# Assessment of GNSS Galileo Contribution to the Modernization of CROPOS's Services
## 2 Materials and Methods
### Mission Planning  
Since the Galileo constellation is still not complete, mission planning was needed to identify and assess the visibility of Galileo satellites. For assessing the visibility of GNSS satellites along with the estimation of PDOP (Position Dilution of Precision) values, two tools were used, both provided by the Trimble company: Planning (version 2.90) and the Trimble GNSS Planning Online, available on Trimble [54]. These tools can be regarded as mutually complementary since certain functionalities present in one tool are not present in the other one. For both tools, the almanac data are essential: for the offline tool, the data have to be downloaded in the appropriate file format (e.g., alm), whereas for the online tool, the almanac data are automatically retrieved. The advantage of the offline tool is that a real horizon can be defined, the duration of the time window can start at any time of the day, and all quantities can be calculated (e.g., various DOP values, elevation, azimuth, and number of visible satellites broken down by each individual system (GPS, GLO, GAL, BDS)). The drawback of the online tool is that it enables the duration of the time window for 6, 12, or 24 h starting at hour (01:00, 02:00, etc. ), but on the other hand, it offers the information about TEC (Total Electron Content), Ionospheric index, and scintillation. The visible (or local) horizon around the station was surveyed using a total station; subsequently, the azimuth and elevation values of the visible horizon were derived and finally imported into the Planning offline tool. The local or visible horizon is the 'line where Earth and sky seem to meet but are blocked by elevated features of the landscape, such as trees or mountains' [55]. The sky plot, produced in the Planning tool and showing the local horizon and Galileo satellites, is shown in Figure 1.  
A great advantage of the offline tool is that different PDOPs (GDOP, PDOP, HDOP, VDOP, TDOP) along with the number of visible satellites above the elevation mask can be calculated and displayed for a 1-min interval or longer. Since the field testing was focused on the assessment of Galileo's contribution to the performance of CROPOS's services, the visibility of Galileo satellites was analyzed. At the time of field GNSS observations (July 2020), GPS and GLONASS systems had a declared Full Operational Capability whereas the Galileo system was under construction, with activities ongoing towards the declaration of FOC. The FOC of the BeiDou system (BDS-3) was declared only two weeks after the conducted measurements, on 31 July 2020 [56]. Focusing on the Galileo satellites' almanac data only, the planning was carried out for the day 14 July 2020 (DOY 196) starting at 8:00 through 17:00 Central European Time (CET). Several time windows were considered with different DOP values and number of visible GAL satellites. Upon analysis, nine time-windows were identified with good, medium, and bad visibility. At the end of the day, a short time window was identified with only four GAL satellites visible, which should prevent the availability of a fixed RTK solution (for OTF ambiguity resolution, the minimum visibility of five satellites is needed). According to the number of visible GAL satellites at the station A above the visible horizon for the time window 8-17 CET, an observation plan was set up and is presented in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Session Planned**} & **Start of Session** & **Expected Number of** & **Expected PDOP** \\  & **(CET)** & **GAL SV Visible** & **Expected PDOP** \\ \hline
1 & 07:50 & 7 & 1.78 \\ \hline
2 & 08:10 & 6 & 2.98 \\ \hline
3 & 09:30 & 6 & 2.37 \\ \hline
4 & 10:30 & 7 & 1.67 \\ \hline
5 & 11:50 & 6 & 2.42 \\ \hline
6 & 12:50 & 7 & 2.73 \\ \hline
7 & 13:30 & 8 & 1.79 \\ \hline
8 & 14:20 & 6 & 3.02 \\ \hline
9 & 16:00 & 7 & 1.74 \\ \hline
10 & 16:30 & 4 & 4.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sessions planned according to the visibility of GAL satellites and expected PDOP values.