  The radio astronomy community is currently building a number of phased array
telescopes. The calibration of these telescopes is hampered by the fact that
covariances of signals from closely spaced antennas are sensitive to noise
coupling and to variations in sky brightness on large spatial scales. These
effects are difficult and computationally expensive to model. We propose to
model them phenomenologically using a non-diagonal noise covariance matrix. The
parameters can be estimated using a weighted alternating least squares (WALS)
algorithm iterating between the calibration parameters and the additive
nuisance parameters. We demonstrate the effectiveness of our method using data
from the low frequency array (LOFAR) prototype station.
