DRS badge small
Duxford Radio Society

Imperial War Museum, Duxford, England 

Equipment Summary


Updated 22-04-2009

Airborne Search Radar RT-5A/APS-4

APS-4 Airborne Radar TX/RX pod

APS-4 Radar TX/RX pod with front and rear radome covers removed

APS-4 on RAF Fairy Firefly

APS-4 radar mounted under the nose of a Fairy Firefly
of the Royal Navy Fleet Air Arm

Photo: Ronald Asplin, RN Retired

Airborne Search Radar RT-5A/APS-4 (USA 1943)

The APS-4 was a light-weight, pod-mounted airborne search Radar which was suitable for either Airborne Interception (AI) or Air-to-Surface-Vessel (ASV) applications.  It was a member of a series of early air-borne radar equipments and was initially designated as AS-H (Air-to Surface, version H).

This very advanced equipment for its time was first used by the USA Navy carrier-borne aircraft (
F6F Hellcat and F4U-2 Corsair), and later by RAF Mosquitos.  In RAF service it was known as AI Mk XV.  Post WW2 it was fitted to the Royal Navy Fairy Firefly and also used by the Swedish Airforce.

The equipment is carried under the wing or the fuselage of the aircraft and is dimensioned to be equivalent to a 500lb bomb.

The Radar dish is designed to scan from side to side for AI applications and can also be commanded to look up and down by a few degrees.  This enables it to search for surface vessels below (ASV) and also aircraft attacking from above.

The frequency used was approximately 9000 MHz, the pulse duration 0.6uS and the pulse repetition frequency adjustable to either 600 or 1000 cpS.  The peak RF output power was 40 to 70 kW according to the version in use.

APS-4 radar system diagram

APS-4 radar system diagram showing all interconnected parts

This particular example of the APS-4 equipment was kindly donated to DRS by Dr. G. E. Winbolt  and was built by  the Western Electric Company in the USA for the US Navy Bureau of Ships and is thought to date from 1943.

The external conservation of this airborne radar TX/RX has begun with inspection and cleaning work.  Approximately 45 valves have been replaced, including the Magnetron transmitting valve.

To date the radar dish assembly has been made to scan from side to side and up and down when powered from an external 24 Volt supply.

The unit is currently on display in Duxford Building 178 and when all work is competed  it is hoped to exhibit this item in  Building 177.



Top of page | Home

Top of page | Home 
  Imperial War Museum Duxford | Duxford Aviation Society

V2.00 25-10-06
Updated 22-04-2009