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The Effelsberg station calibration challenge

Submitter: Stefan Wijnholds
Description: The LOFAR LBA station in Effelsberg has been posing a station calibration challenge ever since it was built, because it is plagued by a mysterious RFI source. Since desperate times call for desperate measures, I decided to try the sky-model-independent redundancy calibration on an array without redundant baselines. Fortunately, the station is very dense, thereby providing a sufficient number of approximately redundant baselines whose baseline vectors differ by at most 60 cm, which is only ~1/10th of a wavelength at 50 MHz.

After calibration, I noticed that the gain solutions seemed to alternate between two possible solutions, where the first solution was obtained when celestial sources were dominating the image and the second solution was found when the RFI was dominating. A nice starting point for further investigation!

The left image shows the difference between the phase solutions in the two situations, interpolated over the aperture of the station. The white crosses indicate the locations of the antennas. A parabolic phase screen is clearly visible, indicating that the RFI source is in the near-field of the station. A more detailed analysis of the curvature indicated that the source is about 215 m away from the station.

The right image puts this in perspective by showing an all-sky map at 51.6 MHz, dominated by the RFI source, projected on a sphere with 215-m radius with the LBA station at its center. Both the color and the opacity of the map reflect the intensity of the radio waves received. The observer is located at an elevation of 30 degrees due North of the station.

This analysis indicates that the RFI is produced by the 100-m Effelsberg telescope, which was confirmed recently by the people in Bonn, who found that the engines of the 100-m dish are producing RFI. Although we seem to have found a way to calibrate the station, we hope to do an RFI-free calibration observation the next time the 100-m telescope is serviced.
Copyright: ASTRON / LOFAR
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