Source Structure in VGOS Fringe-Fitting

Particularly strong natural sources of radio emission are located in space, at locations very distant from the Earth. Radio astronomy uses gigantic antennas to receive radio emission from these sources. These antennas have to be so huge because only a tiny fraction of the emission from these very distant sources arrives on Earth. Geodesy, i.e., the science of measuring the Earth, uses a special kind of radio sources, known as quasars, as fixed points on the sky, in order to measure the Earth, to define coordinate systems, and to determine the instantaneous orientation of the Earth in space. This technology is known as geodetic VLBI. Unfortunately, these quasars are often not perfect points but have an extension, which is also referred to as source structure. This complicates the measurements and deteriorates results. In our project SOFT we develop a novel method to correct radio data for source structure. For this purpose we first inspect images of quasars in order to understand how they typically look like at the wavelengths which are important for us. In the next step we generate artificial data which are affected by a known source structure. These computations are very expensive, which is why we carry them out on the Vienna Scientific Cluster. We develop a computer program to remove the source structure contribution from the radio data. For the following processing steps these data then appear as from point sources. We will investigate how much of the source structure we can remove from the data and to which improvement of the measurements this leads. We will also apply our methods to observational data. Once the source structure of many quasars will be known in reality, the method developed by us will be used to correct the observations and to an improvement of the celestial reference frame.