Side view of an AN/APS-4 radar pod without its shell
The AN/APS-4, originally known as ASH (air-surface, model H) is an early military air-to-air and air-to-surfaceradar used by American and British warplanes during World War II.
APS-4 operated in the X band at 3cm, compared to the 10cm S band 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 service of the Swedish Air Force, it was known as PS-18/A and was equipped on the Saab 18, primarily on the reconnaissance version, S 18A but it was also used on the bomber and torpedo versions B 18A, B 18B, and T 18B.[1] The PS-18/A had slightly improved maritime performance: the beacon navigation function was disabled, and a sea clutter filter, a simple, switchable shortened time constant in the video chain, was introduced.[2]
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 15 miles (24km), and about 5 miles (8.0km) against aircraft. It could also detect coastline at approximately 75 miles (121km).[3]
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.[4] 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 (230kg) bomb.[5] These displays could be set for ranges of 4, 20, 50, and 100 nautical miles (6, 30, 80, and 160km).[6] The radar weighed 180 pounds (82kg).[6]
The APS-4 broadcast in the X-band with a wavelength of 3cm.[6][4] Peak broadcast power varied from 40 to 70kW according to radar version. Pulse repetition frequency was adjustable by the operator to either 600 or 1000 pulses per second.[7]
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°.[5] Results were displayed on one or two 3-inch displays.[4]
An improved version was called the AN/APS-5. A simplified version for single-seat fighters was called the APS-6.[8]
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