RFI mitigation techniques

Numerous techniques have been suggested to perform the challenging task of the excision or mitigation of radio-frequency interference from the data. They include using spatial information to null directions, provided in interferometers or multi-feed systems; removing the RFI by using reference antennae; and blanking out unlikely high values at high time resolutions with the CUSUM method or other thresholding techniques.

A typical example of RFI in a sub-band of a LOFAR observation. While most of the time-frequency diagram is noise-like, the repetitive higher (red) values at constant frequencies are due to narrow-band RFI.

No single method for all RFI

RFI comes in many forms. The strong RFI that is problematic is often either local in frequency or in time. An example of RFI that is local in frequency is shown in the above figure. Such RFI can for example be caused by television stations, aeroplanes and radar, while broadband RFI caused by phenomena such as lightning, high-voltage power cables and sparking electrical fences are often local in time. Sometimes, the frequency of RFI drifts with time. This can be caused by Doppler shifting of a satellite signal, by imperfect transmitters or because the transmitter is intrinsically changing its frequency, such as with certain radar signals when observed at high frequency resolution. A different class of RFI is caused by weakly transmitting but stationary -- and therefore systematic -- devices on site. This class of RFI is hard to recognize, as it might contaminate all the channels in a spectral band. In fringe stopping interferometers, the fringe rotation causes this type of RFI to have a sinusoidal response in the time-frequency domain. It can be recognized and subtracted in various ways, as for example described recently by fringe fitting.

The following subpages will elaborate on some of the methods that are regularly used in the field of radio-astronomical interference mitigation.