# A New Zenith Tropospheric Delay Grid Product for Real-Time PPP Applications over China
## 5 Discussion and Conclusions  
The temporal and spatial variations of ZTD with altitude over China were analyzed on the bases of the latest meteorological reanalysis products (ERA-Interim). Results show that the variations of ZTD with altitude are near exponential, and the temporal variations of exponential function coefficients at specific location mainly contain annual and semi-annual periodical signals. An inverse scale height model was then determined and applied to ZTD solutions derived from CMONOC GPS stations in simulated real-time mode to generate the ZTD grid products. Using 16 stations in CMONOC as test stations, the ZTD derived from the ZTD grid product were compared to ZTD estimated in post-processing mode and the average bias and RMS of the ZTD differences are 0.39 cm and 1.56 cm, respectively.  
All GPS stations in CMONOC were used to generate RtZTD products in simulated real-time mode, and 95 stations with GPS and BDS observations in NBASS were taken as test stations for PPP convergence tests. Data covering one week were processed and the convergence time of PPP with and without RtZTD products used were compared. Regarding the convergence time, the thresholds for convergence were determined based on coordinate error RMS statistics, i.e., 0.4 and 0.2 m of coordinate errors for BDS- and GPS-PPP, respectively, in 95% statistics, while 0.2 and 0.1 m for BDS and GPS, respectively, in 68% statistics. The results show considerable improvements in convergence time by using RtZTD of about 32% (6%) and 65% (65%) in the horizontal and vertical components for BDS-PPP, and 29% (25%) in vertical direction for GPS PPP in 95% (68%) statistics.  
The time delays, including transmission time from reference stations to the data center, data processing time and transmission time from the data center to users, are generally smaller than 10 s in current augmentation systems. For example, the total time delays in the Global Differential GPS (GDGPS) System developed by JPL are smaller than 5 s. Considering the relatively small variations of orbit and clock corrections (<1 cm) and ZTD (<1 mm) in small intervals like 10 s (variations not shown here), these time delays can be safely ignored in real-time applications. This is also the reason why in some satellite-based augmentation system services, e.g., the QZSS L6 signal, which provides centimeter level augmentation services, can broadcast orbit and clock corrections in 30 and 5 s intervals without using any prediction models, and why IGS centers can estimate ZTD as constants in 5 min intervals. Therefore, although all real-time tests in this work were carried out in the simulated mode with time delays ignored, the conclusions can provide important references to real-time applications. As more and more real-time GPS stations are available in China, we can expect better and more robust results in the near future.  
This study is supported by the National Key Research and Development Plan (No. 2016 YFB0501802), and the National Natural Science Foundation of China (41374034 and 41504028). The authors would like to thank ECMWF for providing reanalysis products, and CMONOC and NBASS for providing GNSS observations data.  
[PERSON], [PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] analyzed the data; [PERSON] and [PERSON] contributed analysis tools; [PERSON] wrote the paper; all authors reviewed the manuscript.  
The authors declare no conflict of interest.