  Direction dependent calibration of widefield radio interferometers estimates
the systematic errors along multiple directions in the sky. This is necessary
because with most systematic errors that are caused by effects such as the
ionosphere or the receiver beam shape, there is significant spatial variation.
Fortunately, there is some deterministic behavior of these variations in most
situations. We enforce this underlying smooth spatial behavior of systematic
errors as an additional constraint onto spectrally constrained direction
dependent calibration. Using both analysis and simulations, we show that this
additional spatial constraint improves the performance of multi-frequency
direction dependent calibration.
