Impact of corrections to ionospheric calibrations of legacy geodetic VLBI observations

Over the past decades, the computation of ionosphere calibrations for very long baseline interferometry (VLBI) legacy S/X band observations and especially the derivation of effective frequencies have undergone various changes as a reaction to the continuous development of the VLBI technique. However, we found that almost all of the 4194 databases since 2001.0 serving for the exchange of VLBI data between correlators and analysts distributed via International VLBI Service for Geodesy and Astrometry (IVS) data centers contain inconsistent and in some cases erroneous parameter values. Consequently, effective frequencies and ionospheric calibrations had been computed and/or stored incorrectly resulting in faulty calibrations of VLBI observations. Incorrect values of effective frequencies deviate by as much as 8 MHz with some blunders being even larger than that. Individual ionospheric calibrations differ by multiple tens of picoseconds. To prepare investigations in the impact of the corrections performed, we computed solutions using both currently distributed and state-of-the-art ionospheric calibrations with the Vienna VLBI and Satellite Software (VieVS) and compared them. As a result of a re-processing of 24-h geodetic and astrometric VLBI data of the IVS since 2001.0, we found serious impacts on results, especially from the year 2022 onward. The current solar maximum period amplifies the effects of the wrong ionospheric calibrations and produces, on a session-by-session basis, differences in the estimates of Earth orientation parameters (up to 30 μ μ as/2.0 μ μ s), telescope coordinates (up to 4 mm), global scale determinations (up to 0.4 ppb), and declinations of radio sources in the southern mid latitudes (up to 200 μ μ as). At other time periods, the differences are generally smaller but still noticeable. We like to make the IVS aware of the need for a major re-processing of all S/X band observations and to alert the space geodetic community of imminent changes in VLBI results. For this purpose, we describe the historical developments, point out the issues that will change the results, and discuss their impact.