# Assessment of GNSS Galileo Contribution to the Modernization of CROPOS's Services
## 2 Materials and Methods
### Preparation and Execution of the Field Activities
For both survey styles, a 10\({}^{\circ}\) elevation mask was  
Figure 3: Visible horizon at station A (red line): maximum elevation (26\({}^{\circ}\)) of the visible horizon is in the azimuth 9\({}^{\circ}\); in the portion of the sky 45\({}^{\circ}\) < Azimuth \(<\) 315\({}^{\circ}\), the visible horizon is free above the elevation 18\({}^{\circ}\). applied, with 30 epochs and PDOP mask value set to 6 (default). The survey style related to static observations featured all satellites (GGGB), elevation mask 10\({}^{\circ}\), logging interval 5 s, and session duration 22 min. The duration of a conventional static session was determined according to the rules set forth in [PERSON] et al. [63] for the dual-frequency (L1 + L2) receivers: 20 min + 2 min/km of the baseline length. Although those rules present a conservative approach for session length determination, they were accepted to ensure a reliable baseline determination. In addition to the close (nearby) CORS ZAGR station, observation data for two additional VRS station were downloaded in T02 format and used in baseline processing. All those stations were evenly distributed around the station A at distance < 1 km leading to the session length of 22 min. The CORS ZAGR station and two VRS stations form an approximately equilateral triangle with station A being in its center. The parameters of the survey styles are outlined as follows:  
* VPPS (GGG): GGG, 10\({}^{\circ}\), 30 s, RTCM 3.2 MSM, PDOP 6
* VPPS (GAL-only): GAL, 10\({}^{\circ}\), 30 s, RTCM 3.2 MSM, PDOP 6
* GPPS (GGGB): GGGB, 10\({}^{\circ}\), logging interval 5 s, duration 22 min. Bearing in mind the planned times for observation sessions, the observation schema (sequence) was set as follows: three separate sessions (with individual initialization; each lasting for 30 s) using GGG satellite systems, followed by three separate sessions (with individual initialization; each lasting for 30 s) using Galileo-only signals, and finishing the observation window with 22 min of static observations. The observation session sequence is summarized as follows:  
* VPPS (GGG): 30 s,
* VPPS (GGL-only): 30 s,
* VPPS (GAL-only): 30 s,
* GPPS (GGGB): 22 min. The overall duration of the observation sequence was approximately 27 min. All VPPS sessions were carried out within 4-6 min, providing almost stable (unchanged) observation conditions and satellites visibility. During the field surveying activities, certain problems were encountered related mostly to connecting the rover receiver to the base (fourth session) as well as freezing of the Trimble Access software (sixth session) (see Table 2). All those events have delayed and caused modifications of the planned observation schedule. During the ninth session, VPPS (GGG) observations were carried out as planned, and the measurements with GAL-only observations were taken for only two series (out of three planned). Most likely, the visibility of one out of five visible GAL satellites has fallen below the elevation mask, preventing the acquisition of a fixed solution. All sessions are outlined in Table 2, each series with start times (CET) of observations carried out, along with the information about PDOP and number of satellites contributed to the solution. By comparing the time schedule (Table 1) to the times of sessions as they were carried out (Table 2), it can be easily seen that the first five sessions were performed as planned, but problems that occurred during the sixth session caused the modifications of the further sessions.