Performance of ray-traced VMF3 products in retrieving Zenith Tropospheric Delay over the African tropical region

Abstract

The availability of Global Navigation Satellite Systems (GNSS) networks allowed for the precise determination of the Zenith Tropospheric Delay (ZTD) with high temporal and spatial resolution. For the proper assessment of the atmospheric water vapour, which has a significant impact on weather forecasting and prediction, the availability of high spatial resolution ZTD measurements is absolutely essential. However, the quality and uses of GNSS ZTD products are impacted by the dispersed GNSS receiver stations and the significant data gaps in the GNSS observations over the tropical region of Africa. To solve the issues of data gaps and spatial variability, a number of methods have been devised to estimate the ZTD. The Vienna Mapping Functions 3 (VMF3) which applies the ray-tracing technique to get ZTD is one of the preferred methods for post processing. To evaluate the precision and viability of VMF3 in extracting the ZTD in the tropical region of Africa, validation is necessary. Therefore, the goal of this work is to give a statistical evaluation and performance of the Nevada Geodetic Laboratory (NGL) products and the site dependent VMF3 ZTD (V3GR_FC, V3GR_EI and V3GR_OP) products. To assess the level of agreement between the two datasets, the study used six different statistical criteria. These include the mean bias (MnB), coefficient of determination (R2), root mean square error (RMSE), correlation coefficient (r), the index of agreement (IA) and the Taylor skill score (TSS). The results demonstrate that VMF3 datasets can generate ZTD results that are comparable to NGL ZTD. However, it was discovered that the V3GR_OP ZTD (overall average values, RMSE = 9.77 mm, r = 0.953, R2 = 0.908, IA = 0.971, TSS = 0.948) outperforms the V3GR_EI ZTD (overall average values, RMSE = 13.12 mm, r = 0.912, R2 = 0.832, IA = 0.950, TSS = 0.915) and the V3GR_FC ZTD (overall average values, RMSE = 13.27 mm, r = 0.907, R2 = 0.824, IA = 0.942, TSS = 0.903). The performance of V3GR_EI ZTD and V3GR_FC ZTD varies from station to station.