<s>
Radar	O
,	O
Airborne	B-Device
Interception	I-Device
,	O
Mark	O
20	O
,	O
AI.20	O
for	O
short	O
,	O
also	O
known	O
by	O
its	O
rainbow	O
codename	O
Green	B-Device
Willow	I-Device
,	O
was	O
a	O
prototype	O
Airborne	B-Device
Interception	I-Device
radar	I-Device
developed	O
by	O
EKCO	O
for	O
the	O
English	O
Electric	O
Lightning	O
interceptor	O
aircraft	O
.	O
</s>
<s>
The	O
system	O
was	O
used	O
,	O
almost	O
unchanged	O
,	O
as	O
the	O
basis	O
for	O
the	O
Red	O
Steer	O
tail	B-Device
warning	I-Device
radar	I-Device
used	O
in	O
the	O
Handley	O
Page	O
Victor	O
and	O
Avro	O
Vulcan	O
strategic	O
bombers	O
.	O
</s>
<s>
When	O
the	O
English	O
Electric	O
Lightning	O
project	O
first	O
began	O
,	O
existing	O
Airborne	B-Device
Interception	I-Device
radar	I-Device
systems	O
were	O
generally	O
just	O
more	O
powerful	O
versions	O
of	O
their	O
World	O
War	O
II	O
counterparts	O
,	O
using	O
newer	O
magnetron	B-Device
and	O
klystron-based	O
microwave	O
tubes	O
and	O
more	O
sensitive	O
receivers	O
,	O
but	O
otherwise	O
using	O
the	O
same	O
mechanical	O
scanning	O
concepts	O
and	O
simplified	O
processing	O
that	O
generally	O
required	O
a	O
dedicated	O
crewmember	O
to	O
operate	O
.	O
</s>
<s>
Some	O
British	O
AI	B-Device
radars	I-Device
of	O
the	O
post-war	O
era	O
were	O
hand-me-down	O
units	O
from	O
the	O
US	O
.	O
</s>
<s>
For	O
the	O
Lightning	O
,	O
Ferranti	O
proposed	O
an	O
advanced	O
Airborne	B-Device
Interception	I-Device
radar	I-Device
system	O
that	O
would	O
leapfrog	O
any	O
radar	O
system	O
then	O
in	O
use	O
.	O
</s>
<s>
This	O
was	O
the	O
first	O
true	O
heads	B-Application
up	I-Application
display	I-Application
,	O
a	O
major	O
advance	O
that	O
allowed	O
a	O
single-crew	O
aircraft	O
to	O
be	O
an	O
effective	O
interceptor	O
for	O
the	O
first	O
time	O
.	O
</s>
<s>
The	O
result	O
was	O
AIRPASS	O
,	O
short	O
for	O
"	O
Airborne	B-Device
Interception	I-Device
and	O
Pilot	O
's	O
Attack	O
Sight	O
System	O
"	O
.	O
</s>
<s>
The	O
contract	O
for	O
the	O
backup	O
system	O
was	O
awarded	O
to	O
EKCO	O
in	O
late	O
1953	O
and	O
assigned	O
the	O
rainbow	O
code	O
"	O
Green	B-Device
Willow	I-Device
"	O
.	O
</s>
<s>
To	O
produce	O
bright	O
enough	O
symbols	O
on	O
a	O
cathode	B-Device
ray	I-Device
tube	I-Device
(	O
CRT	O
)	O
required	O
an	O
extremely	O
powerful	O
electron	O
gun	O
.	O
</s>
<s>
In	O
1955	O
,	O
the	O
Royal	O
Aircraft	O
Establishment	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
bombers	O
that	O
had	O
greater	O
performance	O
than	O
the	O
existing	O
Orange	O
Putter	O
that	O
had	O
been	O
developed	O
for	O
the	O
English	O
Electric	O
Canberra	O
.	O
</s>
