On-sky calibration of a SKA1-low station in the presence of mutual coupling

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>This paper deals with the calibration of the analogue chains of a Square Kilometre Array (SKA) phased aperture array station, using embedded element patterns (one per antenna in the array, thus accounting for the full effects of mutual coupling) or average element patterns to generate model visibilities. The array is composed of 256 log-periodic dipole array antennas. A simulator capable of generating such per-baseline model visibility correlation matrices was implemented, which allowed for a direct comparison of calibration results using Statistically Efficient and Fast Calibration (StEFCal) with both pattern types. Calibrating the array with StEFCal using simulator-generated model visibilities was successful and thus constitutes a possible routine for calibration of an SKA phase aperture array station. In addition, results indicate that there was no significant advantage in calibrating with embedded element patterns, with StEFCal successfully retrieving similar per-element coefficients with model visibilities generated with either pattern type. This can be of significant importance for mitigating computational costs for calibration, particularly for the consideration of real-time calibration strategies. Data from the Aperture Array Verification System 1 prototype station in Western Australia was used for demonstration purposes.</jats:p>

Impact of mutual coupling between SKALA4.1 antennas to the spectral smoothness response

“…It is important to notice that, especially at low frequency, mutual coupling is particularly critical for SKA1-Low due to the large dimension of the SKALA4.1 antenna (maximum horizontal length 1.6 m) and to the dense distribution of the antennas. [10][11][12] The reason for the lack of a highly sampled spectral analysis is likely due to the heavy computational demand to simulate one SKA1-Low station. For instance, the full-wave analysis of 256 SKALA4.1 antennas by means of the multilevel fast multipole method along with the method of moment technique requires on the order of days for each frequency point on a multicore workstation.…”

The Aperture Array Verification System 1: System overview and early commissioning results

“…pling(Davidson et al 2019; Di Ninni et al 2019;Davidson et al 2020;Borg et al 2020), verifying the simulations(Paonessa et al 2020), and estimating the impact of the coupling on calibration and downstream signal processing(Wijnholds 2020;Jones & Wayth 2021). …”

The All-Sky SignAl Short-Spacing INterferometer (ASSASSIN) I: Global sky measurements with the Engineering Development Array-2

“…It is an inherent property of closely-spaced interferometers, such as the EDA-2, that internal noise from the receiver electronics will be radiated by an antenna and subsequently received by neighbouring antennas, causing some correlation of internal noise at the correlator (Venumadhav et al 2016;Ung et al 2020;Sutinjo et al 2020). This internal-noise coupling has its greatest effect at short baselines and will cause the interferometric response to change from the ideal case shown in Fig.…”

“…The first step will be to replace the analytic description of the antenna beam pattern by more accurate FEKO models that take into account mutual coupling. Such embedded element patterns (EEPs) have recently been generated by Ung et al (2020) for their work on characterising the noise temperature of the EDA in the presence of mutual coupling. We will investigate the use of individual EEPs for each dipole, as well as using a single, average element pattern, which may prove to be sufficient as was found by (Borg et al 2020) for calibrating the Aperture Array Verification System-1 (Benthem et al, 2020, in preparation), albeit with less-stringent requirements for calibration accuracy than a global 21 cm signal detection will require.…”