## 2 Overview of Multi-Constellation GNSS PPP Technique  
In the beginning, the PPP technique has been carried out by using only GPS observations; however, having the precise products for GLONASS (GLObal'naya Navigatsionnaya Sputnikovaya Sistema) emerged the first multi-constellation GNSS PPP concept in 2007 [5]. In the following period, different organizations and research centers started to produce precise satellite orbit and clock corrections for GLONASS, Galileo (European Global Navigation Satellite System), BeiDou Chinese Global Navigation Satellite System (BDS), and other regional satellite systems. The availability of these precise products provides multi-constellation GNSS PPP concept and allows determination of more accurate, reliable, and robust positioning by using global or regional navigation satellite systems. Within this frame, IGS as a main data provider initiated the IGS Multi-GNSS Experiment (MGEX) Project in 2012 for the tracking, collecting, and analyzing the available GNSS signals [6]. The number of institutions and multi-GNSS stations contributing to the MGEX project has been increasing over time. As of May 2020, 319 IGS multi-GNSS stations spread almost all over the world have been involved in MGEX projects [7]. The geographical distribution of these stations is given in Figure 1.  
Nowadays, seven IGS-ACs (Centre National d'Etudes Spatiales/Collecte Localisations Satellites (CNES/CLS), Center for Orbit Determination in Europe (CODE), German Research Centre for Geosciences (GFZ), Japan Aerospace Exploration Agency (JAXA), Shanghai Astronomical Observatory (SHAO), Technical University of Munich (TUM), and Wuhan University (WHU)) that took part in the MGEX Project have been routinely producing and distributing multi-constellation products including broadcast ephemerides (BRD), precise ephemerides (SP3) and clock products (CLK), Earth orientation parameters (ERP), IGS stations coordinates (SNX), and biases (BIA) for global or regional navigation satellite systems. The routine contributions of the above-mentioned analysis centers are given in Table 1 (as of May 2020). More information about MGEX is available in [7; 9; 10]. When compared to single system PPP, multi-constellation GNSS PPP is expected to increase the positioning performance significantly, since it benefits from better optimized satellite (spatial) geometry and hence improves positioning availability and continuity. Thus, the shortening of the convergence time would be possible since the number of observed satellites increases in the multi-constellation GNSS PPP. This advantage is even more important in studies in which the observations are carried out in tough environments, have limited number of tracked satellites, blocked or sheltered GNSS signals, or poor satellite geometry for positioning [5; 11]. Hence, it can be summarized the benefits of multi-GNSS as in the following [12; 13]:  
* Increased number of visible satellites with stronger satellite geometry. * Improved reliability and redundancy. * Improved position and time accuracies. Figure 1: Multi-GNSS Experiment (MGEX) project stations in the world [8]. * Improved positioning availability and continuity. * Improved signal-to-noise ratio. * Reduced signal acquisition time. * Reduced systematic biases in GNSS observations for positioning application. * Reduced problems, which may be caused by bulky structures such as buildings and trees. The main disadvantages of multi-constellation GNSS PPP positioning are that all the satellite systems have their own time scales and reference frames; moreover, the channels are biased and there are inter-systems biases [14]. These disadvantages can be overcome by using MGEX products. The conventional GNSS code pseudo-range and carrier-phase observation equations that are also the basic mathematical model of the GNSS-PPP algorithm are given by Equations (1) and (2).