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In 1947, an investigation was begun at the U. S. Naval Research Laboratory to study noise in tracking radars. Of particular interest was the noise which was found to be caused by a target when it is of finite size and complex shape; this suggested a new basis for optimum radar design. With the increasing emphasis on precision in radar guidance in the space and missile age, complete instrumentation was prepared to permit investigation of target-scintillation or target-noise phenomena. While extensive theoretical studies were being carried out, the instrumentation was built to include a unique simulator of multiple, finite-size targets with a complete closed-loop-tracking system, an instrumentation radar composed of a dualchannel-tracking radar for measurement of components of target noise under actual tracking conditions, and equipment designed to provide automatic statistical analysis of noise data. The results of these studies showed several ways in which different components of the target noise affect the radar-tracking performance and how these components determine the choice of tracking systems and optimum design of the system. The studies revealed phenomena such as the two-reflector target, which can cause tracking errors of many target spans outside the physical extent, and a new concept of target noise in terms of a tilting of the phase front of the echo signal from a target.