Quantitative analysis of the tropospheric delay difference using empirical mapping functions on VLBI stations

Tropospheric delay is a major error source for the high-accuracy Very Long Baseline Interferometry (VLBI) technique due to rapid water vapor variations. The Niell mapping function (NMF) and Global Mapping Function (GMF) are extensively applied to convert zenith tropospheric delay to slant delays for real-time deep-space VLBI operations. In this study, we quantitatively evaluate the performance of different empirical Mapping Functions for VLBI signal correction, with Vienna Mapping Function 3 (VMF3) and Water Vapor Radiometer (WVR) applied to track Tianwen-1 spacecraft simultaneously. Quantitative tropospheric delay comparisons demonstrate that empirical Global Pressure and Temperature 3 (GPT3) outperform NMF and GMF on BJMY and SHAO, while, GMF performs better at YNKM and CUAO under low elevation angles (1°–30°). Further evaluation of VLBI delay residual across six baselines indicated that at low elevations, the average residual RMS with GPT3 decreases from 5.96 ps to 1.89 ps, representing a 68% improvement relative to GMF. This study provides a reliable reference for tropospheric delay calibration in deep-space tracking and other VLBI-based radio observations, particularly for low-elevation scenarios.