Active mitigation of spaceborne radio frequency interference for VLBI

The proliferation of spaceborne radio frequency interference (RFI) is threatening the operation of VLBI. In particular, mega-constellations, such as Starlink, OneWeb, and Amazon Kuiper, pose a significant threat since some of these satellites will emit signals in the upper-frequency range of the VLBI Global Observing System (VGOS). The power of these signals may saturate the amplifiers of VLBI antennas, causing nonlinearities within the observations. Furthermore, some future InSAR satellites, such as the NISAR mission, might emit signals strong enough to permanently damage the highly sensitive VLBI hardware. We will present our approach for an active mitigation strategy that modifies the current VLBI observing strategy to avoid observations in the direction of spaceborne RFI. However, active avoidance means additional constraints during the observation planning. We will present results of the investigation the impact of these constraints w.r.t. the precision of the geodetic parameters using simulations and compare it to a situation (1) with active avoidance, (2) without active avoidance, where affected observations are lost and simply removed from the simulations, and (3) without active avoidance and no spaceborne RFI being present. Our simulations are based on different VGOS networks and satellite mega-constellation expansion stages. We will discuss to which extent our approach can help to avoid spaceborne RFI but also highlight its limitations and necessary development steps toward operational use. Finally, we will discuss the practical requirements of active mitigation of spaceborne RFI based on scheduling. Here, the accuracy of the orbit prediction seems to play a crucial role. With the current observation strategy of submitting a schedule seven days prior to observation, significant error margins must be applied to the satellite orbits. We will also present recent developments in storing and providing satellite orbit information in the form of two-line-elements (TLE) that will be used in the current scheduling pipelines.