Aircraft Interception Radar
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.
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 1 meters with a power output of 50 watts.

Aircraft Interception Radar
AI Mk I
AI Mk II
AI Mk III
AI Mk IV
AI Mk V
AI Mk VI
AI Mk VII
AI Mk VIII
AI Mk IX
AI Mk X
AI Mk XI
AI Mk XII
AI Mk XIII
AI Mk XIV
AI Mk XV

 


Airborne interception Radar Mk I

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 1 meter, 50 watts
Main items
Remarks Experimental only
Developed by experimental set
Manufactured by handmade, no production

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Airborne interception Radar Mk II

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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Airborne interception Radar Mk III

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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Airborne interception Radar Mk IV

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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Airborne interception Radar Mk V

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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Airborne interception Radar Mk VI

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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Airborne interception Radar Mk VII

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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Airborne interception Radar Mk VIII

Year of issue April 1942

indicator73.jpg (56699 bytes)
Indicator type 73

receiver50.jpg (27789 bytes)
Receiver type 50

receiver184.jpg (39683 bytes)
Receiver type 184

scanner4.jpg (39485 bytes)
Scanner type 4
(which sure needs some restoration....).

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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Airborne interception Radar Mk VIIIA

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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Airborne interception Radar Mk VIIIB

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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Airborne interception Radar Mk IX

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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Airborne interception Radar Mk X

RAF designation ARI 5570 indicator-bc1151.jpg (12702 bytes)
Indicator

controlbox-bc1150.jpg (26598 bytes)
Control box

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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Airborne interception Radar Mk XI

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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Airborne interception Radar Mk XII

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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Airborne interception Radar Mk XIII

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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Airborne interception Radar Mk XIV

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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Airborne interception Radar Mk XV

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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