# Assessment of GNSS Galileo Contribution to the Modernization of CROPOS's Services
## 2 Materials and Methods
### Galileo Satellite System and Its Applicability for High-Accuracy Positioning  
Galileo is the European Global Navigation Satellite System (EGNSS), under civil control, providing PVT (Position, Velocity, and Time) service to users worldwide. The whole program is fully funded and owned by the European Union (EU). It was designed and developed by ESA (European Space Agency) and is operated by EUSPA (European Union Agency for the Space Programme) [31]. Currently, the Galileo system is under development. A comprehensive overview of the early stages of Galileo system development is depicted in Bonnor [32]; an overview of the Galileo Full Operational Capability (FOC) is supplied on the ESA Earth Observation Portal [33]. The information about the current constellation is available on the web site of the European GNSS Service Centre [34]. Galileo transmits several signals and codes on four different carrier frequencies within the range of 1.1-1.6 GHz, namely: E1 (1575.42 MHz; observation codes: C1A, C1B, C1C, C1X, C1Z), E5a (1176.45 MHz; observation codes: C5I, C5Q, C5X), E5b (1207.14 MHz; observation codes: C7I, C7Q, C7X), and E6 (1278.75 MHz; observation codes: C6A, C6B, C6C, C6X, C6Z) and multiplexed together through the AltBOC (Alternative Binary Offset Carrier) scheme [3; 35; 36]. The AltBOC signal, as the unique feature of Galileo in the combined E5a + E5b (known as E5, 1191.795 MHz; observation codes: C8I, C8Q, C8X) band, offers superior noise and multipath performance [37]. Moreover, the wideband E5 signal isexpected to have a much better code (pseudorange) accuracy as compared to its subcarriers E5a and E5b [38]. The bandwidth of E5 is 51.15 MHz, which is far larger than the other signals, and this may explain the superior performance of signal E5 [39]. The very first autonomous determination of ground coordinates using the four Galileo satellites took place in March 2013, marking a milestone in Galileo development [40]. Before that, in January 2013, with four Galileo IOV (In Orbit Validation) satellites in constellation, the short period of their visibility allowed the demonstration of absolute and relative positioning using measurements from GAL satellites only. External orbit and clock information was necessary, since the IOV satellites were not transmitting a valid navigation message at that time. The Galileo frequencies E1 and E5a were observed for absolute positioning, whereas the relative positionings were computed from Galileo E1, E5a, E5b, and E5 AltBOC carrier-phase observations. The test has shown that E1 and E5 carrier-phase measurements will be particularly relevant for future positioning applications due to the possibility of mixed-constellation ambiguity resolution with GPS L1 and L5 signals [41]. The results of RTK positioning based on carrier-phase and code (pseudorange) observations of four Galileo IOV satellites broadcasting navigation message data were presented in [38]. The results of Galileo IOV ambiguity resolution, standalone as well as with GPS, were described for the very first time. The results have demonstrated that the integer ambiguity resolution based on the four IOV satellites needs fewer than three minutes when observables from at least three frequencies are used. Combined with data of four GPS satellites, even instantaneous (single epoch) ambiguity resolution is demonstrated using only two frequencies per constellation (i.e., E1 + E5a and L1 + L2). The Galileo E1 and E5a bands are aligned to GPS L1 and L5, respectively. The advantage of multi-frequency carrier phase measurement is the diversity of carrier wavelengths helping to isolate the correct integer carrier phase ambiguities quickly and reliably. Dual-frequency GNSS observations are sufficient for ionospheric bias estimation/compensation and robust integer carrier phase ambiguity resolution. The addition of carrier phase and code measurements on a third frequency further strengthens ionospheric estimation and the speed and reliability of carrier phase ambiguity resolution [42].