Hierarchical beamforming aspects of OSMA

NFRA is currently developing a 64-element receive only active phased array system as part of the future Square Kilometre Array (SKA). The system will be used to demonstrate concepts of antenna design, RF beamforming, digital beamforming, calibration techniques, adaptive algorithms, RFI suppression and analogue-to-digital conversion. This paper presents some initial results from research on calibration techniques and hierarchical beamforming algorithms. Calibration is an important aspect of all beamforming systems, as it directly affects the ability of an adaptive or non-adaptive beamformer to accurately suppress RFI. The suppression of interference will determine the success of next generation astronomical instruments. The unique dual-hierarchy beamforming structure of the experimental array, called the One Square Metre Array (OSMA), presents two possibilities for RFI suppression. This occurs at the RF level, before digitisation, and afterwards using a digital adaptive beamformer. Additionally, the RF beamforming significantly reduces the calibration requirements of the system, as its inherent averaging process reduces the complexity of the array manifold to a (nearly) predictable pattern.