A Comparative Study of VTEC Estimates Derived From S/X VLBI and VGOS Observations

Abstract The ionosphere, which strongly varies in time and space, is one of the largest error sources for microwave measurements, if not accounted for accurately. Especially for single‐frequency observations, precise ionospheric models are needed to correct for the ionospheric delays. This study explores the utility of Very Long Baseline Interferometry (VLBI) and the VLBI Global Observing System (VGOS) in determining ionospheric delays essential for precise microwave measurements. By comparing Vertical Total Electron Content (VTEC) derived from colocated S/X VLBI and VGOS stations during simultaneous sessions, our analysis reveals good agreement between these two systems and Global Ionosphere Maps (GIM). The results indicate that VGOS yields more stable VTEC estimates, with a smaller RMS increase in differences to the GIM, from 2.48 TECU during solar quiet to 4.19 TECU during solar active conditions, compared to S/X VLBI, which increases from 2.32 to 5.60 TECU. This suggests that VGOS, due to its broader frequency coverage and higher observation density, enables more accurate ionospheric monitoring during active phases. An analysis of long‐term VLBI data indicates good agreement with GIM and corresponds closely to the yearly and solar‐cycle periods. Based on the data used, the bias in the VTEC estimates as derived from S/X VLBI and GIM is −1.5 TECU during solar quiet periods and −4.3 TECU during solar active periods. In relation to VGOS, the bias relative to GIM is −2.2 TECU and −3.4 TECU for the same respective conditions. Overall, S/X VLBI and VGOS can contribute to enhancing our understanding of space weather effects on microwave techniques.