A New Four-Layer Inverse Scale Height Grid Model of China for Zenith Tropospheric Delay Correction

To reduce the convergence time in the real-time precise point positioning (PPP), the zenith tropospheric delay (ZTD) needs to be modeled with high accuracy and sent to the users to be introduced as a priori value. Because of the height difference between the model height and user’s location, the ZTD at the model height must be converted to that at the user’s height. Therefore, it is crucial to investigate an optimal method for this height correction of ZTD. It is normally assumed that the ZTD decreases exponentially with the altitude and an empirical decrease factor is adopted for depicting the ZTD variation with altitude. In this study, the performance of the commonly used single-layer model of the decrease factor was investigated. Based on the investigation of the single-layer model, a new four-layer grid model ( $0.25^{\circ }\times 0.25^{\circ }$ ) over China was constructed with 10-year (from 2009 to 2018) ERA5 data, and its performance was validated. The results indicate that the newly constructed four-layer model has a better performance than the single-layer model with the bias and root mean square (RMS) of the determined ZTD reduced by 54.4% and 27.6%, respectively. With the four- and single-layer grid model and GNSS-derived ZTD, two ZTD models i.e. four-layer ZTD and single-layer ZTD were constructed over China. The experiment shows that the convergence time of PPP can be obviously reduced when introducing these two ZTD models and the four-layer ZTD model has a much better performance than the single-layer ZTD, especially at stations with a high altitude.