# A New Online Service for the Validation of Multi-GNSS Orbits Using SLR
## 2 Materials and Methods
### Capabilities of GOVUS with Respect to the Critical Issues of SLR and Multi-GNSS  
As GOVUS should serve to the scientific community, it is essential to point out the current issues of both the multi-GNSS orbit determination and the SLR tracking network. Most of the current problems with the homogenous multi-GNSS orbit determination are related to the lack of proper models, which could entirely reduce the effects of most important non-gravitational accelerations such as SRP, antenna thrust and both thermal radiation from the Earth and Earth's albedo, as well as proper modeling of satellite attitudes for non-yaw-steering events [1; 44; 45]. Constant monitoring and independent validations are advised by both the MGEX and IGS authorities to help in the description of satellites' perturbations. Moreover, the existence of the orbit-modeling issues may reflect in dependencies between the orbit quality and the elevation angle of the Sun above the orbital plane, the elongation angle, or the latitude of the satellite with respect to the Sun [1]. However, the impact of those dependencies is still not fully understood and eliminated, so that the nature of the perturbations should be constantly investigated. When using GOVUS, the user can restrict the analysis to the desirable satellite system, group of potentially similar objects (common orbital planes) or to focus on particular satellites. According to the last IGS report [43], the analyses of the single satellites are a decent action for the future MGEX development (see Table 10). The major issue of SLR tracking is the heterogeneity of stations' efficiency and accuracy. The stations generate a different number of Normal Points (see Table 10) and are unevenly distributed across the globe [46]. Therefore, the SLR network struggle with the deficiencies in the geometrical distribution of measurements. Regional satellite systems such as QZSS and partly BeiDou-2 are most affected by this fact. The station's performance is dependent on the installed equipment as well as overall system integration, compatibility and proper working of each device. Each technical change or unexpected event [47], which happen at the station, may affect the dataset. GOVUS allows for cross-validation of the log of stations' events and detection of anomalies in the time series of SLR residuals. When performing the SLR validation, the user should be conscious about the station-related issues to not propagate SLR systematic errors to the evaluation of orbit quality. Thanks to the near-real time functionality all discontinuities or degradation of station products' quality may be immediately reported and excluded from analyses. The near-real time functionality may warn the multi-GNSS community about the unexpected behavior of particular satellites. During the last two years, multi-GNSS users realized that a growing number of GLONASS satellites have some problems (e.g., SVN 737 or 736) [1; 48] (see Figure 4). The issue may arise from many possible satellite-related reasons, such as attitude sensors, shifted center of mass with respect to satellite reference frame origin, or a change of the antenna offset. The possibilities of upcoming problems with other GLONASS satellites or any other anomalistic behavior of multi-GNSS satellites can be noticed in near-real time thanks to GOVUS. Such functionality is especially important for users of multi-GNSS post-processed products. This knowledge allows for excluding from their processes those satellites, whose orbit quality exceeds the desired accuracy threshold. The constellation of multi-GNSS is still in constant development. New GNSS satellites complement the deficiencies in constellations or exchange the outdated buses every couple of months. New satellites represent often the new type or block, which may differ from other satellites in shape, size, mass or power of signal transmission. The internal satellite diversity within a satellite system is sufficiently large to impose a problem for regular users of multi-GNSS.