EVLA memo 86 RFI excision in synthesis imaging without a reference signal

The excision of interfering signals is crucial to the continuation of radio astronomical observations into the future. Many algorithms for RFI excision require an estimate of the interference found by observations with a reference system. However, often the best measurements of the interference come from the scientific observations themselves – the sensitivity and sampling are guaranteed to be appropriate. This is similar to the logic of self-calibration whereby the best way to calibrate the telescope is to use the scientific observations. We develop and test an algorithm in which the interference is estimated by a least squares fit to the observations and removed by simple subtraction. We call this technique “RFI selfpartitioning”. Differentiation of interference from signal is dependent on the natural fringe rotation of celestial sources, and the lack of fringe rotation for ground-based interference. Our test is on VLA 333MHz observations of the closely circumpolar radio source NGC6251. The interference source is a radar transmitter at Albuquerque airport, some 200km from the VLA.