## 1 Introduction
Furthermore, for the multi-constellation kinematic PPP approach, an improvement of the positioning accuracy and convergence time was highlighted compared to the single-constellation and dual-constellations cases. In [17] the authors investigated the effectiveness of integrating GPS and GLONASS measures to improve the reliability and accuracy of positioning results using PPP. The authors showed that the integration of GLONASS satellites significantly improved the satellite availability (more than 60%) leading to the enhancement of satellite geometry (in terms of Positional Dilution of Precision--PDOP) by more than 30%. In this way, the performance of precise surveying can be improved in scenarios such as urban areas, dense vegetation, mountains areas, or when the satellite signal is partially obstructed as confirmed also in [18]. A performance analysis of multi-GNSS PPP has been conducted in [19] using DF observations from GPS, GLONASS, Galileo, BeiDou and QZSS systems. Precise multi-GNSS orbit and satellite clock products from GFZ were chosen for six stations selecting data from 150 to 160 days of 2016. If observations were used to eliminate the ionosphere delays. The receiver clock error was computed as an unknown parameter using the least square estimator. Troposphere delay was considered as a random-walk process. Furthermore, the Kalman filter was used to filter and smooth time-dependent parameters. The analysis of the results showed a positioning accuracy improvement at a centimeter level obtained thanks to the multi-GNSS configuration compared to GPS-only. However, it was highlighted that when positioning errors reach a very low level (about 5 mm) the multi-GNSS contribution is rather limited, due to the accuracy of orbit and satellite clock products of the other GNSSs as BeiDou, GLONASS, and Galileo. On the other hand, a significant increasing of the satellite availability was detected when the multi-GNSS approach was considered. Therefore, the same authors suggest that an improvement, both in terms of reliability and availability of positioning results, would be more evident in difficult environments, such as urban areas or partially obstructed places. This same conclusion, also related to the multipath effect, was recently made by other researchers who examined the quad-GNSS PPP performance (GPS, Galileo, GLONASS, BeiDou) [20]. However, from their tests, they found a substantial performance enhancement for the GPS+GLONASS combination with respect to GPS-only. Benefits of multi-GNSS have also been highlighted in [21], in which PPP models were performed using both single and dual frequency measurements from GPS, GLONASS, Galileo, and BeiDou systems and considering three selected stations located in Egypt at different latitudes. The IGS-MGEX final precise products were chosen for the correction of orbital and clock errors, while the final IGS GIMs (Global Ionosphere Maps) were adopted to reduce the effect of the ionospheric delay in the single-frequency (SF) PPP model. For both DF- and SF-PPP models, the results showed that the convergence time for the multi-GNSS configuration was enhanced by 35% with respect to GPS only PPP. It was also highlighted that the positioning accuracy improvement of multi-GNSS PPP is mainly due to the contribution of GLONASS observables, while the contribution of Galileo and Beidou was minimal due to their limited satellite availability. In 2019, an evaluation of availability and performance of PPP positioning with four constellations, GPS, GLONASS, BeiDou, and Galileo, was carried out in [22]. In detail, the performance of quad-constellation DF-PPP, SF-PPP, and SPP are verified using the datasets from 59 globally distributed stations spanning 30 days. Furthermore, in this research, the results have indicated that both the availability and accuracy of positioning solutions were significantly enhanced by integrating multi-constellation signals.