Improving the direction-dependent gain calibration of reflector antenna radio telescopes

[摘要] ENGLISH ABSTRACT: Utilising future radio interferometer arrays, such as the Square Kilometre Array (SKA), to their full potential will require calibrating for various direction-dependent effects, including the radiation pattern (orprimary beam in the parlance of radio astronomers) of each of the antennas in such an array. This requires an accurate characterisation of the radiation patterns at the time ofobservation, as changing operating conditions may cause substantial variation in these patterns. Furthermore, fundamental imaging limits, as well aspractical time constraints, limit the amount of measurement data that can be used to perform such characterisation. Herein three techniques are presented which aim to address this requirement by providing pattern models that usethe least amount of measurement data for an accurate characterisation of theradiation pattern. These methods are demonstrated throughapplication to the MeerKAT Offset Gregorian (OG) dual-reflector antenna.The first technique is based on a novel application of the Jacobi-Bessel series in which the expansion coefficients are solved directly from the secondary pattern. Improving the efficiency of this model in the desired application leads to the development of a different set of basis functions, as well as two constrained solution approaches which reduce the number of pattern measurements required to yield an accurate and unique solution. The second approach extends the application of the recently proposedCharacteristic Basis Function Patterns (CBFPs) to compensate for non-linear pattern variations resulting from mechanical deformations in a reflector antenna system. The superior modelling capabilities of these numerical basisfunctions, which contain most of the pattern features of thegiven antenna design in a single term, over that of analytic basis functions are demonstrated. The final method focusses on an antenna employing a Phased Array Feed(PAF) in which multiple beam patterns are created through the use of a beam-former. Calibration of such systems poses a difficult problem as the radiation pattern shape is susceptible to gain variations. Here we propose a solutionwhich is based on using a Linearly Constrained Minimum Variance (LCMV) beamformer to conform the realised beam pattern to a physics-based analytic function. Results show that the LCMV beamformer successful ly produces circularly symmetric beams that are accurately characterised with a single-term analytic function over a wide FoV.

[发布机构] Stellenbosch University