Design Concepts for a Sky Noise Limited Low Frequency Array

Abstract

The LOw Frequency ARray is an aperture synthesis instrument for the frequency range from10 MHz up to over 300 MHz. There are order thirty stations spanning baselines up to 300 km and distributed such that bandwidth synthesis in the semi-octave observing bands provides sufficient visibiltiy sampling for reliable wide-field imaging. Each antenna station has a few hundred active dipole receptors arranged in a sparse fractal like configuration of about two hundred meter diameter. With appropriate weighting an almost frequency independent station beamwidth is obtained. The effective collecting area scales proportional with wavelength squared approaching at the lowest frequency a square kilometre for all stations together. The digital beamformers at the stations not only perform the spatio-spectral nulling to adaptively remove any interference, but also form the spectrometer part of the array correlator. We propose the F3X correlator architecture where the efficient Fourier butterfly algorithm is not only used to transform a time sequence into a frequency sequence but also to transform a set of station sub-apertures into a set of beams before cross-correlation between antenna stations is performed. Commercial digital signal elements form by 2006 a cost effective implementation solution creating a processing window of opportunity where the signal power of beam-formers and correlators turns out to be matched to the data power of a battery of a few hundred general purpose image engines. We are entering the era where the performance of electronics in phased array antennas can cost effectively meet the performance requirements of a synthesis array.