Sensor Array Systems
An array of sensors distributed over a horizontal plane surface is used to receive a propagating wavefi eld with the following objectives:
(1) To localize a source (2) To receive a message from a distant source (3) To image a medium through which the wavefi eld is propagating
In this chapter we shall study the basic structure of a sensor array system, and in the sequel learn how the above objectives are achieved. The most commonly used array geometries are uniform linear array (ULA) and uniform circular array (UCA). A uniform planar array (UPA), where sensors are placed on an equispaced rectangular grid, is more common in large military-phased array systems. A wavefront that propagates across the array of sensors is picked up by all sensors. Thus, we have not one but many outputs that constitute an array signal. In the simplest case, all components of the array signals are simply delayed replicas of a basic signal waveform. In the worst case, individual sensor outputs are strongly corrupted with noise and other interference, leaving very little resemblance among them. Array processing now involves combining all sensor outputs in some optimal manner so that the coherent signal emitted by the source is received and all other inputs are maximally discarded. The aperture of an array, that is, the spatial extent of the sensor distribution is a limiting factor on resolution. Use of multicomponent or vector sensors has been proposed to effectively increase the array aperture. The aperture can also be synthetically increased by moving a source or sensor. The synthetic aperture concepts are extensively used in mapping radars and sonars. In this chapter, we concentrate on sensor array systems that will form the basic material for the subsequent chapters. Two new sections on distributed sensor array and multicomponent sensor array have been added to widen the scope of the chapter.