RFI Mitigation for the Parkes Galactic All-Sky Survey (GASS)
  The GASS is a survey of Galactic atomic hydrogen (HI) emission in the
southern sky observed with the Parkes 64-m Radio Telescope. With a sensitivity
of 60 mK for a channel width of 1 km/s the GASS is the most sensitive and most
accurate survey of the Galactic HI emission in the southern sky. We discuss RFI
mitigation strategies that have been applied during the data reduction. Most of
the RFI could be cleaned by using prior information on the HI distribution as
well as statistical methods based on median filtering. Narrow line RFI spikes
have been flagged during the first steps of the data processing. Most of these
lines were found to be constant over long periods of time, such data were
replaced by interpolating profiles from the Leiden/Argentine/Bonn (LAB) survey.
Remaining RFI was searched for at any position by a statistical comparison of
all observations within a distance of 0.1 deg. The median and mean of the line
emission was calculated. In cases of significant deviations between both it was
checked in addition whether the associated rms fluctuations exceeded the
typical scatter by a factor of 3. Such data were replaced by the median, which
is found to be least biased by RFI and other artifacts. The median estimator
was found to be equivalent to the mean, which was obtained after rejecting
outliers.
