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.
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. 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. These displays could be set for ranges of 4, 20, 50, and 100 nautical miles (6, 30, 80, and 160 km). The radar weighed 180 lbs.
The APS-4 broadcast in the X-band with a wavelength of 3 cm. 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. 
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°. Results were displayed on one or two 3-inch displays.
An improved version was called the AN/APS-5. A simplifed version for single seat fighters was called the APS-6.
- "HyperWar: Tactical Uses of Radar in Aircraft (RADTWOA) [Part II]". www.ibiblio.org.
- Budge, Kent G. "The Pacific War Online Encyclopedia: ASH Airborne Radar". pwencycl.kgbudge.com.
- "Duxford Radio Society: Restoration: Radar APS-4". www.duxfordradiosociety.org.
- Budge, Kent G. "The Pacific War Online Encyclopedia: APS-6 Airborne Radar". pwencycl.kgbudge.com.
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