Array processing deals with the processing of signals carried by propagating wave phenomena. The received signal is obtained by means of an array of sensors located at different points in space in the field of interest. The aim of array processing is to extract useful characteristics of the received signal field (e. g. , its signature, direction, speed of propagation). The sources of energy responsible for illuminating the array may assume a variety of different forms. They may be noncoherent (i. e. , independent of each other) or coherently related to each other. Equally, as seen from the location of the array, the radiation may be from diffused media and therefore distributed in nature, or it may be from isolated sources of finite angular extent. The array itself takes on a variety of different geometries depending on the application of interest.