# A New Online Service for the Validation of Multi-GNSS Orbits Using SLR
## 2 Materials and Methods  
### System Overview and Description of Processing Flow  
The GOVUS system consists of two main components: (1) server-side algorithms, which execute the daily SLR validation process, prepare the dataset of SLR residuals and summary reports; and (2) web-application for visualization and analyses of SLR residuals. The system constitutes, therefore, a complex, autonomous, and fully automated tool. The web-application is a completely self-sufficient tool for conducting analyses, which do not require any additional software or drivers. All analyses can be done using solely the web browser. The SLR residual is a difference between the distance derived directly from laser ranging and calculated between the fixed station coordinates and the microwave-based position of a satellite. SLR validation at the GNSS altitudes has the largest sensitivity in radial direction [38], because of the limited scope of possible incidence angles in laser ranging, generally up to 15\({}^{\circ}\) (see Figure 1). The SLR validation is only possible when both orbit products and SLR normal points are available. Processing of data is performed using the modified Bernese GNSS Software 5.2 [39]. Figure 2 shows the processing scheme of microwave orbit validation using SLR dataset in Bernese GNSS Software. Currently, only the CODE orbit products are being validated as a representative example of 5-system orbit products delivered in the framework of MGEX. CODE solutions include satellites of the following systems: GPS, GLONASS-M, GLONASS-K, Galileo IOV, Galileo FOC, BeiDou-2 MEO, BeiDou-2 IGSO and QZSS. Besides CODE, there are five other ACs which deliver multi-GNSS orbit products in the framework of MGEX: Centre National d'Etudes Spatiales (CNES/CLS) [4], Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ) [5], Technische Universitat Munchen (TUM), Wuhan University (WU) [40] and Japan Aerospace Exploration Agency (JAXA) [41]. Products from all ACs can be included in the GOVUS service, which is one of the major plans for the future development. Figure 1: (**a**) Satellite-based reference frame; and (**b**) relative geometry of the Earth, satellite and the Sun showing the elevation angle of the Sun over orbital plane \(\beta\), argument of the satellite latitude with respect to the Sun \(\Delta\)u and the elongation angle \(\varepsilon\). The dataset necessary for validation consists of: (1) dataset of SLR normal points (delivered by CDDIS [42]); (2) microwave-based discrete satellite positions (EPH files with orbit ephemerides); and (3) GNSS-derived Earth Rotation Parameters (ERP file). First, the files of Normal Points are converted into separated RINEX files including normal points and meteorological data in turn. In parallel, the continuous 1-day orbits are fitted to the discrete satellite positions from the EPH file. The arc-fit strategy is consistent with the official CODE's MGEX solution processing [5] in an aim to correctly restore the orbit. Minor differences in orbit modeling are absorbed by estimating stochastic pulses in three orbital directions. The summary of observation modeling and the strategy of parameters estimation is included in Table 1.  
The dataset is processed in such a manner to receive SLR residuals with no additional parameter estimation, i.e., with fixing station coordinates, to the ILRS realization of ITRF2014, i.e., SLRF2014, and fixing both orbits and ERP to CODE values. The range measurements are corrected by tidal station displacements, troposphere refraction, general relativity and other corrections recommended by the International Earth Rotation and Reference Systems Service (IERS) 2010 Conventions. Finally, the dataset is subject to the initial analysis, which summarizes the aggregated statistics for each station-satellite pair grouped according to the particular day of observation and stored in a summary file. The output from the validation process consists of a detailed list of SLR residuals together with information about laser station, satellite, residual value and time.