A new polarization calibration technique for a large phased array with maximum baseline ≈220 Km

Long-baseline interferometric arrays are used in radioastronomy to produce polarisation images of the fine structure of radio sources allowing the study of their nature. The multi-element radio-linked interferometer network (MERLIN) is an array of six 25 m dishes connected by microwave links with maximum telescope separation of 217 km, recording both left and right hand circular polarisation (LCP, RCP) at ≈128 Mbits/second in real-time. Accurate calibration is difficult because of phase-instabilities of the incoming wavefront caused by ionospheric and tropospheric effects. In addition, crosstalk between LCP and RCP channels in the MERLIN receivers corrupts the signals and is equivalent to an elliptical feed response. The aim of the calibration process is to reconstruct the LCP and RCP phase and amplitude information and to solve for feed crosstalk which can be corrected in off-line data processing. The standard strategy is to derive corrections from observations of different radio sources with known polarisation characteristics