# Assessment of GNSS Galileo Contribution to the Modernization of CROPOS's Services
## 4 Discussion  
Analysis of the Number of Satellites That Contributed to VPPS Results and Estimated Precision Assessment  
Considering all observations of VPPS series, it is worth analyzing the number of satellites which contributed to the solution along with the PDOP values and estimated Hz and Vt Precision (information obtained from the Trimble Access software, also available in real time during the survey). The range of values (MIN, MAX) is given in Table 3.  
Figure 12: All VPPS solutions obtained: GGG (27 solutions; cyan dots) and GAL-only (26 solutions; green dots); the REFERENCE solution is marked with a red triangle. It can be clearly seen that the precision estimation is better for GGG than for GAL-only solutions. The AVERAGE and MEDIAN values of 'Hz Precision' for GGG solutions are 3.8 mm, whereas the AVERAGE and MEDIAN values of 'Vt Precision' for GGG solutions are 7.1 mm and 6.5 mm, respectively. When GAL-only solutions are considered, the AVERAGE and MEDIAN values are given as: 5.9 mm (AVERAGE) and 5.2 mm (MEDIAN) for Hz Precision and 9.9 mm (AVERAGE) and 8.7 mm (MEDIAN) for Vt Precision. Obviously, AVERAGE and MEDIAN values are almost equal for precision estimation based on 'GGG', whereas for 'GAL-only', there is a small difference (AVERAGE vs. MEDIAN) owing to a weaker (less populated) Galileo constellation. Comparing the number of satellites that contributed to the solution with the number of visible satellites during the static session (22 min), it came into evidence that no BDS satellites contributed to the VPPS solutions. Horizontal precision estimation (Hz Precision) in Trimble Access software was expressed as DRMS (Distance Root Mean Square), whereas the vertical precision estimation (Vt Precision) was given in terms of 1\(\sigma\). DRMS is an estimate of the root mean square of the radial distance between the true position and the observed position [68]. According to the parameters for precision display in Trimble Access software, the Hz Precision is expressed as DRMS (63.2% probability that the estimate is within the range), the Vt Precision is given by 1\(\sigma\) with the probability of 68.3% that the height estimate is within the range from the true value [69; 70]. Horizontal deviation of each VPPS solution from the REFERENCE value (\(\Delta\)d) across all sessions was compared to the 'Hz Precision' estimates given by Trimble Access software. Regarding the VPPS (GAL-only) solutions, the differences 'Hz Precision-\(\Delta\)d' are in the range (\(-0.9\) mm to \(+7.5\) mm), mostly positive (23/26 = 88%), whereas the differences 'Vt Precision-ABS (\(\Delta\)H)' are in the range (\(-3.5\) mm to \(+15.7\) mm), mostly positive (22/26 = 85%), which means that the precision estimates given by Trimble Access software are pretty reliable (in approx. 90% of cases). Regarding the same analysis for the solution obtained with GGG satellites data, the situation is as follows: the differences 'Hz Precision-\(\Delta\)d' are in the range (\(-2.5\) mm to \(+3.4\) mm), mostly positive (16/27 = 59%), whereas the differences 'Vt Precision-ABS (\(\Delta\)H)' are in the range (\(-0.7\) mm to \(+7.7\) mm), mostly positive (26/27 = 96%), which means that the Vt. precision estimates are even more reliable. 'Hz Precision' estimates for VPPS (GAL-only) solutions are on average 2 mm higher (GGG (3.8 mm) vs. GAL-only (5.8 mm), and 'Vt Precision' estimates for VPPS (GAL-only) solutions are on average almost 3 mm higher (GGG (7.1 mm) vs. GAL-only (9.9 mm)). It can be concluded that for GAL-only solutions, the precision estimates for Horizontal and Vertical precision are rather conservative; for the GGG solution, they are more realistic. Nevertheless, in both cases, the Hz and Vt Precision estimates can be regarded as reliable. Across all sessions, the average PDOP value for GGG was 1.325, for GAL-only, it was 2.760, the average of visible GGG satellites was 20, and the average of visible GAL satellites was 6. The improved availability of GGG satellites has led to a better precision estimation compared to GAL-only solutions.