Optimal requirements for determining frame ties using VLBI observations to satellites

Abstract

Observing satellites with Very Long Baseline Interferometry (VLBI) telescopes has been a hot topic in the VLBI community in recent years opening up a wide range of new possibilities, including the determination of the absolute orientation of the orbit in the celestial reference frame. Furthermore, VLBI observations to satellites allow the derivation of station positions of the VLBI antennas in the dynamic satellite frame. These coordinates can be compared to the station coordinates obtained from quasar observations, providing a link between the dynamic satellite frame and the kinematic quasar frame known as frame tie. In this study, we explore the best requirements for estimating station coordinates using VLBI observations to Galileo satellites. The main issue is the number of satellites that need to be outfitted with a VLBI transmitter (VT) in the Galileo space segment to ensure station coordinates are sufficiently accurate. Additionally, there is the matter of how these satellites should be spread across the three planes and what is the best balance between observing satellites and quasars within a schedule. Therefore, we analyse different scenarios with varying numbers of satellites with a VT and their distribution over the planes. For each scenario, several 24 hour schedules are created with different ratios between satellite and quasar observations. These schedules are simulated and analysed using the Vienna VLBI and Satellite Software (VieVS). The different scenarios and ratios are evaluated by the repeatability of the station coordinates in north, east, and up direction. The results indicate that equipping only one satellite does not provide sufficient accuracy for the estimation of station coordinates from a 24 hour session. An interesting discovery is that the optimal placement of two or three satellites with a VT would be in the same plane. This study found that two satellites with a VT in the same plane provide better results than three satellites with a VT in different planes. However, not only proper positioning within the Galileo space segment is critical, but also the balance of satellite and quasar observations within a schedule has a significant impact on the results. For optimal results, this study suggests a balance of approximately 30–40 \% satellite observations out of the total number of observations.