Effects of the 2nd order ionospheric terms on VLBI measurements

Current VLBI software packages take into account ionospheric terms of 1st order only. This research investigates the effects of considering additionally the 2nd order terms on geodetic VLBI measurements. The mathematical algorithm follows the framework that has been presented for GPS in the literature. However, the approximation of the Earth's magnetic field and the assumption of a 400‐km high ionospheric shell are avoided here by using the IGRF and PIM models to calculate the geomagnetic field vectors and electron densities at 100 representative points along the paths of incoming rays. The 2nd order effects are at the level of 10−12 seconds, which we introduced to the VLBI input files to determine the geodetic impact. Results of the VLBI software OCCAM reveal a maximum difference between baseline lengths with and without 2nd order ionospheric terms equivalent to 0.5 mm. Thus, the effect might become visible in VLBI within a few years.