Direct VLBI Observations of Global Navigation Satellite System Signals

We describe an experiment carried out to observe signals emitted by GLONASS (GLObal NAvigation Satellite System) satellites using the Very Long Baseline Interferometry (VLBI) technique. This test was performed on a single baseline and had as its primary goal to evaluate the capability of the VLBI system to observe GNSS (Global Navigation Satellite System) signals in terms of scheduling, tracking, acquisition, recording, correlation and processing of data. The general aim of such observations is to contribute to the connection of the reference frames for GNSS and VLBI by so-called co-location in space, or space-ties, as a complement to the existing so-called local-ties on the Earth's surface. In our experiment we found an interferometric response from both signals emitted by GLONASS satellites and a natural radio source that was observed as a calibrator, using the same VLBI equipment. The derived fringe phase scatters were 80 ps (2.5 cm) and 1.3 ns (39 cm) in 1 s for the GLONASS satellite and the calibrator signals, respectively. This indicates that the accuracy is not limited by GLONASS signals, but by the calibrator. Our results show that VLBI observations of GNSS signals are possible and have the potential to derive the satellite positions on a centimetre level for observing times of just a few minutes. Future experiments should include several baselines and a larger number of calibrators in close angular distance to the satellite tracks to allow frequent switching between calibrator and satellite signals.