P. Noorishad, S.J. Wijnholds, A. van Ardenne and J.M. van der
Hulst, "Redundancy calibration of phased-array stations",
Astronomy & Astrophysics, V545, no. A108, pp1-15, September 2012
Aims. We assess the benefits and limitations of using the
redundant visibility information in regular phased-array systems to
improve the quality of the calibration.
Methods. Regular arrays offer the possibility of using
redundant visibility information to constrain the calibration of the
array independently of a sky model and a beam model of the station
elements. This requires a regular arrangement of the configuration of
array elements and identical beam patterns.
Results. We revised a previously developed calibration method
for phased-array stations using the redundant visibility information
in the system and applied it successfully to a LOFAR station. The
performance and limitations of the method were demonstrated by
comparing its application to real and simulated data. The main
limitation is the mutual coupling between the station elements, which
leads to non-identical beams and stronger baseline-dependent
noise. Comparing the variance in the estimated complex gains with the
Cramer-Rao Bound indicates that redundancy is a stable and optimum
method for calibrating the complex gains of the system.
Conclusions. Our study shows that the use of the redundant
visibility does improve the quality of the calibration in phased-array
systems. In addition, it provides a powerful tool obtaining system
diagnostics. Our results demonstrate that designing redundancy in both
the station layout and the array configuration of future aperture
arrays is strongly recommended. This is particularly true in the case
of the Square Kilometre Array (SKA) with its dynamic range requirement
that surpasses any existing array by an order of magnitude.
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