Definition of Array Receiver Gain and Noise Temperature

With the emergence of aperture and focal plane array systems for application in radio astronomy, there is a need for clear definitions of their gain and noise temperature, preferably in terms of a single antenna/receiver system, to enable proper understanding and comparison of system performance. Although in the literature the noise temperature of a phased array receiver was defined in these terms a long time ago (1966), this paper is not well known, either because it is not easily accessible, or slipped from our common memory. This memo reassesses the original paper with respect to the definitions and places them in the context of radio astronomical applications of arrays with strong coupling between the array elements. Essentially the array antenna and receiver are replaced by a single antenna and single receiver having the overall properties of the antenna array and individual array receivers, respectively (with arbitrary gains and noise temperatures of the individual receivers). An example will be given for the calculation of the gain and noise temperature of an array receiver, starting from the properties of the individual receiver channels in the array environment.