Using embedded element patterns to improve aperture array calibration

Several existing and planned low-frequency (< 350 MHz) radio astronomical facilities exploit subarrays or stations consisting of receiving elements in an irregular configuration. Calibration of the RF path of each receiving element is crucial for accurate beamforming with these stations. Currently used station calibration methods usually assume that the embedded element patterns (EEPs) of the receiving elements within a station are identical. In this contribution, we show that ignoring the inter-element EEP variations causes systematic errors in the calibration results using the stations of the low-frequency receiving systems of the Square Kilometre Array (SKA) and the Low Frequency Array (LOFAR) as examples. We show that the magnitude of these errors increases with increasing EEP variations. We also discuss the challenges faced by SKA and LOFAR to mitigate these errors by exploiting a priori knowledge of the EEPs.