A global zenith tropospheric delay model with ERA5 and GNSS-based ZTD difference correction

Highly accurate tropospheric delay information is essential for global navigation satellite system (GNSS) data processing. However, the current models still have limitations such as a lack of systematic difference correction and appropriate fitting functions. We investigated the performances of different order polynomials in developing ERA5 (the fifth generation of European Center for Medium-Range Weather Forecasts Reanalysis)-based zenith tropospheric delay (ZTD) vertical adjustment model, and it is noticed that a cubic polynomial fitting function is optimal. Considering there is a difference between the ERA5 and GNSS-based ZTD, its characteristic is analyzed using 5 years data. This difference is found to have a noticeable variation of the annual period and is modeled in each window, which depends on ERA5-based ZTD vertical adjustment model. An improved ZTD vertical adjustment model is constructed using the modeled difference, and it has a 20% improvement in accuracy compared with that of ERA5-based ZTD vertical adjustment model. When the proposed ZTD vertical adjustment model is used to establish a ZTD empirical global grid model, the model, which includes systematic differences correction, has an accuracy improvement of 6 and 2%, respectively, compared with GGZTD-H (global grid zenith tropospheric delay) and without correction models. The proposed model also exhibits superior performance in precise point positioning, particularly in the vertical direction. This ZTD empirical global grid model delivers highly accurate ZTD values and can thus be a viable option for GNSS precise positioning.