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The Development of Airborne Pulse Doppler Radar

Ground clutter has been a serious limitation to both ground and airborne radars for years. Moving target indication (MTI) using delay line cancelers, when applied to the airborne case, has not been very successful, because of the spectral spread caused by aircraft motion. As the aircraft flies faster, or looks at larger angles off the velocity vector, the problem becomes overwhelming because the clutter spread approaches the pulse repetition frequency (prf) in width, so little or no clutter-free space remains for detection. Consequently, airborne radars were limited to an uplook or co-altitude geometry, or flight at such high altitude that a target could be detected at ranges shorter than the altitude. This seriously limited the operational usefulness of airborne radar, since the target could fly on the deck and be undetected. What was needed was a look-down capability. With these considerations in mind, the development of pulse doppler (PD) filled a serious need. CW doppler radar [ 11 was developed near the end of World War II. Its range was generally limited by transmitter leak-through and near range mainbeam clutter. To solve this problem, pulsing the transmitter, while gating off the receiver, was a potential solution. The earliest record of a ground-based pulse doppler radar is described by W. W. Hansen in Ridenour "Radar System Engineering," chapter 5-11, in 1947 [21. This radar used a high duty cycle, at a relatively low carrier frequency so that both unambiguous range and dopper were available. The first pulse doppler applied to the airborne problem was the Boeing laboratory Bomarc seeker in the early 1950s. This work, and the more generic work done at Westinghouse thereafter for a number of airborne applications are described. This brief history of three decades is depicted in the flow diagram [3, 41 of Figure 1.