<s>
Airborne	B-Device
Interception	I-Device
radar	I-Device
,	O
or	O
AI	O
for	O
short	O
,	O
is	O
the	O
British	O
term	O
for	O
radar	B-Application
systems	I-Application
used	O
to	O
equip	O
aircraft	O
in	O
air-to-air	O
role	O
.	O
</s>
<s>
These	O
radars	B-Application
are	O
used	O
primarily	O
by	O
Royal	O
Air	O
Force	O
(	O
RAF	O
)	O
and	O
Fleet	O
Air	O
Arm	O
night	O
fighters	O
and	O
interceptors	O
for	O
locating	O
and	O
tracking	O
other	O
aircraft	O
,	O
although	O
most	O
AI	B-Device
radars	I-Device
could	O
also	O
be	O
used	O
in	O
a	O
number	O
of	O
secondary	O
roles	O
as	O
well	O
.	O
</s>
<s>
The	O
term	O
was	O
sometimes	O
used	O
generically	O
for	O
similar	O
radars	B-Application
used	O
in	O
other	O
countries	O
.	O
</s>
<s>
The	O
term	O
was	O
first	O
used	O
circa	O
1936	O
,	O
when	O
a	O
group	O
at	O
the	O
Bawdsey	O
Manor	O
research	O
center	O
began	O
considering	O
how	O
to	O
fit	O
a	O
radar	B-Application
system	I-Application
into	O
an	O
aircraft	O
.	O
</s>
<s>
This	O
work	O
led	O
to	O
the	O
Airborne	B-Device
Interception	I-Device
Mk	O
.	O
</s>
<s>
IV	O
,	O
the	O
first	O
production	O
air-to-air	O
radar	B-Application
system	I-Application
.	O
</s>
<s>
VII	O
,	O
AI	O
moved	O
to	O
microwave	O
frequencies	O
using	O
the	O
cavity	B-Architecture
magnetron	I-Architecture
,	O
greatly	O
improving	O
performance	O
while	O
reducing	O
size	O
and	O
weight	O
.	O
</s>
<s>
A	O
good	O
example	O
is	O
the	O
AI.24	O
radar	B-Application
of	O
the	O
Tornado	O
F.2	O
.	O
</s>
<s>
These	O
radars	B-Application
were	O
often	O
given	O
common	O
names	O
as	O
well	O
,	O
and	O
generally	O
better	O
known	O
by	O
these	O
;	O
the	O
AI.24	O
is	O
almost	O
universally	O
referred	O
to	O
as	O
"	O
Foxhunter	O
"	O
.	O
</s>
<s>
In	O
order	O
to	O
provide	O
the	O
maximum	O
possible	O
warning	O
time	O
of	O
an	O
incoming	O
raid	O
,	O
the	O
RAF	O
's	O
Chain	O
Home	O
(	O
CH	O
)	O
radar	B-Application
stations	I-Application
had	O
been	O
positioned	O
as	O
far	O
forward	O
as	O
possible	O
,	O
right	O
on	O
the	O
coastline	O
.	O
</s>
<s>
Hugh	O
Dowding	O
addressed	O
this	O
through	O
the	O
creation	O
of	O
what	O
is	O
today	O
known	O
as	O
the	O
Dowding	O
system	O
,	O
networking	O
together	O
the	O
radars	B-Application
and	O
observation	O
centres	O
by	O
telephone	O
to	O
a	O
central	O
station	O
.	O
</s>
<s>
Once	O
the	O
enemy	O
aircraft	O
passed	O
the	O
coastline	O
they	O
could	O
not	O
be	O
seen	O
by	O
the	O
radars	B-Application
,	O
and	O
the	O
ROC	O
could	O
not	O
see	O
at	O
night	O
except	O
under	O
ideal	O
conditions	O
with	O
bright	O
moonlight	O
,	O
no	O
cloud	O
cover	O
,	O
and	O
considerable	O
luck	O
.	O
</s>
<s>
The	O
obvious	O
solution	O
would	O
be	O
to	O
mount	O
a	O
small	O
radar	B-Application
on	O
the	O
aircraft	O
,	O
one	O
able	O
to	O
cover	O
the	O
range	O
between	O
the	O
Dowding	O
system	O
's	O
5	O
mile	O
accuracy	O
and	O
the	O
average	O
visual	O
spotting	O
range	O
,	O
about	O
.	O
</s>
<s>
As	O
early	O
as	O
August	O
1936	O
"	O
Taffy	O
"	O
Bowen	O
,	O
one	O
of	O
Robert	O
Watson-Watt	O
'	O
s	O
hand-picked	O
radar	B-Application
development	O
team	O
,	O
personally	O
requested	O
that	O
he	O
be	O
allowed	O
to	O
start	O
research	O
into	O
an	O
airborne	O
radar	B-Application
set	O
for	O
this	O
role	O
.	O
</s>
<s>
At	O
the	O
time	O
,	O
radar	B-Application
development	O
was	O
in	O
its	O
infancy	O
and	O
the	O
other	O
teams	O
were	O
working	O
with	O
long-wavelength	O
transmitters	O
operating	O
around	O
7	O
meters	O
.	O
</s>
<s>
This	O
ability	O
led	O
to	O
the	O
split	O
between	O
AI	O
and	O
air-to-surface	B-Device
vessel	I-Device
radar	I-Device
(	O
ASV	O
)	O
systems	O
,	O
both	O
of	O
which	O
would	O
be	O
widely	O
used	O
during	O
the	O
war	O
.	O
</s>
<s>
Practical	O
ASV	B-Device
radars	I-Device
were	O
operational	O
in	O
1940	O
,	O
but	O
the	O
AI	O
developments	O
proved	O
much	O
more	O
difficult	O
.	O
</s>
<s>
In	O
February	O
1940	O
,	O
John	O
Randall	O
and	O
Harry	O
Boot	O
at	O
Birmingham	O
University	O
successfully	O
ran	O
the	O
first	O
cavity	B-Architecture
magnetron	I-Architecture
,	O
eventually	O
generating	O
1	O
kW	O
at	O
9.8	O
cm	O
(	O
3,060	O
MHz	O
)	O
.	O
</s>
<s>
The	O
narrow	O
beam	O
also	O
meant	O
that	O
the	O
radar	B-Application
could	O
only	O
see	O
targets	O
directly	O
in	O
front	O
of	O
the	O
antenna	O
,	O
unlike	O
the	O
Mk	O
.	O
</s>
<s>
However	O
,	O
the	O
limited	O
power	O
of	O
the	O
magnetron	B-Architecture
,	O
about	O
5	O
kW	O
,	O
provided	O
range	O
of	O
about	O
,	O
not	O
a	O
great	O
improvement	O
over	O
the	O
Mk	O
.	O
</s>
<s>
This	O
run	O
was	O
followed	O
by	O
the	O
production	O
Mark	O
VIII	O
that	O
included	O
the	O
new	O
"	O
strapped	O
magnetron	B-Architecture
"	O
of	O
25	O
kW	O
,	O
improving	O
range	O
to	O
about	O
.	O
</s>
<s>
On	O
1	O
January	O
1942	O
Lovell	O
was	O
sent	O
to	O
work	O
on	O
the	O
H2S	O
radar	B-Application
project	O
and	O
was	O
replaced	O
by	O
Arthur	O
Ernest	O
Downing	O
.	O
</s>
<s>
Window	O
caused	O
false	O
returns	O
on	O
radar	B-Application
displays	O
that	O
made	O
it	O
difficult	O
to	O
tell	O
where	O
the	O
bombers	O
were	O
amid	O
a	O
sea	O
of	O
blips	O
.	O
</s>
<s>
IX	O
might	O
ignore	O
the	O
window	O
completely	O
,	O
as	O
the	O
light	O
metal	O
strips	O
rapidly	O
dispersed	O
from	O
the	O
target	O
being	O
tracked	O
,	O
faster	O
than	O
the	O
radar	B-Application
could	O
follow	O
.	O
</s>
<s>
In	O
testing	O
in	O
1944	O
it	O
was	O
found	O
to	O
be	O
marginally	O
better	O
than	O
the	O
US	O
SCR-720	O
,	O
but	O
with	O
the	O
SCR-720	O
expected	O
to	O
arrive	O
at	O
any	O
moment	O
,	O
the	O
demand	O
for	O
another	O
radar	B-Application
was	O
not	O
pressing	O
.	O
</s>
<s>
The	O
radar	B-Application
antenna	O
was	O
spun	O
around	O
a	O
vertical	O
axis	O
through	O
an	O
entire	O
360	O
degrees	O
10	O
times	O
a	O
second	O
,	O
with	O
the	O
transmitter	O
switching	O
off	O
when	O
the	O
antenna	O
was	O
pointed	O
back	O
towards	O
the	O
aircraft	O
.	O
</s>
<s>
X	O
became	O
one	O
of	O
the	O
UK	O
's	O
most	O
widely	O
used	O
fighter	O
radars	B-Application
,	O
largely	O
because	O
a	O
lack	O
of	O
foreign	O
exchange	O
to	O
purchase	O
newer	O
designs	O
,	O
and	O
the	O
poor	O
economy	O
in	O
general	O
which	O
required	O
the	O
RAF	O
to	O
have	O
a	O
"	O
make	O
do	O
"	O
attitude	O
.	O
</s>
<s>
For	O
the	O
Fleet	O
Air	O
Arm	O
,	O
the	O
TRE	O
developed	O
a	O
series	O
of	O
AI	B-Device
radars	I-Device
operating	O
at	O
the	O
even	O
shorter	O
3	O
cm	O
wavelength	O
,	O
the	O
X	B-Algorithm
band	I-Algorithm
,	O
which	O
further	O
reduced	O
the	O
size	O
of	O
the	O
antennas	O
.	O
</s>
<s>
These	O
designations	O
were	O
given	O
to	O
the	O
US	O
AN/APS	B-Algorithm
-4	I-Algorithm
and	O
AN/APS	O
-6	O
radars	B-Application
,	O
small	O
under-wing	O
X	B-Algorithm
band	I-Algorithm
radars	B-Application
used	O
primarily	O
by	O
naval	O
aircraft	O
.	O
</s>
<s>
The	O
APS-4	B-Algorithm
was	O
originally	O
developed	O
as	O
the	O
ASH	O
,	O
a	O
forward-aimed	O
surface-search	O
system	O
.	O
</s>
<s>
The	O
Fleet	O
Air	O
Arm	O
mounted	O
it	O
on	O
the	O
Fairey	O
Firefly	O
,	O
which	O
had	O
the	O
size	O
to	O
carry	O
a	O
radar	B-Application
operator	O
and	O
the	O
performance	O
to	O
operate	O
as	O
a	O
fighter	O
.	O
</s>
<s>
Considerably	O
later	O
,	O
a	O
single	O
Meteor	O
,	O
EE348''	O
,	O
was	O
fit	O
with	O
an	O
APS-4	B-Algorithm
in	O
a	O
nose	O
mounting	O
as	O
a	O
test	O
vehicle	O
.	O
</s>
<s>
The	O
APS-6	O
was	O
a	O
modification	O
of	O
the	O
APS-4	B-Algorithm
specifically	O
for	O
the	O
interception	O
role	O
.	O
</s>
<s>
Two	O
radar	B-Application
sets	O
competed	O
for	O
the	O
design	O
,	O
the	O
Mk	O
.	O
</s>
<s>
IXC	O
with	O
a	O
number	O
of	O
detail	O
cleanups	O
and	O
a	O
200kW	O
magnetron	B-Architecture
,	O
as	O
well	O
as	O
the	O
ability	O
to	O
cue	O
the	O
"	O
Blue	O
Jay	O
"	O
missile	O
that	O
was	O
then	O
under	O
development	O
.	O
</s>
<s>
18	O
operated	O
in	O
the	O
X	B-Algorithm
band	I-Algorithm
with	O
a	O
180kW	O
peak	O
power	O
,	O
using	O
a	O
parabolic	O
dish	O
that	O
could	O
be	O
pointed	O
±100°	O
in	O
azimuth	O
,	O
+	O
50/	O
-40	O
°	O
in	O
elevation	O
,	O
and	O
could	O
keep	O
a	O
lock	O
at	O
as	O
much	O
as	O
75°	O
in	O
roll	O
.	O
</s>
<s>
The	O
AI	B-Device
Mark	I-Device
20	I-Device
was	O
an	O
X-band	B-Algorithm
radar	I-Algorithm
developed	O
by	O
EKCO	O
Electronics	O
for	O
single	O
seat	O
fighters	O
.	O
</s>
<s>
Code	O
named	O
"	O
Green	B-Device
Willow	I-Device
"	O
by	O
the	O
MoS	O
,	O
it	O
was	O
intended	O
to	O
be	O
a	O
backup	O
system	O
to	O
the	O
AI.23	O
being	O
developed	O
for	O
the	O
English	O
Electric	O
Lightning	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
It	O
is	O
believed	O
that	O
the	O
1953	O
contract	O
was	O
awarded	O
to	O
EKCO	O
due	O
to	O
their	O
already	O
existing	O
work	O
on	O
the	O
Fairey	O
Fireflash	O
missile	O
illumination	O
radar	B-Application
.	O
</s>
<s>
The	O
next	O
year	O
the	O
MoS	O
published	O
a	O
requirement	O
for	O
a	O
new	O
tail	B-Device
warning	I-Device
radar	I-Device
for	O
the	O
V	O
bomber	O
force	O
,	O
replacing	O
the	O
original	O
Orange	O
Putter	O
,	O
and	O
quickly	O
chose	O
the	O
AI.20	O
as	O
its	O
basis	O
.	O
</s>
<s>
As	O
the	O
Javelin	O
ran	O
into	O
delays	O
,	O
it	O
was	O
decided	O
to	O
increase	O
the	O
useful	O
life	O
of	O
the	O
existing	O
Meteor	O
and	O
Vampire	O
night	O
fighters	O
with	O
a	O
new	O
radar	B-Application
.	O
</s>
<s>
The	O
Mark	O
22	O
was	O
the	O
British	O
version	O
of	O
the	O
US	O
AN/APQ	O
-43	O
,	O
This	O
consisted	O
of	O
two	O
radar	B-Application
antennas	O
driven	O
from	O
a	O
common	O
magnetron	B-Architecture
transmitter	O
.	O
</s>
<s>
One	O
used	O
spiral-scan	O
to	O
search	O
for	O
targets	O
,	O
while	O
the	O
second	O
used	O
conical	B-Algorithm
scanning	I-Algorithm
for	O
tracking	O
at	O
close	O
range	O
.	O
</s>
<s>
This	O
was	O
one	O
of	O
the	O
earliest	O
radars	B-Application
to	O
offer	O
track	B-Algorithm
while	I-Algorithm
scan	I-Algorithm
(	O
TWS	O
)	O
operation	O
,	O
although	O
it	O
did	O
so	O
through	O
the	O
use	O
of	O
what	O
was	O
essentially	O
two	O
radars	B-Application
.	O
</s>
<s>
Ferranti	O
's	O
Mark	O
23	O
was	O
an	O
X	B-Algorithm
band	I-Algorithm
design	O
originally	O
designed	O
for	O
the	O
modified	O
Fairey	O
Delta	O
2	O
proposed	O
for	O
the	O
Ministry	O
of	O
Supply	O
's	O
Operational	O
Requirement	O
F.155	O
for	O
a	O
modern	O
interceptor	O
aircraft	O
.	O
</s>
<s>
The	O
AI.23	O
was	O
the	O
world	O
's	O
first	O
operational	O
airborne	O
monopulse	B-Algorithm
radar	I-Algorithm
system	O
.	O
</s>
<s>
The	O
monopulse	B-Algorithm
method	O
allows	O
higher	O
resolution	O
and	O
is	O
far	O
more	O
resistant	O
to	O
common	O
forms	O
of	O
jamming	O
.	O
</s>
<s>
AI.23	O
also	O
included	O
all	O
of	O
the	O
features	O
of	O
earlier	O
AI	B-Device
radars	I-Device
,	O
and	O
more	O
.	O
</s>
<s>
This	O
gave	O
the	O
system	O
its	O
name	O
,	O
AIRPASS	O
,	O
an	O
acronym	O
for	O
airborne	B-Device
interception	I-Device
radar	I-Device
and	O
pilot	O
's	O
attack	O
sight	O
system	O
.	O
</s>
<s>
Further	O
development	O
led	O
to	O
the	O
terrain	O
following	O
radar	B-Application
used	O
in	O
the	O
BAC	O
TSR.2	O
.	O
</s>
<s>
The	O
final	O
radar	B-Application
in	O
the	O
UK	O
series	O
of	O
AI	O
designs	O
to	O
see	O
deployment	O
was	O
the	O
Mark	O
24	O
,	O
better	O
known	O
as	O
"	O
Foxhunter	O
"	O
.	O
</s>
<s>
Development	O
of	O
the	O
ADV	O
began	O
in	O
1976	O
and	O
the	O
radar	B-Application
system	I-Application
contract	O
was	O
eventually	O
won	O
by	O
a	O
curious	O
combined	O
bid	O
;	O
Marconi	O
and	O
Elliot	O
Automation	O
would	O
provide	O
most	O
of	O
the	O
design	O
,	O
while	O
Ferranti	O
built	O
the	O
transmitter	O
section	O
and	O
Antenna	O
Platform	O
.	O
</s>
<s>
Further	O
development	O
slowed	O
,	O
and	O
the	O
radar	B-Application
was	O
still	O
not	O
ready	O
for	O
service	O
by	O
1987	O
,	O
although	O
the	O
aircraft	O
itself	O
was	O
now	O
rolling	O
off	O
the	O
production	O
lines	O
.	O
</s>
<s>
In	O
place	O
of	O
the	O
radar	B-Application
a	O
concrete	O
ballast	O
plug	O
had	O
to	O
be	O
installed	O
in	O
early	O
Tornado	O
ADVs	O
,	O
where	O
it	O
was	O
jokingly	O
known	O
as	O
the	O
"	O
Blue	O
Circle	O
radar	B-Application
"	O
,	O
a	O
pun	O
referring	O
to	O
the	O
Ministry	O
of	O
Supply	O
's	O
rainbow	O
codes	O
,	O
and	O
a	O
local	O
brand	O
of	O
concrete	O
.	O
</s>
<s>
This	O
led	O
to	O
a	O
further	O
series	O
of	O
problems	O
as	O
the	O
radar	B-Application
was	O
adapted	O
to	O
fire	O
this	O
missile	O
.	O
</s>
