Analysis of sky contributions to system temperature for low frequency SKA aperture array geometries

Abstract

The new generation of radio telescopes, such as the proposed Square Kilometer Array (SKA) and the Low-Frequency Array (LOFAR) rely heavily on the use of very large phased aperture arrays operating over wide band-widths at frequency ranges up to approximately 1.4 GHz. The SKA in particular will include aperture arrays consisting of many thousands of elements per station providing un-paralleled survey speeds. Currently two different arrays (from nominally 70 MHz to 450 MHz and from 400 MHz to 1.4 GHz) are being studied for inclusion within the overall SKA configuration. In this paper we aim to analyze the array contribution to system temperature for a number of regular and irregular planar antenna array configurations which are possible geometries for the low-frequency SKA (sparse disconnected arrays). We focus on the sub-500 MHz band where the real sky contribution to system temperature (Tsys) is highly significant and dominants the overall system noise temperature. We compute the sky noise contribution to Tsys by simulating the far field response of a number of SKA stations and then convolve that with the sky brightness temperature distribution from the Haslam 408 MHz survey which is then scaled to observations at 100 MHz. Our analysis of array temperature is carried out by assuming observations of three cold regions above and below the Galactic plane. The results show the advantages of regular arrays when sampled at the Nyquist rate as well as their disadvantages in the form of grating lobes when under-sampled in comparison to non-regular arrays.