## 2 Overview of Multi-Constellation GNSS PPP Technique
The concept of post-processing multi-constellation GNSS PPP has been handled by many researchers in static and/or kinematic applications. Within these studies, contributions of various GNSS satellite constellation combinations to the performance of the post-processing PPP utilizing with different precise products produced by many ACs were investigated with respect to positioning accuracy and convergence time [5; 11; 12; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Their results indicate that multi-GNSS can significantly increase the number of observed GNSS satellites while optimizing the spatial geometry using multi-frequency data. They also concluded that, in general, additional satellite constellations improved the positioning accuracy of PPP compared to that achieved by using a single system. These experimental results also demonstrate that the multi-GNSS configuration reduced the convergence time when compared to single system PPP as an important factor limiting the PPP in some surveying applications. Furthermore, their findings imply that the precise products used during the processing of the data significantly affect the results especially in terms of accuracy. It is worth mentioning that, the datasets within these studies were generally retrieved from the permanent stations of the International GNSS Service (IGS), IGS Multi-GNSS Experiment (MGEX), EUREF Permanent Network (EPN), International GNSS Monitoring & Assessment Service (iGMAS), National Continuously Operating Reference Stations (CORS), and so on. The results given in these studies were based on the processing of the static data as kinematic PPP solutions. As a reminder, generally reference stations for IGS and similar networks are often established in ideal locations where the factors that will negatively affect measurements (low multipath and signal loss) will be minimal and that they can make observations from the all-possible satellites. On the other hand, multi-constellation GNSS PPP in kinematic applications in a realistic scenario were investigated in a few studies. [PERSON] et al. [25] investigated combinations of GPS and GLONASS dual-frequency measurements for both static and kinematic (as car and walk trajectory) applications. Their results concluded that the addition of GLONASS to GPS improved satellite availability and geometry by more than 20%. Furthermore, GPS and GLONASS provided better accuracy compared to GPS-only solutions by processing magicGNSS online GNSS processing software. [PERSON] et al. [26] made kinematic experiments with an aircraft lasting about 4 h. According to this study, adding the GLONASS constellation to the GPS improved the kinematic PPP accuracy and precision. Their results also reveal that, combined GPS and GLONASS observations shortened the convergence time compared to GPS-only. In another study, additional GLONASS constellation data improved the kinematic PPP performance when compared to GPS-only but there was no improvement with additional Galileo constellations [27]. The performance of quad-constellation PPP was investigated in [5] and they reveal that the performance of multi-GNSS PPP positioning showed better performance compared to single and dual-constellations. [PERSON] and [PERSON] [28] carried out kinematic measurements collected from three receivers mounted on a vehicle by observing GPS, BDS, and GLONASS constellations. After processing of the collected data using self-developed software in PPP mode, they found that triple-constellation kinematic PPP obtained better results than those of both GPS-only and dual-constellation PPP in terms of positioning accuracy and convergence time. [PERSON] et al. [29] examined the feasibility of the multi-GNSS PPP in agriculture studies. According to processing of their measurements (static and kinematic), they concluded that multi-GNSS PPP showed better precision and repeatability than GPS-only PPP.