Calibration challenges for low frequency radio astronomy

A major challenge for all high resolution low frequency radio astronomy is measuring and removing the effects of the ionosphere. The isoplanatic patch size for frequencies below a few hundred MHz is generally much smaller than the field of view. In addition, aperture arrays have beams on the sky which vary dramatically with observing geometry. These beams require careful calibration to be stable and known in order to allow imaging. Various approaches to these problems will be discussed.