## 1 Introduction  
Presently, the precise point positioning (PPP) technique is frequently applied for accurate navigation, position, atmospheric sounding, and space weather applications. PPP can be performed in a post-processing method by using accurate satellite orbit and clock products available from the international global navigation satellite systems (GNSS) service (IGS). To address the demands of instantaneous applications (e.g., positioning, atmospheric monitoring, hazard prediction), IGS released the real-time service (RTS), which is attainable from a number of analysis centers, for example, Bundesamt fur Kartographie und Geodasie (BKG), Centre National d'Etudes Spatiales (CNES), Geo Forschungs Zentrum (GFZ), IGS, GNSS Research Center of Wuhan University (WHU), Natural Resources Canada (NRCan), and European Space Agency (ESA). The RTS products contain corrections of multi-GNSS orbits and clocks, as well as code and phase biases through the Radio Technical Commission for Maritime Services (RCTM) and IGS State Space Representation (SSR) formats. Numerous researchers have lately studied the quad-GNSS real-time products' performance in various applications [1; 2; 3; 4; 5; 6; 7]. For example, the precision of products from 10 analysis centers has been explored for kinematic PPP applications [1]. It has been shown that the CNES, WHU, and GFZ products offered stable quad-constellation kinematic PPP solutions with three-dimensional accuracy within the centimeter level. [PERSON] et al. [3] examined the GPS, Beilou, and Galileo real-time PPP solutions' accuracy in both static and kinematic domains using smart phones. Their obtained results indicated that the positioning accuracy was within the decimeter and meter levels for both static and kinematic modes, respectively. More recently, several manufactures have made low-cost dual-frequency quad-constellation GNSS devices, for instance, the U-blox ZED-F9P device. Various quad-constellation GNSS measurements can be tracked by the ZED-F9P module, including GPS L1 C/A and L2C, GLONASS G1 and G2, Galileo E1 and E5b, and Beilou B1 and B2; furthermore, it can be operated in differential, PPP, real-time kinematic (RTK), and RTK-PPP modes. With the ability to collect dual-frequency GNSS observations, the ionosphere-free linear combination can be formed, which mitigates the ionospheric error; therefore, accurate positioning estimates become more feasible. The ZED-F9P device has been evaluated in many applications in the post-processed mode, such as precise point positioning [8; 9; 10; 11; 12; 13; 14; 15; 16] and surveying applications [17; 18; 19; 20]. For example, [PERSON] et al. [11] investigated performance of the ZED-F9P GNSS module in PPP-RTK mode in ambiguity resolution and positioning accuracy domains, and their results indicated that centimeter-level positioning accuracy was obtained from the static PPP-RTK solution; moreover, [PERSON] et al. [17] explored the accuracy of the ZED-F9P device along with low-cost and geodetic-grade antennas in both open-sky and urban areas. It has been found that the low-cost receiver achieved horizontal and vertical accuracies of less than 10 mm and 15 mm, respectively, using relative positioning mode in urban areas, while it has been enhanced to 5 mm in horizontal and vertical directions in open-sky areas. Furthermore, the affordable ZED-F9P module can be utilized in both tropospheric and ionospheric estimation applications [21; 22; 23; 24; 25]. For instance, [PERSON] et al. [25] established a network consisted of 15 low-cost receivers to monitor the ionosphere and its behavior over Canada. The estimated total electron contents (TECs) from the network were compared with the geodetic-based TEC; their results showed that the TEC values had good agreement with those obtained through the geodetic receivers; the ZED-F9P receiver is also investigated in deformation monitoring applications [26; 27; 28; 29; 30; 31; 32; 33]. As an example, [PERSON] et al.