The AN/APS-4, originally known as ASH (air-surface, model H) is an early military air to air and air to surface

radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

used by American and British warplanes during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

. APS-4 operated in the

X band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxim ...

at 3 cm, compared to the 10 cm

S band The S band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a part of the microwave band of the electromagnetic spectrum covering frequencies from 2 to 4 gigahertz (GHz). Thus it crosses the convention ...

used by most radars of the era. This allowed the antenna to be greatly reduced in size and the unit as a whole to fit into a single streamlined fairing that could be mounted to many aircraft. In American service it was used on many aircraft, including the

Douglas C-47 Skytrain The Douglas C-47 Skytrain or Dakota (RAF, RAAF, RCAF, RNZAF, and SAAF designation) is a military transport aircraft developed from the civilian Douglas DC-3 airliner. It was used extensively by the Allies during World War II and remained in ...

, North American P-82D/F/H Twin Mustang, Vought F4U-2\5N Corsair, Grumman F6F-3/5 Hellcat,

Curtiss SB2C-5 Helldiver The Curtiss SB2C Helldiver is a dive bomber developed by Curtiss-Wright during World War II. As a carrier-based bomber with the United States Navy (USN), in Pacific theaters, it supplemented and replaced the Douglas SBD Dauntless. A few surviv ...

and Grumman TBF-3 and TBM-3S Avenger. In

RAF The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...

service it was known as ASV Mark IX and equipped a number of aircraft including the

Fairey Firefly The Fairey Firefly is a Second World War-era carrier-borne fighter aircraft and anti-submarine aircraft that was principally operated by the Fleet Air Arm (FAA). It was developed and built by the British aircraft manufacturer Fairey Aviatio ...

Fairey Barracuda The Fairey Barracuda was a British carrier-borne torpedo and dive bomber designed by Fairey Aviation. It was the first aircraft of this type operated by the Royal Navy's Fleet Air Arm (FAA) to be fabricated entirely from metal. The Barracuda ...

de Havilland Mosquito The de Havilland DH.98 Mosquito is a British twin-engined, shoulder-winged, multirole combat aircraft, introduced during the Second World War. Unusual in that its frame was constructed mostly of wood, it was nicknamed the "Wooden Wonder", or ...

and a small number of

Grumman Avenger The Grumman TBF Avenger (designated TBM for aircraft manufactured by General Motors) is an American World War II-era torpedo bomber developed initially for the United States Navy and Marine Corps, and eventually used by several air and naval av ...

Design

The APS-4 radar is a lightweight air-to-air and air-to-surface radar with a detection range for large ships of about , and about against aircraft. It could also detect coastline at approximately . 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 Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth ...

shape that was similar to a Mk 17 bomb.https://www.aef.se/Avionik/PDF-filer/PS18_USN_APS-4_Tactical_use.pdf These displays could be set for ranges of 4, 20, 50, and 100 nautical miles (6, 30, 80, and 160 km). The radar weighed . The APS-4 broadcast in the

X-band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxim ...

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 simplified version for single-seat fighters was called the APS-6.

References

{{reflist World War II radars Aircraft radars World War II Allied electronics X band radar