# GNSS Real-Time ZTD/PWV Retrieval Based on PPP with Broadcast Ephemerides
###### Abstract  
GNSS precise point positioning (PPP) plays an important role in retrieving atmospheric water vapor values and performing numerical weather prediction. However, traditional PPP relies on real-time orbits and clocks, which require continuous internet or satellite communication. Improved broadcast ephemerides of GNSSs offer new opportunities for PPP with broadcast ephemerides (BE-PPP) instead of using precise ephemeride products. Therefore, we investigated the feasibility of utilizing BE-PPP for retrieving zenith tropospheric delay (ZTD) and precipitable water vapor (PWV) data. We processed the GPS/Galileo observations from 80 ICS stations during a 30-day experiment to retrieve ZTD values using both real-time PPP (RT-PPP) and BE-PPP solutions. Then, we processed observations from 20 EUREF Permanent GNSS Network (EPN) stations to retrieve PWV data. The IGS final tropospheric products were used to validate the ZTD, and radiosonde (RDS) and ERA5 data were used to validate the PWV. The results show that the real-time ZTD from BE-PPP agrees well with that from RT-PPP: the standard deviation (STD) of the ZTD is 1.07 cm when using BE-PPP and 0.6 cm when using RT-PPP. Furthermore, the STD of the PWV is 1.69 mm when using BE-PPP, and 0.96 mm when using RT-PPP, compared to the ERA5-PWV. Compared to the RDS-PWV, the STD is 3.09 mm when using BE-PPP and 1.39 mm when using RT-PPP. In conclusion, the real-time ZTD/PWV products obtained using the BE-PPP solution are consistent with those of RT-PPP and meet the requirements of NWP, so this method can be used as an effective complement to RT-PPP to expand its application potential. e-mail: 202  
and spatial resolution characteristics, of such a method [15]. High-precision ZTD/PWV products can provide powerful supporting data for heavy rainfall forecasting [16] and drought monitoring [17]. Meanwhile, thanks to the real-time service (RTS) of the International GNSS Service (IGS) [18], PPP real-time atmospheric parameter retrieval can help meet the accuracy requirements of time-critical meteorological applications [19]. Based on PPP, high-accuracy real-time ZTD values can be obtained by applying high-accuracy real-time satellite clock and orbit products [20]. [PERSON] et al. retrieved the ZTD based on PPP float ambiguity solutions, and the accuracy varied from 5.4 mm to 10.1 mm across different stations [21]. ZTD accuracy is further improved under the fixed ambiguity solution, with a root mean square (RMS) value of 7 mm compared to the post-processed GPS ZTD product [13]. [PERSON] et al. proposed a real-time linear and nonlinear ZTD (RLNZ) model, and the accuracy of the ZTD estimated by directly using the models is better than that obtained with the Global Pressure and Temperature 3 (GPT3) model and also better than the positioning accuracy of real-time PPP (RT-PPP) [22]. RT-PPP also performs better when retrieving PWV data. The real-time GPS PPP method can obtain PWV data with a root mean square error (RMSE) of less than 3 mm [23]. Using the RDS PWV as a reference value, the RMS of the real-time PPP PWV is less than 2.4 mm [24], and using the fifth-generation ECMWF (European Center for Medium-Range Weather Forecasts) reanalysis (ERA5) PWV as a reference, the RMS is 2.63 mm [25]. However, the retrieval of real-time ZTD/PWV data requires the continuous communication of real-time orbits and clocks via internet links, which can be affected by signal outages. To address this issue, [PERSON] et al. [26] developed a method to utilize IGS ultra-rapid orbits and establish clock corrections. Upon applying this method to the real-time retrieval of ZTD/PWV data, the RMS of only GPS, only Galileo, and only BDS-3 ZTDs are 2.20 cm, 3.07 cm, and 4.21 cm, while the PWVs are 3.42 mm, 4.57 mm, and 6.66 mm, respectively, for a 10 min interruption.