Radio Interferometric Calibration Using The SAGE Algorithm
Monthly Notices of the Royal Astronomical Society(2010)
摘要
The aim of the new generation of radio synthesis arrays such as LOFAR and SKA
is to achieve much higher sensitivity, resolution and frequency coverage than
what is available now, especially at low frequencies. To accomplish this goal,
the accuracy of the calibration techniques used is of considerable importance.
Moreover, since these telescopes produce huge amounts of data, speed of
convergence of calibration is a major bottleneck. The errors in calibration are
due to system noise (sky and instrumental) as well as the estimation errors
introduced by the calibration technique itself, which we call solver noise. We
define solver noise as the distance between the optimal solution (the true
value of the unknowns, uncorrupted by the system noise) and the solution
obtained by calibration. We present the Space Alternating Generalized
Expectation Maximization (SAGE) calibration technique, which is a modification
of the Expectation Maximization algorithm, and compare its performance with the
traditional Least Squares calibration based on the level of solver noise
introduced by each technique. For this purpose, we develop statistical methods
that use the calibrated solutions to estimate the level of solver noise. The
SAGE calibration algorithm yields very promising results both in terms of
accuracy and speed of convergence. The comparison approaches we adopt introduce
a new framework for assessing the performance of different calibration schemes.
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