The operation of all active sensors requires that some of the energy transmitted by the device be reflected off objects within the beam. The characteristics of the reflected signal can then be analyzed in various ways to extract information about the target. It should be stressed that the word 'target' does not necessarily convey the idea that the object is being hunted, though that was almost certainly the root of the word. In this book, target is defined as the object of interest to the sensor, in contrast to clutter, which is defined as any object whose echoes may interfere with the investigation of the target.