page_title: AI Mk. VIII radar
section_title: IFF use
hierarchical_section_title: AI Mk. VIII radar / Description / IFF use
caption_attribution_description: English: Mosquito NF Mark XIII, HK382 ‘RO-T’, of No. 29 Squadron RAF, at Hunsdon, Hertfordshire. The view looking along the lower fuselage towards the tail, showing the aircraft's armament of four forward-firing 20mm Hispano cannon.
context_page_description: Airborne Interception radar, Mark VIII, or AI Mk. VIII for short, was the first operational microwave-frequency air-to-air radar. It was used by Royal Air Force night fighters from late 1941 until the end of World War II. The basic concept, using a moving parabolic antenna to search for targets and track them accurately, remained in use by most airborne radars well into the 1980s.
  Low-level development began in 1939 but was greatly sped after the introduction of the cavity magnetron in early 1940. This operated at 9.1 cm wavelength, much shorter than the 1.5 m wavelength of the earlier AI Mk. IV. Shorter wavelengths allowed it to use smaller and much more directional antennas. Mk. IV was blinded by the reflections off the ground from its wide broadcast pattern, which made it impossible to see targets flying at low altitudes. Mk. VIII could avoid this by keeping the antenna pointed upward, allowing it to see any aircraft at or above its own altitude.
  The design was just beginning to mature in late 1941 when the Luftwaffe began low-level attacks. A prototype version, the Mk. VII, entered service on the Bristol Beaufighter in November 1941.
context_section_description: Mk. VIII was designed to work with IFF Mk. III, a transponder system that listened for pulses between 160 MHz and 190 MHz and responded with longer pulses on a slightly different frequency. Since the magnetron operated at 3.3 GHz, it would not trigger the IFF system, so a second pulsed transmitter system had to be used in the fighters to support this, the interrogator (or interrogator/responder), code-named Lucero.
  Lucero was connected to the Mk. VIII transmitter, and triggered its own 5 µs signal every fifth transmission of the radar. The IFF Mk. III in the remote aircraft contained a receiver circuit fed into a transmitter, causing any received signal to be amplified and stretched out in time. This signal was received by the responder side of Lucero, which mixed it in with the signal from the Mk. VIII's own receiver. Since the Lucero antenna was omnidirectional, the returns were continuous around the entire face of the display and unrelated to the position of the main dish. The result was a series of line segments, spaced every 10 degrees around the display.
  Lucero was designed so its transmission would be triggered more rapidly than the radar's magnetron. This allowed it to send its signal and begin receiving the reply during the time when the main radar pulse was in-flight. This meant that the line segments began at a point indicating closer range than the aircraft sending the response, and ended after it. The radar operator could thus tell which aircraft was sending the IFF responses by looking for blips roughly centred along the line segments.
caption: The IFF/Lucero antenna can be seen projecting downward just behind the guns in this image of the lower fuselage of a Mosquito NF.XIII.
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