VLBI signal transmitter onboard Earth orbiting satellites

Creating an absolute space-tie where all the geodetic methods are onboard is the key for an improved and stable terrestrial reference frame as well as with various scientific applications. Such satellite concepts have already been proposed to achieve an accurate and stable terrestrial reference frame. Next generation Galileo satellites can provide a single well-calibrated platform for the colocation of the space-based geodetic techniques establishing precise and stable ties between the key geodetic techniques. One of the most crucial and novel aspect of such concepts is the VLBI transmitter (VT) which will emit quasar-like signals from the space to be observed by the VLBI ground stations. VT can directly link the terrestrial and celestial reference frames and bring the unique features of VLBI technique to an Earth orbiting satellite. In the context of call for future Galileo payloads a novel VT has been under development. VT shall be compatible both with the legacy and VGOS antennas. Here, we present the progress on ongoing ESA study for VT for Galileo as well as for other future missions.