## 5 Discussion  
Estimating the variances and covariances of measurements in various receivers for the GPS, Galileo, GLONASS, and BeiDou systems yielded remarkable results based on the system and the receiver. As indicated in the figures and tables, the first noticeable issue was that the LEICA GR30 estimations were noisier. Even though this had practically no effect on the estimated parameters, it did significantly impact the precision of the estimated phase variances. Furthermore, among other receivers for phase observations, the SEPT POLARX5 is the most precise. The code measurement findings vary depending on the system and receiver. For example, we may claim that SEPT POLARX5 is the most excellent option for GPS code measurements, while Trimble NETR9 has the most precise code observations for Galileo. LEICA GR30 has the most precise observations for two other systems. The covariances and correlations results revealed strong correlations between two phase and two code measurements, with the values for phase observations being significantly higher. This is because all GNSS satellites, as is well known, have at least two ranging codes. For instance, CA and P-codes are used in GPS. The first is for public use and is available to civilians. An unknown W-code encrypts the latter to generate the Y-code, which is only for military use. There are, however, several methods for tracking the P(Y) signal that does not require knowledge of the W-code. In other words, signal recovery techniques on the L1 frequency can not be used to measure the P-code or the L2 carrier in GPS receivers. With codeless or quasi-codeless techniques, GPS receivers can use L1 and CA-code information to perform measurements on the L2 carrier, and Y-code [31, 56, 57]. The use of these techniques results in a correlation between code and phase observations [42, 58]. Besides that, the correlation values also depend on the method used for the recovery techniques, which varies between different receivers. Furthermore, since the L2 phase tracking loop is not independent of L1 measurements, its SD values are higher. Tables 5-7 back up these claims. GLONASS and BeiDou are identical to GPS in that they both offer public and military positioning services. Unlike more military-oriented systems, however, the Galileo system is intended for civilian and commercial use [59]. Considering all the facts and results, it can be concluded that the precision of GNSS observations is receiver-dependent and system-dependent. That is where estimating a realistic stochastic model becomes vivid. However, in total, SEPT POLARX5 has the lowest correlation among others. To evaluate the effect of applying a realistic stochastic model, the IAR success rate was estimated for two cases: the nominal and realistic stochastic model. When realistic variances and covariances are added into the process, it can be seen that the IAR success rate increases for each system as well as multi-GNSS data processing. The introduction of the realistic stochastic model can also lead to a realistic standard deviation of the baseline components. Compared to the basic stochastic model, the results of employing the realistic model indicated a reduction in these values. Although the changes are minor, given the baseline length, it can be claimed that using a realistic stochastic model in this experiment