| Aircraft Interception Radar
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| Airborne Interception equipment are airborne radar sets installed in
night fighters (usually two-seater aircraft) to enable them to detect and
intercept enemy aircraft under conditions of poor visibility. In the
practice the Radio Navigator operates the AI and interprets the display to
the pilot. Since the maximum range of detection of existing forms of AI is
small (4 to 6 miles) most interception involve GCI control in the early
stages, but "free-lance" AI aircraft have achieved some measure
of success and as the maximum ranges of equipments are improved,
"free-lancing" will probably assume greater importance as a
method of interception. Owing to the unsatisfactory state of the art of radar identification, the fighter relies on visual identification of the target before attempting to shoot. However, AI equipments with blind firing facilities have been developed, but were not in use. |
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| In 1936 a team of scientists, led by Dr. E.G.Bowen, commenced trials on equipment that
lead to the development of airborne radar. These first trials were initially carried out
on a wavelength of 7 meters with only the receiver actually fitted in the aircraft, the
target being illuminated by a separate ground transmitter. At later trials the transmitter was also added to the aircraft and intensive development during 1937 led to a reduction in size to allow the equipment to be fitted into smaller aircraft. It was during this period that the development of airborne radar began to diverge into two classes of equipment, ASV (Air to Surface Vessel) radar for the detection of shipping and Airborne Interception (AI) radar for the detection of aircraft. By December 1937 Dr Bowen was proposing an AI equipment operating on a wavelength of 1½ meters with a power output of 50 watts. |
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In May 1939 the first AI was flown for the first time in a Fairy Battle aircraft. Target position was shown on two cathode ray tube displays showing azimuth and elevation. The equipment was able to detect targets down to a minimum range of 900 feet and out to a maximum range limited by the reception of ground returns which were displayed on the screens at ranges corresponding to the height of the radar carrying aircraft. These installations were complete hand constructed.
| Year of issue | May 1939 | |
| RAF designation | ||
| Frequency | ||
| Wave length | 1½ meter, 50 watts | |
| Main items | ||
| Remarks | Experimental only | |
| Developed by | experimental set | |
| Manufactured by | handmade, no production |
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AI Mk II was fitted into Bristol Blenheim twin engine fighters, and commenced service trials in November 1939, the early results were limited due to the temperamental nature of the equipment. Interesting is that the receiver was based on one of the commercial Pye TV chassis.
| Year of issue | November 1939 | |
| RAF designation | ARI 5535 or 5933 | |
| Frequency | ||
| Wave length | 1.5m | |
| Main items | R 3041 T 3042 |
|
| Remarks | ||
| Developed by | Production version of Mk.I | |
| Manufactured by | Transmitters by Metropolitan Receivers by Pye. |
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Transmission of the AI Mk III was via a double dipole aerial arranged to look like an arrow head. Reception was by two swept back dipole aerials on one of the wings giving elevation, and two vertical dipole aerials on the other wing giving azimuth. Due to the temperamental design the equipment was unreliable and only a few successful interceptions were made.
| Year of issue | late 1939 | |
| RAF designation | ||
| Frequency | ||
| Wave length | 1.5m | |
| Main items | Receiver R 3069 Transmitter T 3045 / T 3062 Indicator type 19 |
|
| Remarks | Only used for training | |
| Developed by | ||
| Manufactured by |
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GEC designed a small powerful transmitter valve and EMI developed a new modulator which made the AI Mk IV the first successful Airborne Interception radar. The equipment could detect targets from a maximum range of 20.000 feet down to a minimum range of 400 feet. AI Mk IV quickly became the standard fit in RAF night fighters and was installed in the new Bristol Beaufighter which became operational just in time to take part in the defense of Britain during the Blitz in 1941.
| Year of issue | July 1940 | |
| RAF designation | ARI 5003 | |
| Frequency | 188 -198 Mc/s | |
| Wavelength | 1.5 m | |
| Main items | Receiver R 3066 or R 3102 Transmitter T 3065 Indicator unit 20 or 48 Modulator type 20 Voltage control panel type 3 Azimuth aerial type 21 & 25 Transmitter aerial type 19 Elevation aerial type 25 Impedance matching unit type 35 |
|
| Test equipment | TS 36 TS 127 |
|
| Remarks | First version to be used in combat | |
| Developed by | Air Ministry Research Establishment | |
| Manufactured by | Pye & EMI |
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Single seat aircraft had the AI Mk IV modified by the inclusion of a pilot indicating tube and was redesignated Mk V and Mk VI.
| Year of issue | ||
| RAF designation | ARI 5005 / 5526 | |
| Frequency | ||
| Wave length | 1.5m | |
| Main items | Receiver R 3085 Transmitter T 3065 or T 3100 Indicator 41 or 42 Modulator type 29 Control unit 87 |
|
| Test equipment | TS 36 | |
| Remarks | Mk.IV for use by a pilot alone | |
| Developed by | ||
| Manufactured by |
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Single seat aircraft had the AI Mk IV modified by the inclusion of a pilot indicating tube and was redesignated Mk V and Mk VI.
| Year of issue | ||
| RAF designation | ARI 5006 | |
| Frequency | ||
| Wave length | 1.5m | |
| Main items | Receiver R 3075 Rx 36 Transmitter T 3074 Indicator 30 or 32 Modulator 16 or 176 Control unit 96, 97, 67 Oscillator 214 Aerials 29 & 49 |
|
| Test equipment | TS 31 TS 33 TS 36 |
|
| Remarks | Mk.IV for use by a pilot alone | |
| Developed by | ||
| Manufactured by |
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The AI Mk VII was the first AI which could operate at centrimetric wavelengths. The AI Mk VII had also a new aerial system consisting of a fixed aerial around which the scanner revolved
| Year of issue | March 1941 | |
| RAF designation | ARI 5046 or 5049 | |
| Frequency | ||
| Wave length | 9,1cm S-band | |
| Main items | Receiver R 3124 Transmitter T 3130 Indicator 56 Modulator 42 Power unit 149 |
|
| Remarks | First centimeter radar set, only small number of sets produced. | |
| Developed by | ||
| Manufactured by |
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| Year of issue | April 1942 |
|
| RAF designation | ARI 5093 | |
| Frequency | 3 GHz | |
| Wavelength | 9,1 cm | |
| Main items | Tx/Rx TR 3151 or 3152 Tx unit type 3549 Rx unit type 50 / 184 Indictor unit type 73 Modulator unit type 53 Power unit type 225 Control unit type 5 & 462 Junction Box type 67 |
|
| Test equipment | TS 247 | |
| Remarks | ||
| Developed by | TRE (Telecommunications Research Establishment) | |
| Manufactured by | Ecko & GEC |
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| Year of issue | ||
| RAF designation | ARI 5093A | |
| Frequency | ||
| Wavelength | ||
| Main items | ||
| Test equipment | ||
| Remarks | Pre-production of AI Mk VIII | |
| Developed by | ||
| Manufactured by |
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| Year of issue | ||
| RAF designation | ARI 5588 / ARI 5645 | |
| Frequency | ||
| Wavelength | ||
| Main items | ||
| Test equipment | ||
| Remarks | ARI 5645 = AI Mk VIIIB with Lucero. This equipment incorporates Lucero for use with 1,5 metre Beacons, BABS and IFF |
|
| Developed by | ||
| Manufactured by |
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| Year of issue | ||
| RAF designation | ARI 5765 | |
| US designation | AN/APX 7 | ??? |
| Frequency | ||
| Wave length | ||
| Main items | ||
| Remarks | Frequency-tunable Automatic lock-follow facilities |
|
| Test equipment | TS 248 TS 300 |
|
| Developed by | ||
| Manufactured by |
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| RAF designation | ARI 5570 |  Indicator
|
| US designation | SCR 720 | |
| Year of issue | 1942 / 1943 | |
| Function | AI MkX is a modified version of the American airborne radar installation SCR-720B.It is designed to provide facilities for the interception and ranging other aircraft, beacon or homing services and also for use as a navigational aid | |
| Wavelength | 9.1cm, S-band | |
| Pulse width | 3/4 microsecond for radar 2 1/4 microsecond for beacon |
|
| Pulse recurrence frequency | 1600 for radar 400 for beacon |
|
| Range scales | 5, 10, 20, 100 miles | |
| Maximum range | 8-10 miles for radar | |
| Minimum range | 375 ft for radar 3/4 mile for beacon |
|
| Scanner speed | 360 r.p.m. for radar 100 r.p.m. for beacon |
|
| Beam width | 10 deg. approx. | |
| Peak power | 70 kW | |
| Power supplies | 24V DC aircraft supply and 80V AC from type U Alternator | |
| Power consumption | 500 watts DC, 900 watts AC | |
| Main items | TR 3529 or TR 3530 RF unit BC 1091 Modulator BC 1142 Synchronizer BC 1148 (type C) BC 1149 Indicator BC 1151, 1152 Control unit BC 1150 Scanner unit BC-94D Junction box type 253 Antenna equipment RC-94-C (Aerial system 341) Rectifier RA-90-A, RA-88-A Inverter unit PE-218-A Power unit PE-158 Junction box JB-98-A (power) JB-87-A |
|
| Remarks | Replaced the SCR-520. SCR 720 is very often combined with SCR 729 |
|
| Used on Mosquito's, P-61, P-70. | ||
| Developed by | Radiation Laboratory, Massachusetts Institute of Technology | |
| Manufactured by | Made by Western Electric, but Bell Labs also involved. |
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| Year of issue | ||
| RAF designation | ARI 5552 (or 5502) | |
| Frequency | ||
| Wave length | 3cm | |
| Main items | TR 3529 (3198) or TR 3526 (3503) |
|
| Remarks | Developed for Fleet Air Arm | |
| Developed by | ||
| Manufactured by |
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| Year of issue | ||
| RAF designation | ARI 5654 | |
| Frequency | ||
| Wave length | ||
| Main items | ||
| Remarks | Improved version of AI Mk XI | |
| Developed by | ||
| Manufactured by |
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| Year of issue | ||
| RAF designation | ARI 5647 | |
| Frequency | ||
| Wave length | ||
| Main items | ||
| Test equipment | ||
| Remarks | Lightweight AI Mk XI | |
| Developed by | ||
| Manufactured by |
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| Year of issue | ||
| RAF designation | ARI 5692 (or 5751) | |
| US designation | AN/APS 6 AIA-1 (Navy) |
|
| Frequency | ||
| Wave length | ||
| Main items | ||
| Test equipment | ||
| Remarks | Pilot operated | |
| Developed by | ||
| Manufactured by |
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| Year of issue | 1944 | |
| RAF designation | ARI 5578 | |
| US designation | AN/APS-4 ASH (Air Surface H) | ?? |
| Frequency | 10 GHz | |
| Wavelength | 3 cm | |
| Main items | ||
| Remarks | ||
| Developed by | Radiation Laboratory, Massachusetts Institute of Technology | |
| Manufactured by | Western Electric Company |
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