Saturday 28 December 2013

RFI investigations

On Wednesday, we reported on some strange RFI (radio-frequency interference). Today we've a few more leads on our investigation.

The following aspects have been noted:
  • The events appear equally in both X- and Y-polarisations. 
  • The events last between 5 and 10 minutes. However, they are not always the same duration. 
  • The events are broadband, but seem to be stronger at some frequencies. This is not linear nor monotonic across the different frequencies. 
  • The rise is generally quite gradual, although sometimes there is an initial step function to start. 
  • The end of the event is extremely abrupt.
During the observations, correlator files were being acquired. This allows us to check on the direction of the RFI by forming an all-sky image. The following is an example showing an RFI event.

It seems that the source is always the same direction (az = approx. 150 degrees) and is always on the horizon. Using the KAIRA landscape in Stellarium, we can check on this direction.

The beamsize at the frequency of the all-sky image above is approx. 8 degrees. This is not huge, but is sufficient to show that the source is unlikely to be Saana (there is a radio transition station on the mountain there at az=131).

However, at az=150, there are LOTS of potential sources: the customs (Tulli) station, IRIS, Retkeilykeskus, the village centre, the Biol.Asema, et al..

Investigations continue...

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