Trajectory Based RFI Subtraction and Calibration for Radio Interferometry

TL;DR

This paper introduces tabascal, a novel algorithm that jointly models antenna gains and satellite-based RFI in radio interferometry data, improving calibration accuracy and data retention compared to traditional flagging methods.

Contribution

The paper presents tabascal, a new method for joint RFI subtraction and calibration that enhances data quality and reduces data loss in radio interferometry observations contaminated by satellite RFI.

Findings

Gain estimates are unbiased and better constrained.

RFI subtraction reduces noise and improves flux recovery.

Significantly less data loss compared to flagging alone.

Abstract

Radio interferometry calibration and Radio Frequency Interference (RFI) removal are usually done separately. Here we show that jointly modelling the antenna gains and RFI has significant benefits when the RFI follows precise trajectories, such as for satellites. One surprising benefit is improved calibration solutions, by leveraging the RFI signal itself. We present tabascal (TrAjectory BAsed RFI Subtraction and CALibration), a new algorithm that jointly models the RFI and calibration parameters in visibilities. We test tabascal on simulated MeerKAT calibration observations contaminated by satellite-based RFI. We obtain gain estimates that are both unbiased and up to an order of magnitude better constrained compared to uncontaminated data. When combined with an ad hoc RFI subtraction scheme, tabascal solutions can be further applied to an adjacent target observation: 5 minutes of calibration data results in an image with about a third the noise achieved when using flagging alone. The recovered flux distribution of RFI subtracted data was on par with uncontaminated data. In contrast, RFI flagging alone resulted in a higher detection threshold and consistent underestimation of source fluxes. For a mean RFI amplitude of 17 Jy, using RFI subtraction leads to less than 1% loss of data compared to 75% data loss from an ideal $3\sigma$ flagging algorithm, a very significant increase in data available for science analysis. Although we have examined the case of satellite RFI, tabascal should work for any RFI moving on parameterizable trajectories, relative to the phase centre, such as planes and/or objects fixed to the ground.