RFI and radio astronomy

Modern telescopes in radio astronomy, 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 incredible sensitive devices that observe the sky with enormous depth and detail. The observed bandwidth of telescopes has dramatically increased over the last decades, and often overlaps with parts of the radio spectrum that have not been reserved for radio astronomy. Simultaneously, the radio spectrum is becoming more crowded because of technological advancement. Therefore, radio observations are affected by man-made radio transmitters, which can be several orders of magnitude stronger than the weak celestial signals of interest. This kind of interference, which seriously disturbs radio observations, is called radio-frequency interference (RFI).

RFI overshadows astronomical signals

If the frequency at which is observed contains RFI, thus overlaps with the frequency at which other devices transmit, the recorded data will be corrupted and can not be used directly. For example, because RFI can be many orders of magnitude stronger than the signal of interest, it might not be possible to calibrate contaminated data. Moreover, because of the difference in strength, the signal of interest will be overshadowed by the RFI, and the signal can not be extracted.

Increase of electromagnetic devices

Around 1980 when the radio spectrum was becoming more and more occupied as a result of technical advancements (V. Pankonin and R. M. Price, 1981), radio observers started to notice RFI caused by electronic equipment (Thompson, Gergely & Vanden Bout, 1991), and started to develop methods to mitigate it. The first techniques to deal with contaminated data were performed by the data reducing scientist, and consisted of manual selection of good data. Examples are to manually remove data from particular antennae or time and frequency ranges at which the interference was received. Although these techniques can be tedious, they have been sufficient in most situations during the last few decades. Building telescopes at radio quiet sites would improve this situation, but this is not always feasible.

Radio astronomy is advancing

Now that the radio telescopes of the next generation are coming into operation, the dawn of software-driven telescopes producing terabyte sized data sets has begun. Because of that, data reduction in radio astronomy is entering a new era in which more emphasis is put on automated data processing and pipelining the various steps in the data reduction. Dealing with RFI is an important step in the reduction process. As the volume of data and the required sensitivity of observations increases significantly, and the contamination of RFI through an increased usage of electronic equipment grows, more sophisticated automated flagging procedures are required for the next generation of telescopes.

The RFI mitigation page describes several techniques to deal with RFI.

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.