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The right tool for the right job

Submitter: Peter Fridman
Description: LOFAR observes the sky at radio frequencies (20-240 MHz) that are also used for a wide range of other human activities. The resulting Radio Frequency Interference (RFI) in the LOFAR signal must be detected and removed. Fortunately, there are many ways to approach this.

The raw LOFAR data has many narrow frequency channels. In order to minimize processing and storage, these are then averaged as soon as possible to a smaller number of channels with a wider bandwidth. But the high time - frequency resolution data can be used to "mitigate" RFI.

a) The power spectrum of the LOFAR signal in band 25 (203 MHz), with a frequency resolution of 5 kHz (32 channels).

b) The same signal, with a frequency resolution of 152.5 Hz (1024 channels). The RFI is much better to see, and filtering out the contaminated fragments will leave useful data.

c) The spectrum after the RFI mitigation from (b) with a robust statistical algorithm, and averaging to the frequency resolution of (a).

Caveat: Despite the beautiful result shown here, RFI mitigation techniques do have inherent limitations. For example, excised data cause some loss of sensitivity and information. Furthermore, we always have to guard against introducing low-level artifacts. However, there is a lot we can do, and what we can do is rather impressive.
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