USAF Frequency Diversity Program

From “Long-Range UHF Radars for Ground Control of Airborne Interceptors” by William W. Ward and F. Robert Naka, in the Lincoln Laboratory Journal, Volume 12, Number 2.

In June 1955 Rome Air Development Center, Griffiss Air Force Base, New York, let design-study contracts for six new ground control of intercepts (GCI) radars, each to operate in a segment of the frequency range 214 to 5900 MHz. At that time, Air Force GCI radars were moving through attrition toward occupancy of only two frequency bands: the AN/ FPS-7 surveillance and height-finding radar with stacked beams operated at 1300 MHz, and the AN/FPS-6 height-finding radar operated at 2900 MHz. These two radars, lineal descendants of radars developed during World War II, constituted what amounted to a single-frequency air-defense radar system. The frequency-diversity (FD) radar program was to reverse that trend.

The spread of operating frequencies to be provided by the FD radar program promised to make it more costly in terms of payload for an airborne intruder to penetrate and survive in the defensive radar environment, as discussed in the main text. At the same time, the new program would enhance the Air Force's GCI capabilities, in particular its ability to feed high-quality data to the Semi-Automatic Ground Environment (SAGE) air-defense system.

Five of the six proposed radars were selected for prototype development, and four were produced in quantity. These five systems in their prototype forms were installed for testing and evaluation at operational Air Force sites in Alabama, Louisiana, and Mississippi, part of the Mobile, Alabama, Air-Defense Sector. Their test programs began in 1959.

In addition, the AN/GLA-8 signal processing system, built by Airborne Instrument Laboratory, was an important common adjunct to each frequency-diversity radar. This equipment included a special anti-jamming console used by the radar's human counter-countermeasures (CCMs) operator. As discussed in the main text, CCMs such as frequency hopping and PRF jitter/stagger are useful in reducing the effectiveness of both passive countermeasures (chaff, for example) and active countermeasures (spot and noise jamming, and signal repeaters). The wise use of the many features of a highly flexible FD radar required special skills and sophisticated technological support.