VLBI Celestial Reference Frames and Assessment with Gaia
Abstract
Finding the exact positions of objects in the sky, also called astrometry, has a long tradition, which dates back thousands of years. For most of that time, the visible stars were used as reference points. However, nowadays observations are accurate enough to observe proper motions of these stars, which makes them inadequate reference points. A new type of extragalactic object, the quasar, revolutionized the field with its discovery in the 1960s. These quasars are billions of light years away, which means that they are very stable and hence the ideal reference point. They are observed with a technique called Very Long Baseline Interferometry (VLBI), which uses an array of radio telescopes to measure the accurate positions of these objects. Only a couple of decades after their discovery a catalog of quasars replaced the most accurate star catalog as recommended reference frame. For three decades the accuracy of this frame was out of reach for techniques operating in the optical part of the electromagnetic spectrum. This has changed with the Gaia satellite mission from the European Space Agency (ESA), which was launched in 2013 with the aim of mapping a billion stars with an unprecedented accuracy. Among the objects measured by Gaia are about half a million quasars with positional accuracies comparable to VLBI. For the first time in the history of VLBI a comparable celestial reference frame exists which can be used to compare positions. The aim of this thesis is to investigate the influence of different modeling and analysis strategies on the celestial reference frame estimated with VLBI. Systematic changes in the VLBI reference frame are assessed with the help of the Gaia reference frame. Before the influence of models and analysis strategies on the celestial reference frame can be assessed a suitable solution has to be compiled. In this case the Vienna VLBI and Satellite Software (VieVS) was used to derive a catalog of source positions in X/S band with more than 4500 sources. This solution is comparable in scope and accuracy with the official International Celestial Reference Frame 3 (ICRF3) solution. It has a noise floor of about 30 µas and deformations below 15 µas with respect to ICRF3. In order to compare two astrometric catalogs one has to find intersecting sources and calculate the difference of their coordinates. A robust approach of outlier determination has to be found and utilized. Then a suitable method to extract large-scale systematic effects from this difference scatter has to be applied. The state of the art technique used for comparing astrometric catalogs is the so-called Vector Spherical Harmonic (VSH) decomposition. It expands a vector field using orthogonal basis functions up to a certain degree using the difference vector field of the two catalogs. The degree of expansion depends on the level of investigated detail. Since large scale systematic effects are of interest in this thesis, the expansion was stopped after degree 2. Degree 1 can be further split into a global rotation and deformation (also called glide). Investigating large-scale systematics in the VLBI technique with the help of the Gaia reference frame revealed that certain choices made by the analyst are affecting source coordinates significantly. First, it was found that the correction of galactic aberration removes most of the glide between the VLBI and the Gaia reference frame. Second, the D3-parameter which is directly connected to the source declination was found to be affected by many models and estimation choices. This indicates that this parameter is rather unstable, which has to do with the weak VLBI observing network in the Southern Hemisphere. Third, it was found that using ray-tracing to calculate a priori tropospheric delays succeeds in reducing the most significant VSH parameter (the a20 parameter of degree 2) between these reference frames. In conclusion one can say that the VLBI and the Gaia reference frame do agree on a level, which will facilitate future studies about the reference points.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- December 2018
- Bibcode:
- 2018PhDT.......126M
- Keywords:
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- VLBI;
- Gaia;
- ICRF;
- VSH;
- celestial reference frame