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An AN/APS-4 radar
An-APS-4 side view

The AN/APS-4 radar is an early military air to air and air to surface radar used by American and British warplanes during World War 2.


APS-4 radar is a lightweight air to air and air to surface radar with a detection range for large ships of c. 15 miles, and for an aircraft, about 5 miles. It could also detect coastline at approximately 75 miles.[1]

Physically, the APS-4 consists of a control box, one or two indicators, the same number of indicator-amplifiers, an antenna, a transmitter-receiver, and a cable junction box.[2] The antenna and transmitter-receiver were typically housed externally below one wing, in a fiberglass shape that was similar to a Mk 17 500 pound bomb.[3] These displays could be set for ranges of 4, 20, 50, and 100 nautical miles (6, 30, 80, and 160 km).[4] The radar weighed 180 lbs.[4]

The APS-4 broadcast in the X-band with a wavelength of 3 cm.[4][2] Peak broadcast power varied from 40 to 70 kW according to radar version. Pulse repetition frequency was adjustable by the operator to either 600 or 1000 pulses per second. [5]

The APS-4 emitted a radio beam in the form of a 6° cone. The beam could be directed in three modes, manual, search and intercept. In manual mode the beam was aimed by operator control from 10° above, to 30° below the longitudinal axis of the aircraft. In search mode, the radar beam scans through 150° in azimuth, and while doing so would run two lines scans, each separated by 4°. This caused the beam to cover 10° in a vertical plane. In intercept mode, the beam executes a four-line scan, with 6° between lines, to cover a vertical plane of 24°.[3] Results were displayed on one or two 3-inch displays.[2]

An improved version was called the AN/APS-5. A simplifed version for single seat fighters was called the APS-6.[6]


  1. ^>
  2. ^ a b c "HyperWar: Tactical Uses of Radar in Aircraft (RADTWOA) [Part II]".
  3. ^ a b
  4. ^ a b c Budge, Kent G. "The Pacific War Online Encyclopedia: ASH Airborne Radar".
  5. ^ "Duxford Radio Society: Restoration: Radar APS-4".
  6. ^ Budge, Kent G. "The Pacific War Online Encyclopedia: APS-6 Airborne Radar".