Type 242

The Type 242 is a secondary surveillance radar (SSR) interrogator, operating on a fixed frequency within the VHF band (165-185 MHz) as part of the standard Mark III Identification Friend or Foe (IFF) system. It features a pulse repetition frequency (PRF) of 125 or 50 Hz, a pulsewidth of 6 µs, a peak power of 1 kW, and a beamwidth of 60°. The Type 242 is typically paired with radar systems such as the Type 275 and Type 291, while the Type 242M variant is used with the Type 277 series and Type 293 radars. The Type 242P/Q is commonly integrated with the Type 960/982 and the Type 983 height-finder. The interrogator's PRF is derived from the main radar equipment's PRF, with a 4:1 or 10:1 countdown in the modulator, which also triggers a displaced trace on the A-scope for displaying inverted interrogator signals received by the responser unit alongside normal radar echoes. Target identification is achieved through the coincidence of the interrogator pulse and the radar echo, with associated transponders being the shipborne Type 253 series or Mark III IFF units in aircraft.

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