, where we use a 3D-ionosphere model and take also the altitude of the two satellites into account.

How to cite: Müller, L., Rothacher, M., and Chen, K.: Systematic biases in the on-board navigation solution of a CubeSat GNSS payload board , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7660, https://doi.org/10.5194/egusphere-egu21-7660, 2021.

Besides the natural extra-galactic radio sources, observing an artificial Earth-orbiting radio source with the Very Long Baseline Interferometry (VLBI) permits to extend the geodetic and geodynamic applications of this highly accurate interferometric technique. Furthermore, combining aforementioned observations provides a promising method to determine the satellite orbit and delivers the new type of observations such as group delay and delay rate which might be employed to validate the orbit independent of other space geodetic techniques.

In this research, the potential of the interferometric satellite tracking for the Precise Orbit Determination (POD) has been explored based on simulated observations for different scenarios with various VLBI networks, satellite orbits (eccentric low Earth orbits or circular medium Earth orbits) and error sources. POD of the Earth-orbiting satellites is studied on the basis of daily VLBI sessions where satellite observations are scheduled together with the quasar observation for regionally or globally distributed legacy as well as next generation VLBI station networks. In order to simulate VLBI to satellite observations, the influence of the most prominent random error sources in VLBI as well as mismodelling of different force models acting on the satellite are utilized. This study indicates that POD is feasible with VLBI observations and the accuracy mainly depends on the observation geometry.

How to cite: Mammadaliyev, N., Schreiner, P., Glaser, S., Neumayer, K. H., Koenig, R., Heinkelmann, R., and Schuh, H.: Potential of VLBI observations to satellites for precise orbit determination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12458, https://doi.org/10.5194/egusphere-egu21-12458, 2021.

Precise orbit determination (POD) of LEO satellites is done with a geodetic grade GPS receiver measuring carrier-phase between a LEO and GPS satellites, and in some cases this is supported with a DORIS instrument measuring Doppler between LEO and ground DORIS stations. Over the last 20 years we have demonstrated 1-2 cm accurate LEO POD and about 1 mm for inter-satellite distance. In order to increase the accuracy of the single satellite POD or satellites in LEO formation we propose an “optical GNSS receiver”, a cw-laser on a LEO satellite to measure Doppler between a LEO and GNSS satellite(s) equipped with SLR arrays and to develop it for the next gravity field mission.      

The objective of the ESA mission NGGM-MAGIC (Next Generation Gravity Mission - Mass-change and Geosciences International Constellation) is the long-term monitoring of the temporal variations of Earth’s gravity field at high resolution in time (3 days) and space (100 km), complementing the GRACE-FO mission from NASA at 45° orbit inclination. Currently, the GRACE-type mission design is based on optical carrier-phase measurements between two LEO satellites flying in a formation and separated by 200 km.

We propose an extension of the GRACE-type LEO-LEO concept by the “optical GNSS receiver” to provide Doppler measurements between a LEO satellite and GNSS satellite(s) equipped with SLR corner cubes by means of a cw-laser onboard a LEO satellite. Such a “vertical” LEO-GNSS observable is missing in the classical GRACE-type LEO-LEO concept. If Doppler measurements are carried out from the two GRACE-type satellites in the LEO orbit to the same GNSS satellite and by forming single-differences to that GNSS satellite one can remove any GNSS-orbit related error in the measured LEO-GNSS Doppler. In this way, radial orbit difference can be obtained between the two GRACE-type satellites (free of all GNSS orbit errors) and complement “horizontal” LEO-LEO measurements between the two GRACE-type satellites in the LEO orbit.

The non-mechanical laser beam steering has been developed for an angle window of -40° to +40° and it does not require a rotating and a big telescope in LEO (no clouds and atmosphere turbulences in LEO). Therefore, in such a beam-steering window, one could always observe with a fiber cw-laser one GNSS satellite close to the zenith from both GRACE-type satellites. The non-mechanical beam steering concept in zenith direction can be supported by a small 10-cm like (fixed) Ritchey-Chrétien telescope (COTS), a Cassegrain reflector design widely used for LEO satellites, e.g., for James Webb Space Telescope or for an optical Earth imaging with Cubesats with the 50 cm resolution.

Considering that several GNSS satellites in the field of view could be observed from a LEO satellite with this approach (including LAGEOS-1/2 and Etalon satellites) and the non-mechanical laser beam steering could be extended towards the LEO horizon, an “optical” GNSS receiver is a new concept for POD of LEO satellites. Here, we provide simulations of this new concept for LEO POD with GNSS/SLR constellations equipped with SLR arrays and discuss all new applications this new concept could bring.

How to cite: Svehla, D.: Optical GNSS Receiver for the ESA's NGGM-MAGIC Mission and for LEO Satellites with the Highest Orbit Accuracy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9072, https://doi.org/10.5194/egusphere-egu21-9072, 2021.