Satellite Observations with Radio Telescopes for Superior Reference Frame Interconnections

SORTS: FWF I2204 and DFG (PI: Axel Nothnagel)

Project-Leader: Johannes Böhm (1 January 2016 – 31 December 2019)

Summary for public relations work:

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.

Further reading:

  • Hellerschmied A. , McCallum L. , McCallum J. , Sun J., Böhm J. , Cao J. (2018). Observing APOD with the AusScope VLBI Array, Sensors, 18, 5, pp. 1587 – 1607.
  • Plank L., Hellerschmied A., McCallum J., Böhm J., Lovell J. (2017). VLBI observations of GNSS satellites: from scheduling to analysis, Journal of Geodesy, 91, 7; pp. 867 – 880.