## 4 Discussion and Conclusions  
This research aimed to consider the use of relative weighting in combined orbit determination when two or more observation techniques are used. We performed simulations with various input parameters to deeply investigate the impact of used VCE approaches when different ISL range biases are present and distinct ground station sets are used. Using the Helmert or Forstner approach of the VCE method may be beneficial in future processing schemes exploiting ISLs and GNSS measurements in the Galileo system. In [PERSON] et al. (2015), the VCE algorithm was already applied to the parameter estimation of the satellite orbit determination in BeiDou with ISLs. The results in [PERSON] et al. (2015) confirm that orbit accuracy was improved after the adjustment of the VCE algorithm in the fusion data processing including 10 ground stations located in China in addition to ISLs. For different processing settings (e.g., constellation, station placement, and errors), the general conclusions about the VCE exploitation also reveal that the orbit determination accuracy is improved. An initial check of possibility for ISL range bias estimation has been conducted in [PERSON] et al. (2014) and it was suggested that accurate ISL range bias estimation is possible because of the GNSS observations. In this research, we analyze 44 and 16 stations uniformly distributed around the world, and 16 stations located only in Europe to verify the impact of the number of ground stations and their placement on the VCE approaches and the ISL range bias estimation possibilities. In addition, estimation of ISL range biases was investigated in [PERSON] et al. (2015) and it was shown that ISL range biases up to 5 mm have no significant impact on ISL residuals and clock estimation errors, but ISL range biases introduce a considerable error in the radial component. [PERSON] et al. (2015) suggest implementing precise ground calibration of the instruments rather than estimation of range biases as estimated ISL range biases may deviate from the simulated values due  
Figure 9: (**a**) SISRE(orth) and (**b**) SISRE values computed under different simulation scenarios. Please note that values for 16E-GNSS are presented with different vertical axis in cm. to modelling errors. In contrast, in [PERSON] et al. [17] it was suggested that calibration is not necessary as the estimation provides results with sufficient accuracy. However, it is worth paying attention to the simulation settings and estimation properties, which are distinct in these works. Still, the best-suited ISL range bias calibration remains uncertain as different analyses based on various simulation approaches or estimation settings provide incompatible results. This will be a challenge that is difficult to solve in simulations as the onboard hardware in orbit may face conditions that are considerably different from the current assumptions. Ground calibration of ISL range biases remains an open question that we would like to address in future analysis. For the Galileo constellation, we can only test the hypothesis concerning ISL processing with the use of simulated measurements. Simulations are a great tool to check possible solutions, as we know exactly which effects were introduced. In this research on employing ISLs in Galileo, we tested three interlacing cases using relative weighting methods: (1) comparison under a strict (Helmert) and simplified approach (Forstner) to the VCE, (2) results of bias estimation in view of the weighting method, and (3) possible impact of the number of ground stations and their location on Earth. Firstly, we found that the VCE helps to minimize errors for satellite positions without impacting clock estimation. However, the total gain is no more than 20% compared with the Nominal weighting method when the least optimal simulation scenario was used. Forstner and Helmert approaches to the VCE perform similarly.