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Description of the radar set, tactical-technical characteristics

Figure 1: AN/FPS-118

Figure 1: AN/FPS-118, OTH-B transmitting array

frequency: 5 … 28 MHz
pulse repetition time (PRT):
pulse repetition frequency (PRF):
pulsewidth (τ):
receiving time:
dead time:
peak power:
average power: 1 MW
instrumented range: 1 800 NM (≙ 3 333 km)
range resolution:
beamwidth: Tx: 7.5°; Rx: 2.5°
hits per scan:
antenna rotation:


AN/FPS-118 was an early warning radar system designed to detect low-flying enemy aircraft. It is a bistatic and FMCW Doppler radar operating in HF-band as Over-the-Horizon Backscatter Radar System (OTH-B). This radar was developed in the 1970s and 1980s to provide all-altitude, long-range surveillance of aerial approaches to the United States. Each AN/FPS-118 system consists of an operations center, a receive site, and a transmit site, all geographically separated to prevent interference. The AN/FPS-118 system employed a series of large transmitter arrays that fired off signals which bounced off the ionosphere, are backscattered by an airplane and sent a signal back to the equally large receiver arrays. AN/FPS-118 covers a 60 degree arc between 900 to 3 300 km. The transmit site of a given AN/FPS-118 was located near Moscow, Maine, USA (45°09'39.4" N   69°51'17.1" W). The associated receive site was near Columbia Falls, Maine, USA (44°42'33.2" N   67°43'41.5" W). All OTH-B radar transmitter and receiver facilities have been demolished.

Picture gallery of AN/FPS-118

Figure 2: AN/FPS-118 receiver array