Reduced Ambiguity Calibration for LOFAR

TL;DR

This paper introduces a calibration method for LOFAR that removes non-unitary ambiguities caused by ionospheric distortions, enabling more accurate modeling of the sky and propagation effects.

Contribution

The paper presents a novel calibration approach that eliminates ambiguities in LOFAR data, with exact ambiguity characterization and conditions for successful application.

Findings

Method effectively removes ambiguities under certain conditions.

Simulation results demonstrate improved calibration accuracy.

Degeneracy cases identified where the method fails, suggesting diversity exploitation.

Abstract

Interferometric calibration always yields non unique solutions. It is therefore essential to remove these ambiguities before the solutions could be used in any further modeling of the sky, the instrument or propagation effects such as the ionosphere. We present a method for LOFAR calibration which does not yield a unitary ambiguity, especially under ionospheric distortions. We also present exact ambiguities we get in our solutions, in closed form. Casting this as an optimization problem, we also present conditions for this approach to work. The proposed method enables us to use the solutions obtained via calibration for further modeling of instrumental and propagation effects. We provide extensive simulation results on the performance of our method. Moreover, we also give cases where due to degeneracy, this method fails to perform as expected and in such cases, we suggest exploiting diversity in time, space and frequency.