RFI and radio astronomy

Radio Frequency Interference and Radio Astronomy

Modern radio observatories such as the Jansky Very Large Array (VLA) in New Mexico, the Low-Frequency Array (LOFAR) in the Netherlands and the Murchison Widefield Array (MWA) in Australia are extraordinarily sensitive instruments that map the sky with remarkable depth and resolution. Their observed bandwidth has grown sharply over recent decades and frequently overlaps with spectrum bands that were never set aside for astronomy. At the same time, the airwaves are getting busier as technology advances. As a result, astronomical recordings are disturbed by human-made transmitters that can be far stronger than the faint cosmic signals researchers are trying to capture. This disturbance is known as radio-frequency interference, or RFI.

How RFI Obscures Astronomical Signals

When the observed frequency range contains RFI and therefore overlaps with the frequencies used by other transmitting devices, the recorded data becomes corrupted and cannot be used as-is. Because interference may be many orders of magnitude stronger than the target signal, calibration of contaminated data can become impossible. The sheer difference in strength means the astronomical signal is buried beneath the interference and cannot be extracted.

The Growth of Electromagnetic Devices

Around 1980, as the radio spectrum grew increasingly occupied through technical progress (V. Pankonin and R. M. Price, 1981), observers began noticing interference produced by electronic equipment (Thompson, Gergely & Vanden Bout, 1991) and started devising ways to counter it. The earliest approaches were carried out by the scientists reducing the data and involved manually selecting clean data — for instance, discarding data from particular antennas, or from the time and frequency ranges where interference was picked up. Though laborious, these methods proved adequate in most situations over the past few decades. Placing telescopes at radio-quiet locations would help, but that is not always practical.

Radio Astronomy Moves Forward

With next-generation radio telescopes now entering service, the era of software-driven instruments producing terabyte-scale data sets has arrived. Data reduction in radio astronomy is therefore entering a new phase that emphasises automated processing and pipelining of the various reduction steps. Handling RFI is a key part of that process. As data volumes and required sensitivity rise sharply, and interference from growing use of electronic devices increases, the next generation of telescopes demands more sophisticated automated flagging procedures. The RFI mitigation page describes several techniques for dealing with RFI.

AOFlagger software interface used for automated RFI detection and flagging — Automated flagging with AOFlagger

LOFAR high-band antenna array with a person standing beside it — A LOFAR HBA antenna up close

References

V. Pankonin and R. M. Price (1981): "Radio Astronomy and Spectrum Management: The Impact of WARC79". IEEE Trans. on Electromagnetic Compatibility, EMC-23:308–317.
A. R. Thompson, T. E. Gergely, and P. A. Vanden Bout (1991): "Interference and Radioastronomy". Physics Today, 44:41–49.

Explore the RFI mitigation techniques page, or follow the telescope links for the VLA, LOFAR and MWA.

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