Comprehensive VLBI observations of Galileo satellites with the AuScope array

Abstract

Interest in the topic of geodetic co-location in space and space ties has recently intensified within the geodetic community, particularly following the approval of the European Space Agency's (ESA) Genesis mission. From the perspective of Very Long Baseline Interferometry (VLBI), observations of Earth-orbiting satellites are not standard practice yet. To enable VLBI support for future colocation satellite missions, such observations must be integrated into the VLBI processing chain. In this study, we present comprehensive VLBI observations of Galileo navigation satellites conducted with the Australian AuScope VLBI array. Using the 12-m antennas in Hobart, Katherine and Yarragadee equipped with VLBI Global Observing System (VGOS) instrumentation, Galileo E1 and E6 signals were observed in test experiments and a series of four full-scale 24-hour observing sessions. We present the estimation of VLBI station coordinates from observations to navigation satellites, thereby demonstrating, for the first time, inter-technique ties between the VLBI and Global Navigation Satellite System (GNSS) frame. We describe the processing strategy, including correlation, fringe fitting, precision assessment and satellite tracking approach. Delay observables achieve precisions of a few picoseconds in the E1 band and several tens of picoseconds in the E6 band for 1-s integration times. However, unmodelled signals on the order of several hundred picoseconds are found in the residual delays. Estimated station coordinates agree with a priori values at the metre level, while baseline lengths agree at the sub-metre level. These results demonstrate the feasibility of large-scale VLBI observations to GNSS satellites and provide critical groundwork for future co-location satellite missions such as Genesis.

Figures (20)

• Figure 1: Locations of the antennas used in the experiments of this work in Hobart (Hb), Katherine (Ke) and Yarragadee (Yg).
• Figure 2: Common visibility of Galileo satellites at elevations larger than 7$^{\circ}$ from the antennas in Hobart, Katherine and Yarragadee over three days.
• Figure 3: Illustration of the stepwise satellite tracking approach employed in the experiments of this work. At time epoch $t_i$, the antenna points ahead of the satellite in along-track direction at the midpoint of the orbit arc between the time epochs $t_i$ and $t_{i+1}$ to reduce the maximum angular separation between the topocentric satellite direction and the antenna pointing.
• Figure 4: Simplified illustration of the L-band, S-band and VGOS signal chains at the 12-m VLBI antennas in Hobart, Katherine and Yarragadee.
• Figure 5: Overview of the applied recording modes Rec_4x32_2 and Rec_2x64_8 (left) and the allocated frequency channels to the Galileo frequency bands (right). Information is given about the channel numbers, exact frequency range, bit-depth and corresponding Galileo frequency bands. The vertical upwards pointing arrows represent the respective band centre frequencies.