## 2 Materials and Methods
### Data Sources  
Observation data from 18 GNSS CORS in Hong Kong, China, during June 2021 (DOY 173-DOY 179) and August 2021 (DOY 229-DOY 235) were selected. The data download website is: [[https://www.geodetic.gov.hk](https://www.geodetic.gov.hk)]([https://www.geodetic.gov.hk](https://www.geodetic.gov.hk)) (accessed on 14 September 2022). The location distribution map of the selected stations is shown in Figure 2. The coordinates list of Hong Kong CORS is shown in Table 1.  
Figure 1: GNSS station real-time atmospheric water vapor inversion process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Station** & **Latitude (\({}^{\circ}\))** & **Longitude (\({}^{\circ}\))** & **Antenna** \\ \hline HKCL & 22.2958 & 113.9077 & TRM59800.00 \\ HKFN & 22.4946 & 114.1381 & LEIAT504 \\ HKKT & 22.3678 & 114.3119 & TRM59800.00 \\ HKKS & 22.4449 & 114.0665 & LEIAR25.R4 \\ HKLM & 22.2189 & 114.1200 & TRM59800.00 \\ HKLT & 22.4181 & 113.9966 & LEIAR25.R4 \\ HKMW & 22.2558 & 114.0031 & LEIAR25.R4 \\ HKNP & 22.2490 & 113.8938 & LEIAR25.R4 \\ HKOH & 22.2476 & 114.2285 & LEIAR25.R4 \\ HKPC & 22.2849 & 114.0378 & LEIAR25.R4 \\ HKQT & 22.2910 & 114.2132 & TRM59800.00 \\ HKSC & 22.322 & 114.1411 & LEIAR25.R4 \\ HKSL & 22.3720 & 113.9279 & LEIAR25.R4 \\ HKSS & 22.4310 & 114.2693 & LEIAR25.R4 \\ HKST & 22.3952 & 114.1842 & LEIAR25.R4 \\ HKTK & 22.5465 & 114.2232 & TRM59800.00 \\ HKWS & 22.4342 & 114.3353 & LEIAR25.R4 \\ T430 & 22.4947 & 114.1382 & TRM59800.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hong Kong CORS coordinates list. Figure 2: GNSS station and RS station position distribution map. Because the IGS final product has a delay of about 12-18 weeks, the IGS fast product also has a delay of about 17-41 h, which generally leads to the PPP adopting the post-processing mode. Real-time PPP can also be implemented using the prediction part of IGS ultra-fast products, but its accuracy decreases as the prediction time increases, and real-time PPP implemented in this way has poor performance. The IGS RTPP was officially released in 2013, providing real-time services using real-time products formed by hundreds of real-time tracking stations around the world [24]. At present, several analysis centers can provide real-time service (RTS) products, but most of them only support single GPS systems or dual GPS/GLONASS systems. The RTS products provided by CNES can support four GNSS systems (GPS, GLONASS, Galileo, and BDS), so this paper uses its products for analysis. Real-time CNT products released by CNES can be downloaded from [[http://www.ppp-wizard.net/products/REAL_TIME](http://www.ppp-wizard.net/products/REAL_TIME)]([http://www.ppp-wizard.net/products/REAL_TIME](http://www.ppp-wizard.net/products/REAL_TIME)) (accessed on 14 September 2022); the website can provide sp3/clock products with a sampling rate of 5 min, and the file is named cntwwwwwwd.sp3/clk. In addition, it also provides a high-sampling-rate clock product with a sampling rate of 5 s. The acquisition delay of both files is 1 d [25]. The radiosonde data were from the Integrated Global Radiosonde Archive Version 2 (IGRA2) dataset generated by the National Climate Data Center of the United States in August 2016. The location distribution is shown in Figure 2. The experiment used radiosonde data from June 2021 (DOY 173-DOY 179) and August 2021 (DOY 229-DOY 235). The PWV of the radiosonde station was used to evaluate the accuracy of real-time PPP PWV. The PWV data of the radiosonde station were updated twice a day at 08:00 (UTC 00:00) and 20:00 (UTC 12:00). Data download address: [[ftp://ftp.ncdc.noaa.gov/pub/data/igra/derived/derived-Por](ftp://ftp.ncdc.noaa.gov/pub/data/igra/derived/derived-Por)]([ftp://ftp.ncdc.noaa.gov/pub/data/igra/derived/derived-Por](ftp://ftp.ncdc.noaa.gov/pub/data/igra/derived/derived-Por)) (accessed on 18 September 2022) [26]. The ERA5 pressure stratification and single-layer data were derived from the ERA5 reanalysis dataset provided by the European Centre for Medium-Range Weather Forecasts (ECMWF).