Altitude Estimation of Radio Frequency Interference Sources via Interferometric Near Field Corrections

TL;DR

This paper introduces a method to estimate the altitude of RFI sources using near-field corrections in radio interferometry, enabling better RFI subtraction and data preservation.

Contribution

It presents a novel technique for altitude estimation of RFI sources through near-field corrections, improving RFI mitigation strategies in radio astronomy.

Findings

Successfully estimated RFI source altitude as 11.7 km in a test case.

Identified an object as an airplane based on altitude and speed.

Consistently detected airplanes as RFI sources in multiple observations.

Abstract

Radio-frequency interference (RFI) presents a significant obstacle to current radio interferometry experiments aimed at the Epoch of Reionization. RFI contamination is often several orders of magnitude brighter than the astrophysical signals of interest, necessitating highly precise identification and flagging. Although existing RFI flagging tools have achieved some success, the pervasive nature of this contamination leads to the rejection of excessive data volumes. In this work, we present a way to estimate an RFI emitter's altitude using near-field corrections. Being able to obtain the precise location of such an emitter could shift the strategy from merely flagging to subtracting or peeling the RFI, allowing us to preserve a higher fraction of usable data. We conduct a preliminary study using a two-minute observation from the Murchison-Widefield Array (MWA) in which an unknown object briefly crosses the field of view, reflecting RFI signals into the array. By applying near-field corrections that bring the object into focus, we are able to estimate its approximate altitude and speed to be $11.7$ km and $792$ km/h, respectively. This allows us to confidently conclude that the object in question is in fact an airplane. We further validate our technique through the analysis of two additional RFI-containing MWA observations, where we are consistently able to identify airplanes as the source of the interference.