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For microwave-based space geodetic techniques, co-located instruments within a few hundred meters are considered to share the same troposphere, and thus intra-/inter-technique tropospheric ties can be applied to improve the multi-technique integrated solution. The concept of applying tropospheric ties assumes that tropospheric parameters from each technique represent the true tropospheric state, i.e., (1) there should be no technique-related systematic bias, and (2) the technique-specific stochastic noise that leaks into atmospheric delay estimates should be minimized. However, this is usually not the case in reality, as the modeling and parameter estimation in the analysis of space geodetic techniques naturally contain systematic and stochastic uncertainty. For example, the multi-path effect and antenna phase center variation (PCV) error could introduce systematic biases in Global Navigation Satellite Systems (GNSS) tropospheric parameters, and the limitation of observation geometry in Very Long Baseline Interferometry (VLBI) could bias the horizontal gradients. Therefore, it is critical to identify and handle the systematic biases, and properly handle the stochastic part. In this study, we demonstrate that tropospheric parameter biases could be large, e.g., up to 5 mm in zenith total delay (ZTD). Ignoring systematic biases distorts the ground network, especially the VLBI network. For example, the systematic bias in ZTD shifts the network scale, and the bias in horizontal gradients rotates the network and deteriorates earth orientation parameters (EOP). We demonstrate that the GNSS and VLBI integrated solution can be improved after properly handling the tropospheric ties., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Accurate tropospheric delays from Numerical Weather Models (NWM) are an important input to space geodetic techniques, especially for precise real‐time Global Navigation Satellite Systems, which are indispensable to earthquake and tsunami early warning systems as well as weather forecasting. The NWM‐based tropospheric delays are currently provided either site‐specific with a limited spatial coverage, or on two‐dimensional grids close to the Earth surface, which cannot be used for high altitudes. We introduce a new method of representing NWM‐derived tropospheric zenith hydrostatic and wet delays. A large volume of NWM‐derived data is parameterized with surface values and additional two or three coefficients for their vertical scaling to heights up to 14 km. A precision of 1–2 mm is achieved for reconstructing delays to the NWM‐determined delays at any altitudes. The method can efficiently deliver NWM‐derived tropospheric delays to a broader community of space geodetic techniques., Plain Language Summary: Precise positioning with microwave‐based space geodetic techniques, such as Global Navigation Satellite Systems (GNSS), requires accurate modeling of the atmospheric refraction. Numerical Weather Models (NWM) can provide tropospheric delays with an accuracy of 1–2 cm in zenith direction and are therefore useful for improving the data analysis. However, due to the large data volume to handle, NWM‐based products are typically provided only for selected sites, or on a global grid referring to a specific height. We provide an efficient method to represent the vertical profile of tropospheric delay from the Earth surface up to 14 km altitude with a precision of 1–2 mm. The method is used to preserve the precision of NWM‐derived tropospheric delays at the altitudes using three to four coefficients per geographic location (longitude, latitude) at the ground. This paves the way of applying the NWM‐based accurate tropospheric delays in space geodetic data analysis, especially for global augmentations of real‐time GNSS, which play a critical role in the rapid characterization and early warning of geohazards such as earthquake and tsunami, as well as kinematic platforms of high altitudes., Key Points: New method for precise modeling of the zenith hydrostatic and wet delays from the Earth surface up to an altitude of 14 km. Tropospheric delay vertical modeling precision of better than 3 mm is achieved on a global scale. The method provides numerical weather model‐derived precise tropospheric augmentation correction for real‐time space geodetic techniques., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, China Scholarship Council CSC, Helmholtz OCPC Program, https://cds.climate.copernicus.eu/cdsapp

The orbit accuracy of the navigation satellites relies on the accurate knowledge of the forces on the spacecraft, in particular the non-conservative perturbations. This study focuses on the Inclined Geosynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites of the regional Chinese BeiDou Navigation Satellite System (BDS-2), for which apparent deficiencies of non-conservative models are identified and evidenced in the Satellite Laser Ranging (SLR) residuals. The orbit errors derived from the empirical 5-parameter Extended CODE Orbit Model (ECOM) as well as a semi-analytical adjustable box-wing model show prominent dependency on the Sun elongation angle, even in the yaw-steering attitude mode. Hence, a periodic acceleration in the normal direction of the +X surface, presumably generated by the mismodeled thermal radiation pressure, is introduced. The SLR validations reveal that the Sun elongation angle-dependent systematic errors were significantly reduced, and the orbit accuracy was improved by 10–30% to approximately 4.5 cm and 3.0 cm for the BDS-2 IGSO and MEO satellites, respectively.