Applicability and limitations of redundancy cal. in phased array stations

Parisa Noorishad, Stefan Wijnholds, Arnold van Ardenne & Thijs van der HulstYERAC2011, Manchester, 18th -20th July. 2011

A few words about new radio telescopes

Introduction & Motivation

[1] J. E. Noordam & A. G. de Bruyn, Nature, 14 oct. 1982

[2] M. Wieringa, PhD thesis, Leiden Univ., 1991

Introduction:

Introduction:

• Independent of a sky model,

• Less sensitive to RFIs,

• Computationally cheap and fast,

• Simple to implement (important for SKA),

• Could also be used as a diagnostic tool.

• Regularity in a station configuration e.g. in MWA, HBA, EMBRACE.

Why redundancy?

Redundancy in HBAs

Beam identicalness:

CAESAR: Computationally Advanced and Efficient Simulator for Arrays [Rob Maaskant et al.]

When redundancy fails:

When redundancy holds:

Checking the influence of non-identical sidelobes

The systematic error due to non-identical sidelobes

The potential calibrator sources for RS208, on

24th of Nov. 2009 at 21:29:04 UTC and the

systematic errors in the amplitudes and the

phases of the redundant visibility due to the

contribution of other strong sources through

non-identical sidelobes.

Quantifying that systematic error:

The plots of the residuals for corrected redundant visibilities

Redundancy cal. Quality

CRB and the actual variance of the estimated parameters:

We tracked CasA on 24th November 2009 from 15:25:43 UTC until 22:12:19 UTC.

Redundancy cal. Limitations

• Station configuration

• Mutual coupling between the station elements which leads to:

– Non-identical beams of the station elements

– Presence of baseline dependent noise

• SNR of the observed sky

Limitations of the redundancy calibration method:

Station configuration and the condition of redundancy calibration

One more point

• The mutual coupling seems to be a non-dismissible effect (for phased array stations).

• We need beam model of each individual station elements e.g. each tile at HBAs.

Model based cal. method:

[1] Wijnholds & van der Veen, 2009

[2] Wijnholds & van der Veen, 2010

Summary & Conclusion

• Summary:– Applicability and limitations of the redundancy cal. for phased

array station was investigated.

• Conclusions:– Redundancy cal. is a stand-alone gain cal. method.– Using redundancy requires taking into account the parameters

which affects its applicability and performance.– Identical beam; Mutual coupling seems to be a non-dismissible

effect, no matter which method we use.

• Next Step:– To do more rigorous CRB and Monte-Carlo analysis for two

existing implementations of the redundancy method.– To evaluate the merit of the redundancy in UVW-space at the

whole array level using SKA configuration.