Introduction

A single element telescope with a steerable paraboloidal reflecting surface is the simplest kind of radio telescope that is commonly used. Such a telescope gives an angular resolution $\sim \lambda / D$, where D is the diameter of the aperture and $\lambda$ is the wavelength of observation. For example, for a radio telescope of 100 m diameter (which is about the largest that is practically feasible for a mechanically steerable telescope), operating at a wavelength of 1 m, the resolution is $\sim 30 ~arc ~min$. This is a rather coarse resolution and is much less than the resolution of ground based optical telescopes.

Use of antenna arrays is one way of increasing the effective resolution and collecting area of a radio telescope. An array usually consists of several discrete antenna elements arranged in a particular configuration. Most often this configuration produces an unfilled aperture antenna, where only part of the overall aperture is filled by the antenna structure. The array elements can range in complexity from simple, fixed dipoles to fully steerable, parabolic reflector antennas. The outputs (voltage signals) from the array elements can be combined in various ways to achieve different results. For example, the outputs may be combined, with appropriate phase shifts, to obtain a single, total power signal from the array - such an array is generally referred to as a phased array. If the outputs are multiplied in distinct pairs in a correlator and processed further to make an image of the sky brightness distribution, the array is generally referred to as a correlator array (or an interferometer). Here we will primarily be concerned with the study of phased arrays, with direct comparison of the performance with correlator arrays, where relevant.