On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green
  Bank Telescope
  We use observational data to show that 21 cm line profiles measured with the
Green Bank Telescope (GBT) are subject to significant inaccuracy. These include
~10% errors in the calibrated gain and significant contribution from distant
sidelobes. In addition, there are ~60% variations between the GBT and
Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur
because of the high main-beam efficiency of the GBT. Stokes V profiles from the
GBT contain inaccuracies that are related to the distant sidelobes.
  We illustrate these problems, define physically motivated components for the
sidelobes, and provide numerical results showing the inaccuracies. We provide a
correction scheme for Stokes I 21 cm line profiles that is fairly successful
and provide some rule-of-thumb comments concerning the accuracy of Stokes V
profiles.
