The Theory and Design of Chirp Radars

A recent declassification makes it possible to publish information concerning an important advance in military electronics that has been pursued in classified work at Bell Telephone Laboratories since 1951. Traditionally, warfare has been a continuing competition between armament and firepower. This competition has carried over to the electronics of warfare. In particular, radar has been hard put to keep pace with the requirements determined by faster and higher-flying aircraft and missiles and the various radar countermeasures that they require. Therefore, anything that enhances the capability of radar, either in range, range resolution or rate of acquisition has always been most welcome. Simple pulsed radar is limited in range by the average power radiated, in resolution by the pulse length and in acquisition time by the beam width. The design of any radar involves a compromise among these three factors--range, resolution and speed. Anything that eases this compromise is of great interest. During most of the war, the principal emphasis in development of radar techniques and components was centered on increasing range and range resolution performance by increasing transmitter peak power and reducing pulse length. Thus, shortly after the end of the war, transmitting tubes were in existence with average power capabilities in excess of 100 times that which could be used in straightforward short-pulse operation. At about this time a number of investigators realized that targets separated in range by a distance AR could be resolved by using an appropriately modulated pulse provided that c/2A/ ^ AR, where c is the velocity of propagation and A/ is the width of the frequency spectrum of the transmitted pulse.