G2.1

G2 EDI

Precise orbit determination is of central importance for many applications of geodesy and earth science. The challenge is to determine satellite orbits in an absolute sense at the centimeter or even sub-centimeter level, and at the millimeter or even sub-millimeter level in a relative sense. Four constellations of GNSS satellites are available and numerous position-critical missions (e.g. altimetry, gravity, SAR and SLR missions) are currently in orbit. Altogether, outstanding data are available offering new opportunities to push orbit determination to the limit and to explore new applications.

This session aims to make accessible the technical challenges of orbit determination and modelling to the wider community and to quantify the nature of the impact of dynamics errors on the various applications. Contributions are solicited from, but not limited to, the following areas: (1) precise orbit determination and validation; (2) satellite surface force modelling; (3) advances in modelling atmospheric density and in atmospheric gravity; (4) advances in modelling earth radiation fluxes and their interaction with space vehicles; (5) analysis of changes in geodetic parameters/earth models resulting from improved force modelling/orbit determination methods; (6) improvements in observable modelling for all tracking systems, e.g. SLR, DORIS, GNSS and their impact on orbit determination; (7) advances in combining the different tracking systems for orbit determination; (8) the impact of improved clock modelling methods/space clocks on precise orbit determination; (9) advances in modelling satellite attitude.

Convener: Adrian Jaeggi | Co-conveners: Alexandre CouhertECSECS, Urs Hugentobler, Oliver Montenbruck, Heike Peter
Presentations
| Thu, 26 May, 15:55–18:24 (CEST)
 
Room -2.16

Presentations: Thu, 26 May | Room -2.16

Chairpersons: Adrian Jaeggi, Oliver Montenbruck, Urs Hugentobler
15:55–16:02
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EGU22-7653
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On-site presentation
Florian Dilssner, Francisco Gonzalez, Erik Schönemann, Tim Springer, and Werner Enderle

The Y-bias as present on most global navigation satellite system (GNSS) spacecraft plays an important role in precise orbit determination and prediction. Accurate knowledge about the Y-bias and its temporal variability is particularly relevant for the Galileo system in order to fulfil its once-in-a-lifetime station-keeping maneuver requirements. Despite the widely recognized importance, however, no consensus has been reached on the physical mechanism that is responsible for the Y-bias. In this presentation, we shed light on the origins of the Galileo Y-bias using temperature and attitude data series from spacecraft telemetry to analytically determine Y-bias time histories for different Galileo satellites. We start by calculating the thermal radiation pressure forces generated by the two surface radiators at the main body's +Y and -Y sides of satellite GSAT0204 over a period of five years, from the activation of the spacecraft's search and rescue payload in early 2016 to the deactivation of its navigation payload in December 2017 and beyond. The net force from both radiators yields the Y-bias as it evolves over time, with some striking discontinuities due to abrupt changes in the amount of dissipated heat after the payload units have been turned on or off. Comparison against empirical Y-bias estimates from satellite laser ranging long arc analyses proves the correctness of our Y-bias model. In addition, we report on yearly variations in the Y-bias acceleration of GSAT0101 between -0.10 nm/s² and +0.05 nm/s², leading to a secular increase in the satellite orbit's semi-major axis since January 2016. Yaw error measurements from the spacecraft's fine sun sensor (FSS) spanning 2016-2019 provide compelling evidence that these Y-bias variations originate from an attitude-related mispointing of the satellite's solar panels by a few tenths of a degree. Least square fitting of the FSS measurements led to the development of a refined yaw model for GSAT0101. As a result of this new model, estimates of the Y-bias parameter are significantly reduced in magnitude and less dependent upon the position of the sun relative to the orbit plane. Overall, our analyses provide the first hard evidence that the Galileo Y-bias is primarily of thermal origin and, contrary to popular belief, that solar panel orientation errors only play a secondary role. The implications for precise orbit determination will be discussed. In addition, our results confirm the long-standing hypothesis that Y-bias and solar panel orientation error are linearly related.

How to cite: Dilssner, F., Gonzalez, F., Schönemann, E., Springer, T., and Enderle, W.: GNSS Satellite Force Modeling: Unveiling the Origins of the Galileo Y-bias, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7653, https://doi.org/10.5194/egusphere-egu22-7653, 2022.

16:02–16:09
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EGU22-2832
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ECS
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On-site presentation
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Bingbing Duan and Urs Hugentobler

An accurate model of all the forces acting on a satellite is an essential precondition of achieving high orbit accuracy. Solar radiation pressure (SRP), the largest non-gravitational perturbation for GNSS satellites is typically modeled by an empirical model (i.e., Empirical CODE Orbit Model, ECOM/ECOM2). If satellite metadata information is available, an analytical box-wing model can be formed to reinforce the ECOM models. However, the current GNSS satellite orbits show notable degradation during eclipse seasons in particular for long-arc solutions and orbit predictions. The reason is proven to be mostly due to the ignoring of the thermal imbalanced forces (i.e., radiator emission and thermal radiation of solar panels). The ECOM parameters can compensate these thermal radiation forces fairly well outside eclipse seasons, while this is not true when satellites are inside eclipse seasons, because the Earth’s shadowing of a satellite in orbit causes periodic changes of the thermal environment. On one hand, these thermal imbalanced forces contribute also inside the shadow while inside the shadow all the ECOM parameters are deactivated. On the other hand, satellite attitude could be far from the nominal inside the shadow, making that these thermal imbalanced forces cannot be well absorbed by the ECOM parameters. To capture these thermal forces, we set up physical thermal force models for each Block type of GNSS satellites. In the absence of published thermal properties, we estimate necessary thermal modeling parameters using tracking data over long time period. With the use of the physical thermal force models, satellite orbits inside eclipse seasons are greatly improved. For instance, orbit misclosures are improved by a factor of two for BDS-3 and Galileo satellites when using the 5-parameter ECOM model.

How to cite: Duan, B. and Hugentobler, U.: Impact of thermal imbalanced radiation forces on GNSS satellite orbits, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2832, https://doi.org/10.5194/egusphere-egu22-2832, 2022.

16:09–16:16
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EGU22-5884
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Virtual presentation
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David Lucchesi, Marco Cinelli, Alessandro Di Marco, Emiliano Fiorenza, Carlo Lefevre, Pasqualino Loffredo, Marco Lucente, Carmelo Magnafico, Roberto Peron, Francesco Santoli, Feliciana Sapio, and Massimo Visco

G4S_2.0 is a project funded by the Italian Space Agency aiming to perform a set of Fundamental Physics measurements using the two Galileo FOC satellites GSAT0201 (Doresa) and GSAT0202 (Milena). Indeed, the orbits of these satellites are characterized by a relatively high eccentricity, about 0.16, which represents a good prerequisite for a series of tests and measurements concerning the predictions of different theories of gravitation, as compared with the General Relativity (GR) ones. The main objectives include a new measurement of the gravitational redshift effect of the on-board atomic clocks --- thanks to its modulation with the orbital period due to the high eccentricity of the orbits --- and the measurement of the main precessions of relativistic origin, primarily the Schwarzschild one.

To achieve these significant results, and possibly improve the current constraints of several theories of gravitation with respect to GR, it is of fundamental importance to take a step forward --- compared to the state of the art --- in the reliability of the dynamic model used for the orbits of the satellites and, as a direct consequence of this, in their precise orbit determination (POD). In this context, non-gravitational perturbations (NGPs) are the most subtle and difficult to model because of the complex shape of the Galileo satellites and their attitude law. In this regard, the main challenge is represented by a more refined and reliable model for the direct solar radiation pressure (SRP), the largest NGP on Galileo satellites, as well as on every satellite of every GNSS constellation.

Our final goal is to build a finite element model (FEM) of the Galileo FOC spacecraft, as refined as possible, and apply a dedicated raytracing technique to it to compute the perturbing accelerations due to radiation pressure. In view of this, we have already developed a 3D-CAD model of the spacecraft. As an intermediate step, we have built a Box-Wing (BW) model based on the relatively poor information presently available on the geometrical and physical properties of the spacecraft. This BW model has been used to compute the perturbing accelerations due to the direct SRP and to the Earth's albedo and infrared radiation.

The results obtained for the accelerations, to be included in the POD process, will be presented in various cases. Then, by computing the residuals in the orbital elements, it will be possible to verify the goodness of the POD results and observe the expected progressive improvement starting from the BW model towards the FEM one. The present analyses were made using the nominal attitude law of the Galileo FOC spacecraft; the application of this law will be discussed in the case of satellites in elliptical orbit. We finally highlight that the results of G4S_2.0 in terms of POD improvements are particularly useful for all applications of the Galileo FOC satellites in the fields of space Geodesy and Geophysics.

How to cite: Lucchesi, D., Cinelli, M., Di Marco, A., Fiorenza, E., Lefevre, C., Loffredo, P., Lucente, M., Magnafico, C., Peron, R., Santoli, F., Sapio, F., and Visco, M.: The Galileo for Science (G4S_2.0) project: Precise Orbit Determination for Fundamental Physics and Space Geodesy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5884, https://doi.org/10.5194/egusphere-egu22-5884, 2022.

16:16–16:23
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EGU22-6796
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ECS
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Virtual presentation
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Shi Huang, Yongqiang Yuan, Keke Zhang, and Xingxing Li

The constellation of China’s BeiDou navigation satellite system (BDS) has been fully constructed since July 2020 and provides open services for worldwide users. Due to the natural sensitivity of satellite technique to geocenter motion, BDS has the capability to determine the time series of geocenter coordinates (GCCs). The purpose of this study is to assess the impact of solar radiation pressure (SRP) modeling on the BDS-derived geocenter motion. To that end, 3-year sets of daily GCCs have been determined with data of BDS. The data was recorded over the period 2019-2021 by a global network of 93 iGMAS stations. Different SRP models including the empirical CODE orbit model (ECOM/ECOM2) and the a prior box-wing model have been applied for BDS geocenter estimation, respectively. We find that under the purely empirical SRP model, the peak-to-peak amplitude of geocenter z-coordinates (GCC-Zs) can reach to 10 cm. In additional, IGSOs would bring obvious jumps to GCC-Zs during earth eclipse periods. The introduction of an a priori box-wing model can largely mitigate the spurious signals in the spectra of GCC-Zs, presenting (13.0, 4.5, 2.1, 2.4) mm for the amplitude of the 1, 3, 5, 7 cpy signals, compared to (26.2, 5.9, 1.2, 2.0) mm in the ECOM case. However, the jumps brought by IGSOs still remains, which could be caused by distortion of optical properties. Therefore, we simultaneously estimate the optical properties together with other parameters in the processing. This model, known as a prior adjustable bow-wing model (ABW), appears to improve the orbit modeling in the eclipsing season and eliminate the negative influence of IGSOs on GCC-Zs, which is reflected in the decrease of spurious signal at periods other than annual one and the amplitude of the 1, 3, 5, 7 cpy signals for GCC-Zs are (16.2, 3.8, 1.4, 0.3) mm. The ABW solution is thus closer to the geocenter motions determined with other space-geodetic techniques.

How to cite: Huang, S., Yuan, Y., Zhang, K., and Li, X.: Geocenter motions derived from BDS: The impact of solar radiation force model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6796, https://doi.org/10.5194/egusphere-egu22-6796, 2022.

16:23–16:30
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EGU22-2383
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ECS
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On-site presentation
Hanane Ait-Lakbir, Alvaro Santamaria, Félix Perosanz, and Jim Ray

Day-boundary orbit comparison is one of the criteria used to assess the performance of GNSS orbits. The overall statistics of orbit discontinuities such as RMS are usually computed to assess dynamical modeling and the processing configurations. Additional information about the systematic orbit errors is also accessible through their spectral content.

A particular feature is the flicker or 1/f noise describing the low-frequency band, indicating time-correlated orbital errors. This type of noise is observed not only in the orbits but also in other GNSS-derived geodetic time series such as in station positions, Earth rotation parameters,... The sources explaining this feature, either from the GNSS orbit modeling or from unaccounted orbital perturbations, are not well understood. By computing simulated orbits, we look at possible causes in the orbit determination processing.

How to cite: Ait-Lakbir, H., Santamaria, A., Perosanz, F., and Ray, J.: Investigation of the flicker nature of the day-boundary differences of GNSS orbits, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2383, https://doi.org/10.5194/egusphere-egu22-2383, 2022.

16:30–16:37
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EGU22-6929
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ECS
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On-site presentation
Angel Navarro Trastoy, Sebastian Strasser, Lauri Tuppi, Maksym Vasiuta, Sanam Motlaghzadeh, Markku Poutanen, Torsten Mayer-Gürr, and Heikki Järvinen

Neutral gas atmosphere bends and delays propagation of microwave signals in satellite-based navigation. Weather prediction models can be used to estimate these effects by providing 3-dimensional refraction fields for signal delay computation. In this study, a global numerical weather prediction model (Open Integrated Forecasting System (OpenIFS) licensed for Academic use by the European Centre for Medium-Range Weather Forecast) is used to generate the refraction fields. The slant delays are produced using a Least Travel Time (LTT) ray-tracer. Finally, the GNSS satellite orbits are solved using the GROOPS (Gravity Recovery Object Oriented Programming System) software toolkit of the Technical University of Graz which applies the raw observation method. Specifically, our implementation supplies the slant delays directly to the orbit solver without an intermediate mapping step, i.e., mapping of zenith delay to a prescribed functional form of azimuth and elevation angles. Essentially, this removes the assumption that signal delays follow some functional form, and allows hence to take full advantage of local refraction field asymmetries in GNSS signal processing that are partially lost in the mapping procedure. Our results indicate that this has clear benefits, both in terms of accuracy of the tropospheric correction and stream-lining the information flow in GNSS processing. Our view is that this new framework exposes the synergies in space geodesy and meteorology better than the earlier approaches.

How to cite: Navarro Trastoy, A., Strasser, S., Tuppi, L., Vasiuta, M., Motlaghzadeh, S., Poutanen, M., Mayer-Gürr, T., and Järvinen, H.: Tropospheric corrections in GNSS orbit determination without the mapping step, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6929, https://doi.org/10.5194/egusphere-egu22-6929, 2022.

Coffee break
Chairpersons: Adrian Jaeggi, Oliver Montenbruck, Alexandre Couhert
17:00–17:07
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EGU22-1336
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ECS
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Presentation form not yet defined
Modernizing Canadian Geodetic Survey’s precise GNSS orbit and clock system
(withdrawn)
Mohammad Ali Goudarzi
17:07–17:14
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EGU22-794
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On-site presentation
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Zhiguo Deng, Jungang Wang, and Maorong Ge

Global Navigation Satellite Systems (GNSS) play a critical role for providing real-time positioning and navigation services, and the precise satellite orbit and clock products are essential for the high-precision GNSS applications. The International GNSS Service (IGS) and its Analysis Centers (ACs) have been working on the study on precise GNSS data processing and provision of the precise products. The German Research Center for Geosciences (GFZ), as one of the ACs, also provides the multi-GNSS rapid products: the GBM product. We introduce the GBM data processing strategy, analyze the precision of GBM multi-GNSS orbits from 2015 to 2021, and present the impact of applying the undifferenced ambiguity resolution on satellite orbits. The GPS orbits of GBM products agree with the IGS final orbits at the level of 11-13 mm in the three directions, and the GPS orbit 6-hour prediction precision is around 6 cm. The 6-hour prediction precision of GLONASS is around 12 cm, slightly worse than that of GALILEO, which hold an average value of 10 cm in the same period but shows a significant improvement to around 5 cm after end of 2016. The prediction precision of BDS MEO satellites are around 10 cm, and that of the BDS GEO satellites and QZSS satellites are at the level of 1 to 3 meter. The Satellite Laser Ranging (SLR) residuals show that the orbit precision of GALILEO, GLONASS, and BD3-MEO is 23 mm, 41 mm, and 47 mm, respectively. Moreover, comparing the double-differenced ambiguity resolution, adopting the undifferenced ambiguity resolution improves the 6-hour orbit prediction precision by 9-15%15-18%11-13%6-17%14-25% for the GPS, GLONASS, GALILEO, BDS-2 and BDS-3 MEO satellites, respectively.

How to cite: Deng, Z., Wang, J., and Ge, M.: The GBM Rapid Product and the Improvement from Undifferenced Ambiguity Resolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-794, https://doi.org/10.5194/egusphere-egu22-794, 2022.

17:14–17:21
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EGU22-4834
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ECS
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On-site presentation
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Helene Wolf, Johannes Böhm, Axel Nothnagel, Urs Hugentobler, and Matthias Schartner

Observing Earth-orbiting satellites additionally to natural extra-galactic radio sources with Very Long Baseline Interferometry (VLBI) radio telescopes offers a variety of new possibilities and allows expanding the research activities of this highly accurate technique. The combination of observations to satellites and quasars permit the determination of the satellite orbit from VLBI observations in the terrestrial as well as in the International Celestial Reference Frame. The latter is enabled by the unique capability of VLBI to determine Universal Time UT1.

In this contribution for the first time, the precision of short satellite orbital arcs determined with simulated VLBI observations to Galileo satellites for different observation geometries using various VLBI networks and arc lengths is investigated. For this purpose, schedules including both, observations to quasars and satellites, are created using the scheduling software VieSched++. The simulations of the scheduled observations and the estimation of the satellite arcs are carried out using the Vienna VLBI and Satellite Software (VieVS). The quality of the estimated orbits is investigated and assessed based on the mean formal errors and the repeatabilities of the individual components of the satellite positions based on Monte Carlo simulations. 

How to cite: Wolf, H., Böhm, J., Nothnagel, A., Hugentobler, U., and Schartner, M.: Precision of Galileo satellite orbits obtained from simulated VLBI observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4834, https://doi.org/10.5194/egusphere-egu22-4834, 2022.

17:21–17:28
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EGU22-4985
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Virtual presentation
Jaime Fernandez Sanchez, heike Peter, Marc Fernandez, Pierre Femenias, and Yago Andres

The Copernicus POD (Precise Orbit Determination) Service is a consortium responsible for providing orbit 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. Products and data are also made available to external users through the ESA Copernicus Open Access Hub.

Sentinel-6 Michael Freilich launched in November 2020 is the newest satellite in the Copernicus POD Service operations. A near real-time orbit product computed based on GNSS data is delivered to EUMETSAT, acting as backup the DORIS DIODE aboard. For the first time, a combined GPS and Galileo receiver is used for POD. In addition, the satellite is equipped with a DORIS receiver and a Laser Retro Reflector for Satellite Laser Ranging (SLR). Additional GPS observations usable for POD are delivered from the POD antenna of the GNSS-RO (radio occultation) instrument. All these observations allow for various cross-comparisons between orbits from the different observation techniques and instruments. The quarterly and yearly Regular Service Reviews include validation of post-processed Sentinel-6 orbit solutions from various members of the Copernicus POD Quality Working Group (QWG).  

This contribution focuses on post-processed POD based on the combined GPS & Galileo receiver and validation with SLR done at the Copernicus POD Service. Precise orbits may be derived as single-system or combined solutions. Integer ambiguity resolution is a key technique to obtain highest accuracy orbits.

Precise orbit determination results from GPS-only, Galileo-only and combined GPS & Galileo observations with resolved integer ambiguities are presented. Cross-comparison between the different solutions, SLR validation, and comparison to other orbit solutions provided by members of the Copernicus POD QWG are shown and analysed.

How to cite: Fernandez Sanchez, J., Peter, H., Fernandez, M., Femenias, P., and Andres, Y.: Sentinel-6 Orbit Determination at the Copernicus POD Service, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4985, https://doi.org/10.5194/egusphere-egu22-4985, 2022.

17:28–17:35
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EGU22-7136
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ECS
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Virtual presentation
Cyril Kobel, Daniel Arnold, and Adrian Jäggi

The Copernicus Sentinel Earth observation satellites provide crucial earth observation measurements, e.g., sea surface-height. It is of highest importance that the underlying precise orbit determination (POD) of these low Earth orbiters (LEOs) is of high accuracy. The POD is based on observations from Global Navigational Satellite Systems (GNSS). All Sentinel satellites collect measurements from the Global Positioning System (GPS), whereas Sentinel-6A additionally collects measurements from the Galileo system. To achieve highly accurate POD, it is of crucial importance to have exact knowledge of the phase center position of the LEO receiver antenna for both the GPS and Galileo measurements. The phase center position is composed of the antenna reference point (ARP) and frequency-dependent phase center offsets (PCOs) and phase center variations (PCVs). It is known that the pre-launch characterization of the LEO receiver antennas is difficult and corresponding estimates are therefore less precise than those of the ARP. This makes it necessary to apply in-flight determined corrections to the initial pre-launch values of the PCO.

Previous studies have shown that there are deficits in the PCOs of the Sentinel-1 GPS antennas. For example, different estimates of empirical orbit parameters of similar satellites point to such deficits. The aim of this study is to determine corrections to the currently used PCOs of the Sentinel-1,2,3 and 6A satellites and to investigate their variability and reliability. Initial results show that non-negligible corrections result for the PCOs of the satellites studied.

The estimation of the corrections of the PCOs is performed as part of the POD process, which is performed with the Bernese GNSS software. The application of single receiver ambiguity resolution is necessary because it improves the stability of the estimated PCOs. It is of high importance that the modeling of non-gravitational forces acting on the satellite is as accurate as possible because modeling deficits may degrade the estimation of PCOs. The influence of such modeling deficits on the PCO estimation is investigated in this study. The estimation of PCO corrections can thus serve to not only get a better accuracy of observation modelling, but also to identify potential non-gravitational force modeling deficits.

Since the Sentinel-1,2,3 satellites are identical in construction in pairs (A and B), a direct comparison of the estimated corrections of the PCOs is possible. This can serve as a measure for the plausibility of the PCO correction estimation. Because the Sentinel-3 and Sentinel-6A satellites are altimetry satellites, the radial direction is of particular importance. Therefore, it is important to investigate the possible changes in radial levelling by applying corrections to the PCO. This can be done by analyzing Satellite Laser Ranging (SLR) measurements. The Sentinel-3 and Sentinel-6A satellites are equipped with SLR retroreflectors, which allows for SLR validations, which serves as a reliability test of the PCO correction estimations.

How to cite: Kobel, C., Arnold, D., and Jäggi, A.: Estimation of phase center offset corrections for Sentinel satellites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7136, https://doi.org/10.5194/egusphere-egu22-7136, 2022.

17:35–17:42
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EGU22-7335
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ECS
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Virtual presentation
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Xinyuan Mao, Daniel Arnold, and Adrian Jäggi

Low Earth orbiting (LEO) satellites require routine maneuvers to maintain the predefined trajectories. However, spaceborne scientific instruments might suffer from data discontinuities or even anomalies due to instantaneous orbit changes caused by the performed maneuvers. With the advances of spaceborne Global Navigation Satellite System (GNSS) technique, the high-low satellite-to-satellite tracking observations enable us to generate high precision satellite orbits for the nominal orbit operation periods, and more importantly, also for the maneuver periods. This research will outline the recent developments of Precise Orbit Determination (POD) for  maneuvering LEO satellites at the Astronomical Institute of the University of Bern (AIUB). The Sentinel-3 mission, an European Space Agency (ESA) Earth observation satellite formation devoted to oceanography and land-vegetation monitoring, is used as test example.

A prerequisite input for this research is the maneuver information collected by the telemetry measures which clarify the maneuver time span and accelerations. Due to unavoidable in-flight software delays and hardware performance accuracy, the maneuver information may not be perfect and needs to be improved  in the POD process. Essentially two solutions are made in this research: a. estimating the full accelerations or corrections to the known maneuver accelerations, b. estimating instantaneous velocity pulses at the requested epochs. Both algorithms are tested using the Bernese GNSS Software and POD performances for the maneuver days during 2018-2020 will be assessed. Results reveal that the post-fit carrier phase residuals can be significantly reduced, ensuring better internal consistency between the reduced-dynamic and kinematic orbit solutions. Besides, a few institution members from the Copernicus POD Quality Working Group (QWG) have been routinely generating orbit products for the maneuver days, allowing for the direct cross-validations with our new AIUB products. This research implies promising benefits to the Sentinel-3 POD and scientific research community.

How to cite: Mao, X., Arnold, D., and Jäggi, A.: Precise orbit determination for the maneuvering Sentinel-3 satellites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7335, https://doi.org/10.5194/egusphere-egu22-7335, 2022.

17:42–17:49
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EGU22-1614
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On-site presentation
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Ulrich Meyer, Heike Peter, Martin Lasser, and Adrian Jäggi

The precise orbit determination (POD) of Low Earth Orbiters (LEO), e.g. the Copernicus Sentinel Earth observation satellites, relies on the precise knowledge of the Earth gravity field and its variations with time. The most precise observation of time-variable gravity on a global scale is currently provided by the GRACE-FO satellites. But the monthly gravity field solutions are released with a latency of approx. 2 months, therefore they cannot be used for operational POD.

We present a deterministic signal model (DSM) that is fitted to the time-series of COST-G combined monthly gravity fields and describe the differences with respect to the available long-term gravity models including seasonal and secular time-variations. To validate the DSM, dynamic POD of the Sentinel-2B, -3B and -6A satellites is performed based on long-term or monthly gravity field models, and on the COST-G DSM. We evaluate the model quality on the basis of carrier phase residuals, orbit overlap analysis and independent satellite laser ranging observations, and study the limitation on orbit altitude posed by the reduced spherical harmonic resolution of the monthly models and the DSM.

The COST-G DSM is updated quarterly with the most recent GRACE-FO combined monthly gravity fields. It is foreseen to apply a sliding window approach with flexible window length to allow for an optimal adjustment in case of singular events like major earthquakes.

How to cite: Meyer, U., Peter, H., Lasser, M., and Jäggi, A.: The new COST-G deterministic signal model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1614, https://doi.org/10.5194/egusphere-egu22-1614, 2022.

17:49–17:56
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EGU22-774
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ECS
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Presentation form not yet defined
Updated inverse estimation of systematic errors in Earth radiation data from spaceborne accelerometery
(withdrawn)
Kristin Vielberg and Jürgen Kusche
17:56–18:03
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EGU22-7681
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ECS
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On-site presentation
Lukas Müller, Kangkang Chen, and Markus Rothacher

The number of low Earth orbit (LEO) satellites equipped with Global Navigation Satellite System (GNSS) receivers is rapidly increasing. GNSS observations in space are no longer limited to a small number of Earth observation satellites, but the rapid development of large nanosatellite constellations enables a dense network of GNSS observations around the Earth. An example of this is the Astrocast CubeSat constellation, to which we contribute with our low-cost multi-GNSS payload board. The first 10 satellites of the Astrocast constellation have successfully been launched on 24 January 2021 (5 satellites) and 30 June 2021 (5 satellites). Further Astrocast CubeSats equipped with dual-frequency GNSS receivers will be launched in the coming years, completing a constellation of 100 satellites by 2024.

The formation of a homogeneous and highly dynamic GNSS network in space holds great potential for geodetic Earth observation, as it has some advantages over a ground-based GNSS network and GNSS observations on board single or formation-flying satellites: A space-based GNSS network can be autonomously processed in a double-difference mode without the need for ground observations, thus, GNSS signals are not affected by tropospheric refraction, and it provides a better observation geometry improving the sensitivity to certain geodetic parameters. In this study, we investigate the feasibility of forming such a space-based GNSS network for estimating geodetic parameters, namely the orbit parameters of the LEO and GNSS satellites, the antenna phase center corrections of the GNSS satellites, and the low-degree coefficients of the Earth’s gravity field including the geocenter coordinates.

We consider 3 different constellation scenarios: (1) A LEO constellation of 36 satellites uniformly distributed over 6 orbital planes with an inclination of 55° and (2) the expected configuration of the complete Astrocast constellation, with sun-synchronous polar orbits and equatorial orbits. In both cases (1) and (2), the GNSS observations are simulated with the Bernese GNSS software based on the given orbit specifications. (3) In a third scenario, we use real GNSS observations from various existing Earth observation missions, including GRACE, OSTM/Jason-2 and Swarm, which are combined to a pseudo-constellation.

For each scenario, the number of possible GNSS single- and double-differences and the corresponding baseline lengths will be computed. Based on these observations, we will examine, how well carrier-phase ambiguities can be resolved and how this depends on the constellation configuration. With a network processing of GNSS double-difference observations, we will estimate concrete parameters related to the LEO orbits, the GNSS antenna phase center corrections and the Earth’s gravity field. To estimate the expected accuracy for these parameters, we examine their sensitivity to small errors in the observation data resulting from, e.g., the force model, once-per-revolution parameters, stochastic pulses or small accelerations like ocean tide or Earth albedo effects. Based on this research, we will draw conclusions about the potential of large satellite constellations to complement or replace the existing geodetic Earth observation missions in the future.

How to cite: Müller, L., Chen, K., and Rothacher, M.: Formation of a GNSS network in space based on LEO satellite constellations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7681, https://doi.org/10.5194/egusphere-egu22-7681, 2022.

18:03–18:10
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EGU22-1607
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ECS
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Virtual presentation
Xingchi He, Urs Hugentobler, Anja Schlicht, Yufeng Nie, and Bingbing Duan

Since 2010s, many companies such as SpaceX, OneWeb, Amazon and Samsung showed their interests to launch hundreds and even thousands of low Earth orbit (LEO) satellites for global internet service. Due to their unique characteristics compared to medium Earth orbit (MEO) and geostationary Earth orbit (GEO) satellites, these LEO mega-constellations soon draw much attention from the scientific community. Studies from constellation design, to applications such as positioning, ionosphere modelling and gravity recovery are investigated by many researchers.

Orbit determination is a key to many applications. Traditionally, onboard Global Navigation Satellite System (GNSS) receivers are used to determine LEO satellite orbits. However, with thousands of satellites in space in the future, an independent system without relying on GNSS is worth to be studied. Since these LEO satellites are intended for internet service, connections between the satellites and to the ground are available by nature. But how would the distribution of a station network affect the orbit accuracy? How many stations would be sufficient to determine a precise orbit? Besides observations from ground stations, inter-satellite link (ISL) is also proposed and implemented by many current GNSSs. It already showed its potential to improve the orbits. Could this technique also be applied to the orbit determination of LEO satellites?

This simulation study investigates the influence of ground station distribution to orbit determination, as well as the benefit from ISL observations. By using a constellation with 60 LEO satellites, we show that for regional station networks, a high latitude network leads to worse orbit accuracy than a middle or low latitude network. With the help of ISL observations, orbit errors reach the same level as a global station network. We further investigate the influence of different number of stations contained in the network. The results prove that although increasing the station number could improve orbits, the improvement is minimal when the global network contains more than 16 stations. While for a regional network, even with 60 stations, the orbit errors are 1.5 times larger than for a small global network with 6 stations. This proves that the ground station distribution is more important than the number of observations. Furthermore, if the ISL technique is adopted, even a regional station network with 16 stations could be sufficient to determine an accurate orbit.

How to cite: He, X., Hugentobler, U., Schlicht, A., Nie, Y., and Duan, B.: Influence of ground station network distribution on orbit accuracy of low Earth orbit (LEO) satellites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1607, https://doi.org/10.5194/egusphere-egu22-1607, 2022.

18:10–18:17
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EGU22-2215
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ECS
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Virtual presentation
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Amir Allahvirdi-Zadeh and Ahmed El-Mowafy

The use of CubeSats is expanding in space and earth science applications due to the low costs of building and the possibility of launching them in a large low-earth orbits (LEO) constellation. Such constellation can serve as an augmentation system for positioning, navigation and timing. However, real-time precise orbit determination (POD) is still one of the challenges for this application. Real-time reduced-dynamic POD requires more processing capability than what is available in current CubeSats, and the kinematic POD highly depends on the number and the quality of the signals from Global Navigation Satellite Systems (GNSS). In this study, an approach is proposed to increase the orbital accuracy by implementing the precise inter-satellite ranges in the Kinematic POD. The precise orbits of a set of CubeSats from the Spire Global constellation that are determined using the reduced-dynamic POD is to be used to generate the precise inter-satellite ranges. These ranges vary from hundreds to thousands of kilometres and are constrained in the relative kinematic POD between the tested CubeSats. The results, which depend on the length of the inter-satellite ranges, show the improvement of the orbital accuracy in all directions. An initial architecture for implementing such a method in a smart CubeSats constellation is proposed and the limitations and remedies are discussed.

How to cite: Allahvirdi-Zadeh, A. and El-Mowafy, A.: The impact of precise inter-satellite ranges on relative precise orbit determination in a smart CubeSats constellation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2215, https://doi.org/10.5194/egusphere-egu22-2215, 2022.

18:17–18:24
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EGU22-12288
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ECS
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On-site presentation
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Drazen Svehla

At the last EGU and AGU conferences, we have proposed and demonstrated the feasibility of a laser GNSS receiver in the LEO orbit in order to provide carrier-phase measurements on a CW laser between a LEO satellite and GNSS satellites equipped with SLR arrays. This is a novel approach in space geodesy for precise orbit determination (POD) of LEO satellites and the gravity field mapping from space. Considering that the wet delay in signal propagation is typically 67x smaller for optical than for microwaves, we have extended this laser GNSS receiver to laser occultation for atmosphere sounding where use of a modulation on a CW laser could be applied to combine this method with the GNSS radio-occultation (GNSS-RO). In that case, one could compare in LEO orbit microwave GNSS measurements and CW laser measurements between a LEO and GNSS satellites from the top of the atmosphere down to the clouds and the lower troposphere.

 

Here we propose to further extend the laser GNSS approach in space geodesy, and to demonstrate the combination of a CW laser and GNSS measurements with a ground parabolic antenna of about 60 cm diameter. The CW laser and the receiving photodiode is to be placed in the optical center and collocated with the phase center of the parabolic GNSS antenna. If the same parabolic mirror is used as an antenna to track laser and microwave GNSS measurements to a single GNSS satellite in the zenith direction, all geometry effects can be removed (geometry-free), ending up with the Galileo satellite clock and GNSS receiver clock parameter being the only parameters of such a geometry-free ground-to-space optical/microwave metrology link for Galileo. Considering that optical frequency of a CW laser, stabilized by an internal cavity, can be provided with the frequency stability of <7×10-16, it can be transformed into a microwave band (with frequency comb) and with the same level of stability used as a reference frequency of the GNSS receiver. Therefore, one can use optical frequency of a CW laser via microwave Galileo signal to compare frequency of Galileo satellite clocks or optical clocks in the timing labs. Atmosphere effects for optical band (CW laser) can be applied a priori, whereas for microwave GNSS, troposphere zenith delays (TZDs) need to be estimated with the noise level of about a few millimeters in the zenith direction. Therefore, by selecting one Galileo satellite, close to zenith from two optical clocks on the ground, all Galileo satellite-related errors will be removed including Galileo satellite clock parameter, and time and frequency of optical clocks could be compared at the 10-17 - 10-18 frequency uncertainty level. This opens up the possibility of using Galileo by the timing labs for the generation of the official time (TAI, UTC) and for metrology in space, along with the laser GNSS applications in LEO orbit for POD and atmosphere sounding that very nicely complement the microwave GNSS.

How to cite: Svehla, D.: Laser GNSS Receiver for LEO POD, Laser Occultation and Time & Frequency Transfer of Optical Clocks in the Timing Labs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12288, https://doi.org/10.5194/egusphere-egu22-12288, 2022.