## Materials and Methods
### RTKLib Software  
To perform the PPP-static technique, RTKLib version 2.4.2 p13 software is used for the data processing [40]. RTKLib is an open source positioning software developed by Dr. [PERSON]. All the data and corrections are entered as inputs using the software Graphic User Interface. It supports standard and precise positioning algorithms with GPS, GLONASS, Galileo, QZSS, BeiDou, and SBAS. Different positioning modes are allowed using GNSS data for both real-time and post-processing such as single point positioning (absolute technique), DGNSS (differential code approach) Kinematic, Static, Moving-Baseline, Fixed (carrier-phases-based relative approach), PPP-Kinematic, PPP-Static, and PPP-Fixed. The software consists of a program library and a series of executable files that use this library. It is possible to use the RTKLib package through the graphical interface, or the \"main program\" called RTKLAUNCH (Figure 2) or by running the individual CUI (command-line user interface) APs (application programs). The aim of the work is to evaluate the performance of a PPP-static positioning mode, a method mainly used for scientific purposes, taking into account the incidence of the various biases, the various error correction products and the contribution of the available satellite constellations. Although the  
Figure 2: Screenshot of graphical user interface of the RTKLAUNCH module. main problem of this method is related to the long convergence time of the solutions, with a multi-GNSS integration it is possible to reduce time and also improve positioning accuracy [3; 41; 42]. For static PPP processing, the RTKPOST is used, which is the RTKLib module for the post processing of GNSS data. RTKPOST requires, as input, the RINEX files (containing the observations, the navigation data and also the precise orbit products if available) and returns, as output, the \".pos\" format file (Figure 3). In addition to the positioning solution, the output file also provides the number of satellites used for the solution computation, the type of solution obtained, and the standard deviations estimates. In the output files, the solutions can optionally be expressed in different formats: the geographic coordinates (lat, lon, h), the Cartesian geocentric coordinates (X, Y, Z), baseline (E, N, h), and the message NMEA0183 (Figure 4). Based on the adopted positioning mode, RTKPOST allows one to select different settings such as the carrier frequencies (L1, L1 + L2, L1 + L2 + L5), filter type (Forward, Backward, Combined), elevation and SNR (signal noise ratio) masks, the navigation satellite systems, and several errors correction models (i.e., Earth Tides, ionosphere, troposphere) to be applied. In addition, as it is previously described, to process the observation data collected by a receiver in PPP mode using RTKPOST, several correction models must be applied selecting specified settings such as precise satellite ephemeris and clock corrections, satellite and receiver antenna phase center variations (PCV), corrections of the  
Figure 4: Output options interface with the RTKPOST module. Figure 3: Example of obtained solution extract with the RTKPOST module. tropospheric and ionospheric effects, SBAS corrections, solid earth tides, ocean tide loading, and pole tides can be also modeled. To verify the performance improvements for PPP-static mode obtained thanks to multi-GNSS, focusing the attention on Galileo, precise satellite products for GPS, GLONASS, and Galileo are downloaded by MGEX [43]. In particular, precise products computed by CNES/CLS are selected in the current analysis. This choice is also related to the aim of testing the PPP-AR (Ambiguity Resolution) function for the resolution of the phase ambiguity that can be selected in RTKLib software. However, the results obtained applying the PPP-AR method in RTKLib have highlighted that the considered ambiguity resolution technique provides unstable and inaccurate solution with respect to standard PPP.