Instrumental and systematic effects in VGOS differential ionospheric total electron content

The new generation of geodetic Very Long Baseline Interferometry (VLBI) network, named the VLBI Global Observing System (VGOS), aims to provide high-accuracy data, with the goal of 1 mm position accuracy over a 24-h observing session and 0.1 mm/yr velocity accuracy. The precision of the VGOS ionosphere product, measured as a differential Total Electron Content (dTEC) at the observing baseline, is notably higher compared to the dTEC determined with traditional geodetic S/X VLBI observations. However, the new VGOS instrumental and data processing strategy may introduce some effects on the estimated dTEC and the group delay, potentially degrading the geodetic parameters. To evaluate the VGOS ionosphere product dTEC, we used the Center for Orbit Determination in Europe (CODE) Global Ionosphere Model (GIM) as a reference. VGOS sessions observed between 2019 and 2022, along with corresponding CODE GIM maps, were analysed. The linear regression fitted between VGOS dTEC and CODE GIM dTEC reveals a systematic slope and an offset between the two datasets. The detected dTEC offset indicates the presence of systematic errors due to data processing and/or instrumental effects in the VGOS data. We found that residual phase corrections applied during VGOS data post-processing affect the VGOS dTEC and cause the offset.