Satellite Observations by Radio Telescopes

Satellite observations by radio telescopes have the potential to dramatically improve the frame ties between the International Celestial Reference Frame (ICRF) realized with Very Long Baseline Interferometry (VLBI) observations to extragalactic radio sources and satellite orbits realizing celestial frames dynamically. At the same time, the consistency of the International Terrestrial Reference Frame (ITRF) as a multi-technique solution from VLBI and satellite geodesy observations will benefit substantially, which otherwise solely relies on local tie measurements at co-located terrestrial observing sites. Accurate frame ties are fundamental for precise navigation in space and are a prerequisite for the observation of sea level rise and other global geodynamic processes. In project SORTS, we cover all technical aspects which are necessary for the realization and analysis of VLBI-like observations to satellites dedicated to such space-tie projects. Now is the optimal time frame for such a project because proposals for satellites which carry a VLBI transponder, laser reflectors and a GNSS (Global Navigation Satellite Systems) receiver like GRASP or MicroGEM are close to acceptance. In project SORTS, we will investigate the application of noise versus modulated signals in terms of signal-to-noise ratio and projected precision of group and phase delays. Next, we will set up a fully realistic scheduling tool taking into account all station- and source-specific restrictions like slewing rates and integration times and we will close all gaps from scheduling to the actual observations of satellites with radio telescopes allowing for automated procedures. Particular emphasis will be put on joint schedules with alternate observations to quasars and satellites. To generate group and phase delays from the observations by radio telescopes to satellites at low Earth orbits, we will adapt all models in the correlation software. Finally, we will expand existing geodetic analysis software for the purpose of optimally combining quasar and satellite observations. With this software, we will use realistically simulated observations to determine orbital arcs from various types of radio telescope observations and we will run series of Monte-Carlo simulations to provide feedback to earlier steps in the processing chain like the scheduling and to assess the impact on the frames ties. Project SORTS is a joint endeavor of the Vienna University of Technology and the University of Bonn with the strengths of both groups greatly complementing each other. Furthermore, in close co- operation with partners at the VLBI stations at Wettzell in Germany and Onsala in Sweden, we will be able to validate our developments with real observations already at a very early stage, for example with VLBI observations to GNSS satellites.

Global geodetic reference frames (GGRF) are essential for all kinds of positioning and navigation on Earth and in space as well as for geodynamic studies like the observation of sea level rise, where utmost accuracies are required to reliably determine a sea level rise at the level of 3 mm/year. We typically estimate GGRF, such as International Terrestrial Reference Frame 2014 (ITRF2014), in a combination of Global Navigation Satellite Systems (GNSS), Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) observations. The individual four techniques are connected with so-called local tie measurements at the co-location sites between the antennas from the techniques. Although these local measurements are very precise in principle, very often residuals at the few centimeter level show up if compared to results from space geodesy (GNSS, VLBI, SLR, DORIS). Thus, the idea of the geodetic community is to connect the technique observations with a well calibrated satellite equipped with all four techniques. While this concept is straightforward for GNSS, SLR, and DORIS, the observation of satellite signals with VLBI radio telescopes and the derivation of geodetic observables is new. In project SORTS we have successfully realized the complete process chain from scheduling VLBI observations to satellites, carrying out the observations, correlating and fringe-fitting the raw data with a so-called correlator, and analyzing the observables with geodetic software. We have performed these tasks for VLBI observations to GNSS (L1- and L2-band signals from GPS and GLONASS satellites) on the Australian baseline Hobart to Ceduna. Additionally, Warkworth in New Zealand joined the last observing session. In a second case study, we observed the Chinese low Earth orbiter APOD-A with the Auscope network in Australia including the telescopes in Hobart, Katherine and Yarragadee which are run by colleagues at the University of Tasmania. While APOD-A nanosatellite is a prototype of a co-location satellite sending special DOR-tones at X- and S-band, the low orbit at about 450 km complicates the observations and the analyses. In both test cases, i.e., for the VLBI observations to GNSS satellites and the APOD-A nanosatellite, we could retrieve observations residuals at the nanosecond level. In summary, the findings of project SORTS form a very good basis for future research activities in that area.