Calibration of stations of the low band antenna system of the LOw Frequency ARray (LOFAR-LBA) by Holography
N.V.S.S. Sivadevuni (TU Delft - Electrical Engineering, Mathematics and Computer Science)
A. G. Yarovoy – Mentor (TU Delft - Microwave Sensing, Signals & Systems)
Stefan Wijnholds – Graduation committee member (Netherlands Institute for Radio Astronomy (ASTRON))
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Abstract
The LOw Frequency ARray (LOFAR) radio telescope in the Netherlands covers an area of 300,000 sq.metre. LOFAR is considered as a pathfinder to the new and the largest radio telescope under construction, the Square Kilometer Array (SKA) covering an area of 1000,000 sq. metre. The similarities between the systems of the LOFAR and the SKA allow extension of calibration strategies from LOFAR to SKA. The calibration strategy employed for the LOFAR radio telescope assumes identical embedded element patterns (EEPs) among the antenna elements, which leads to systematic calibration errors. The aim of this thesis is to investigate the impact of this assumption on calibration errors for the low band antenna system of the LOFAR radio telescope (LOFAR-LBA). To this end, a simulation of calibration of the stations of the LOFAR-LBA system by holography is developed and implemented during this thesis. Results from the simulation are compared to investigate the impact of calibration errors with respect to the systemic errors from a LOFAR-LBA observation. Calibration methodologies that allow the inclusion of individual EEPs such as holography, superstation calibration and stand alone station calibration are briefly explored to facilitate comparison of computational costs involved in their respective implementations based on the systemic structure of LOFAR. Holography is identified and chosen for development of an instrument model with the inclusion of individual EEPs and implemented both as a simulation and on LOFAR-LBA measurement data. Holography on LOFAR-LBA is first implemented through simulation within the observed range of the signal-to-noise ratio from the measurement data. Results do not indicate appreciable errors in calibration after including individual EEPs in the instrument model owing to the low signal-to-noise ratio. An attempt was made to implement holography on LOFAR-LBA measurement data and several issues with the data were identified. The identified issues with LOFAR-LBA data were resolved and the code from the implementation of holography on LOFAR-HBA is made compatible to LOFAR-LBA data. Improvements in calibration results were expected with increasing SNR during the simulation. However, it was observed that the incorporation of individual EEPs into the instrument model to perform holography resulted in gain estimates very close to the gain estimates with the assumption of identical EEPs. This illustrates that while incorporation of individual EEPs does have an impact on the calibration results, the additional computational effort is not justified.