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Description of the radar set, tactical-technical characteristics

Figure 1: AN/FPS-31 and AN/FPS-6 heightfinder (left)

Figure 1: AN/FPS-31 and AN/FPS-6 heightfinder

frequency: 420 … 430 MHz
pulse repetition time (PRT):
pulse repetition frequency (PRF): 300 Hz (± 10 Hz)
pulsewidth (τ): 6 µs
receive time:
dead time:
peak power: 2 MW
average power: 3.6 kW
instrumented range: 250 NM (≙ 460 km)
range resolution:
hits per scan:
antenna rotation: 6 rpm


AN/FPS-31 was operating in UHF-Band Ground-controlled Interception (GCI) radar that was a part of the Experimental SAGE Subsector (ESS). A prototype only was operational in the test site West Bath in Maine (43°54'07.0" N   69°50'43.0" W) in October 1955. The site was closed in November 1962. The tower is still used as a mobile phone mast.

The antenna's reflector was 120 feet wide and 16 feet high. The transmitter was a liquid-cooled magnetron of type QK508 built by Raytheon.


  1. William W. Ward and F. Robert Naka ''Long-Range UHF Radars for Ground Control of Airborne Interceptors'', Lincoln Laboratory Journal, Volume 12, Number 2, 2000, S.209