Watchman

The Watchman is an S-band (2,750-3,050 MHz) medium-range air traffic control radar developed by Plessey Radar Ltd in the 1970s for terminal area/approach and ground-controlled approach surveillance. It features a configurable modular design, offering options such as transportable or static aluminum/carbon-fiber antennas, adaptive or eight-filter Moving Target Detector (MTD) signal processing, and plot or track outputs. The radar utilizes a traveling-wave tube (TWT) based transmitter, providing frequency diversity and agility, with a peak power of 58 kW and an average power of 1.3 kW, enabling detection ranges up to 222 km (120 NM). It employs a dual pulse train: a 0.4 µs pulse for high discrimination at short ranges and a 20 µs pulse for long-range detection of small radar cross-section aircraft, compressed in the receiver to 0.4 µs, yielding a range resolution of 60 m. Signal separation is achieved by using different radio frequencies for the long and short pulses, with target detection further enhanced by interchanging frequencies at nine pulse repetition intervals (approximately 1,100 Hz). The system incorporates a CFAR (Constant False Alarm Rate) circuit and a high-definition clutter map using alpha smoothing to compensate for noise and clutter. The Watchman system, with an antenna rotation of 15 rpm (4 seconds per rotation) and a beamwidth of 1.5° by 30°, provides air traffic services for the majority of UK Ministry of Defence operational stations globally, with over 40 systems currently in service, including tactical variants for rapid deployment.

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