Potential of UT1–UTC transfer to the Galileo constellation using onboard VLBI transmitters

UT1–UTC is one of the Earth orientation parameters (EOP) that can only be determined by very long baseline interferometry (VLBI), observing distant celestial sources to measure the Earth rotation angle. Earth orbiting satellites tracked from Earth are insensitive to this angle, and the orbit determination and time synchronization (ODTS) procedure for GNSS satellites hence requires the UT1–UTC as an input. Today, UT1–UTC is provided by the International Earth Rotation and Reference Systems Service (IERS). A VLBI transmitter (VT) onboard GNSS satellites would, as an alternative way, allow the direct transfer of this information as an integrated step to the ODTS process thanks to the space-tie established between the VLBI and GNSS techniques. Here, we investigate the transfer quality of the UT1–UTC in such a concept by considering different VLBI baselines. In the simulations, we assume observations from a VT onboard a Galileo satellite together with quasar observations acquired with the same VLBI ground stations during a session and therewith allowing to directly transfer UT1–UTC to the GNSS constellation. The geometrical setting of the Galileo satellite with respect to the ground stations is quantified by the UT1–UTC dilution of precision (UDOP). Our simulations show that it is feasible to transfer UT1–UTC with a precision of about 30μs for a long VLBI baseline where the UT1–UTC precision estimated from quasar observations is 20μs. Using VLBI networks instead of a single baseline can improve the transfer precision further by more than 20 % depending on the baseline selection.