## 5 Analysis of Results
### Real-Time PWV Determination Validation  
To explore the precision of the ZED-F9P device in tropospheric applications, GNSS observations are collected simultaneously from both the device and the geodetic-grade receiver over the three previously selected days. The ZED-F9P's observations are processed in real-time PPP mode utilizing BKG, CNE, GFZ, IGS, and WHU products, while the post-processed PPP mode is utilized for the Trimble R4s observations; thereafter, the ZTD is calculated as a by-product from the proposed PPP solutions. The estimated ZTDs from real-time GPS, GLONASS, Galileo, and BeiDou PPP solutions are referenced to the geodetic-grade PPP extracted ZTD values. The estimated ZTDs from the GPS, GLONASS, Galileo, and BeiDou PPP models are depicted in Figure 9, compared with the obtained ZTD values through geodetic-grade solutions over DOY 225 as an example. It is seen that the extracted real-time ZTDs from both the GPS and BeiDou solutions utilizing the five products are close enough to the extracted geodetic-grade ZTD with slight fluctuations in the first 40 min. This is due to the PPP solution's convergence time, and thereby the ZTD values; however, the obtained real-time ZTDs through GLONASS PPP solutions have large discrepancies with respect to the extracted geodetic-grade ZTD; also, the computed real-time ZTDs from BeiDou solutions have small discrepancies compared with to the extracted geodetic-grade ZTD. This is attributed to the accuracy of the real-time GLONASS and BeiDou PPP solutions. Figure 8: CDF for 3D positioning errors for various PPP models. Figure 9: _Cont._The performance of the ZED-F9P module in atmospheric sounding applications is further examined through the computation of the real-time precipitable water vapor. The estimated real-time PWV from the four proposed PPP processing solutions are validated with the estimated PWV from the geodetic-grade PPP solution. Figure 10 represents the PWV's time series derived from ZED-F9P PPP and geodetic-grade PPP solutions. It is clear that the derived PWVs from the GPS PPP models are more precise than the others; furthermore, the calculated real-time PWVs from the Galileo PPP models relatively agree with geodetic-grade PWVs. An exception is the IGS-extracted PWVs. Similarly to ZTD, both the GLONASS-based and BeiDou-based PWVs have differences in comparison with the geodetic-grade PWVs. To further analyze the PWV's precision on daily basis, the standard deviation (STD) for the differences between the PWVs and the derived geodetic-grade PWVs are calculated and given in Figure 11. It is shown that the STD values are smaller than 4 mm and 10 mm for GPS and the other solutions, respectively; also, it is seen that the STD values on the third day are greater than on the other two days for all solutions. This may be due to variations in water vapor because of surface temperature increment on that day. Figure 10: _Cont._  
Figure 9: Real-time ZTD values from different PPP solutions on DOY 225. Furthermore, the WHU-derived GPS PWV values are more precise than the other products over the three days. For GLONASS-derived PWVs, there are variations in STD values over the three days for each product, with minor changes for BKG and CNE products. This might be attributed to the daily GLONASS product's accuracy for each analysis center; on contrary, the STD values are quite small over the first two days, with large differences in the third day for Galileo-derived PWVs. Additionally, the IGS-derived PWVs have greater STDs than the other products. The variability in the BeiDou-derived PWV precision is obvious, particularly on DOY 226 and 227; in addition, WHU products offer more precise PWV values compared with GFZ and IGS products. Figure 11: _Cont._  
Figure 10: Real-time PWV values from different PPP solutions on DOY 225. Furthermore, the CDF of the PWV differences is determined (Figure 12).