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.
A. Hellerschmied, L. McCallum, J. McCallum, J. Sun, J. Böhm, J. Cao: Observing APOD with the AusScope VLBI Array, Sensors, 18, 5, pp. 1587 – 1607, 2018.
L. Plank, A. Hellerschmied, J. McCallum, J. Böhm, J. Lovell: VLBI observations of GNSS satellites: from scheduling to analysis, Journal of Geodesy, 91, 7; pp. 867 – 880, 2017.
Raytraced Delays in the Atmosphere for Geodetic VLBI
Radiate: FWF P25320
Project-Leader: Johannes Böhm (1 May 2013 – 30 June 2017)
Summary for public relations work:
While atmospheric effects on the propagation of radio waves take place in the whole atmosphere, the troposphere as the lowest part plays a key role with its weather phenomena and the highly variable distribution of humidity. Modelling the delays in the troposphere of the signals from extragalactic radio sources in the analysis of geodetic Very Long Baseline Interferometry (VLBI) observations and of the signals from Global Navigation Satellite Systems (GNSS), such as the U.S. Global Positioning System (GPS) or the European Galileo, is a major error source influencing the accuracy of these space geodetic techniques. In particular, insufficient models may affect station coordinates and the terrestrial reference frame. The goal of the project RADIATE VLBI was to challenge the existing tropospheric delay models, thereby developing improved and new methods for the tropospheric calibration.
We used the concept of “ray-tracing" to determine values of those delays through applying electromagnetic wave equations onto data of numerical weather models provided by the European Centre for Medium-range Weather Forecasts (ECMWF). In the first part of project RADIATE VLBI we developed a sophisticated and fast ray-tracing program called RADIATE. This program was then used to determine slant tropospheric delays for all (more than 10 million) geodetic VLBI observations and to derive improved tropospheric delay models, such as the Vienna Mapping Function 3 (VMF3).
We found that the application of ray-traced delays in VLBI analysis in particular improves the solution of VLBI sessions with a small number of observations or if no tropospheric gradients are estimated. The newly developed VMF3 slightly improves the accuracy of the terrestrial reference frame with an impact on station heights up to two millimetres. Ray-traced delays and tropospheric gradients, the latter derived together with VMF3, do have a significant impact on the celestial reference frame with source declination changes up to more than 50 microarcseconds. In parallel to 6-hourly coefficients of the VMF3, we also derived so-called empirical delay models such as GPT3, which is fully consistent with VMF3 but only contains annual and semi-annual terms.
Although we could confirm that existing tropospheric delay models are already at a very high level of accuracy, we do provide new models (VMF3, GPT3, gradients) which improve the accuracy of geodetic parameters. However, it should be stressed that further research in tropospheric delay modelling is required to reach the goal of one millimetre accuracy in daily station coordinates. On the other hand, all models developed in project RADIATE VLBI may be implemented in positioning and navigation devices with GNSS receivers such as smartphones.
Armin Hofmeister, Johannes Böhm, Application of ray-traced tropospheric slant delays to geodetic VLBI analysis, Journal of Geodesy Vol. 91(8), doi:10.1007/s00190-017-1000-7, pp. 945-964, 2017.
Daniel Landskron, Johannes Böhm, VMF3/GPT3: Refined Discrete and Empirical Troposphere Mapping Functions, Journal of Geodesy, doi: 10.1007/s00190-017-1066-2, 2017.
Project-Leader: Johannes Böhm (1 July 2011 – 31 December 2014)
Hana Krásná, Johannes Böhm, Harald Schuh, Free core nutation observed by VLBI, Astronomy and Astrophysics, 555, A29, 2013.
Hana Krásná, Johannes Böhm, Harald Schuh, Tidal Love and Shida numbers estimated by geodetic VLBI, Journal of Geodynamics, Vol. 70, pp. 21-27, 2013.
Optimum design of geodetic VLBI networks and observing strategies
VLBI2010: FWF P18404
Project-Leader: Johannes Böhm (1 October 2005 – 31 December 2008)
J. Wresnik, J. Böhm, A. Pany, H. Schuh, Towards a new VLBI System for Geodesy and Astrometry, Advances in Geosciences, Vol. 13: Solid Earth (2007), pp. 167-180, 2009.