<s>
An	O
early-warning	B-Algorithm
radar	I-Algorithm
is	O
any	O
radar	B-Application
system	I-Application
used	O
primarily	O
for	O
the	O
long-range	O
detection	O
of	O
its	O
targets	O
,	O
i.e.	O
,	O
allowing	O
defences	O
to	O
be	O
alerted	O
as	O
early	O
as	O
possible	O
before	O
the	O
intruder	O
reaches	O
its	O
target	O
,	O
giving	O
the	O
air	O
defences	O
the	O
maximum	O
time	O
in	O
which	O
to	O
operate	O
.	O
</s>
<s>
EW	O
radars	B-Application
tend	O
to	O
share	O
a	O
number	O
of	O
design	O
features	O
that	O
improve	O
their	O
performance	O
in	O
the	O
role	O
.	O
</s>
<s>
For	O
instance	O
,	O
EW	O
radar	B-Application
typically	O
operates	O
at	O
lower	O
frequencies	O
,	O
and	O
thus	O
longer	O
wavelengths	O
,	O
than	O
other	O
types	O
.	O
</s>
<s>
Likewise	O
,	O
EW	O
radars	B-Application
often	O
use	O
much	O
lower	O
pulse	O
repetition	O
frequency	O
to	O
maximize	O
their	O
range	O
,	O
at	O
the	O
cost	O
of	O
signal	O
strength	O
,	O
and	O
offset	O
this	O
with	O
long	O
pulse	O
widths	O
,	O
which	O
increases	O
the	O
signal	O
at	O
the	O
cost	O
of	O
lowering	O
range	O
resolution	O
.	O
</s>
<s>
The	O
canonical	O
EW	O
radar	B-Application
is	O
the	O
British	O
Chain	O
Home	O
system	O
,	O
which	O
entered	O
full-time	O
service	O
in	O
1938	O
.	O
</s>
<s>
Since	O
then	O
,	O
improvements	O
in	O
receiver	O
electronics	O
has	O
greatly	O
reduced	O
the	O
amount	O
of	O
signal	O
needed	O
to	O
produce	O
an	O
accurate	O
image	O
,	O
and	O
modern	O
examples	O
the	O
transmitted	O
power	O
is	O
much	O
less	O
;	O
the	O
AN/FPS	B-Application
-117	I-Application
offers	O
range	O
from	O
25kW	O
.	O
</s>
<s>
EW	O
radars	B-Application
are	O
also	O
highly	O
susceptible	O
to	O
radar	B-Application
jamming	O
and	O
often	O
include	O
advanced	O
frequency	B-Protocol
hopping	I-Protocol
systems	O
to	O
reduce	O
this	O
problem	O
.	O
</s>
<s>
The	O
first	O
early-warning	B-Algorithm
radars	I-Algorithm
were	O
the	O
British	O
Chain	O
Home	O
,	O
the	O
German	O
Freya	O
,	O
the	O
US	O
CXAM	O
(	O
Navy	O
)	O
and	O
SCR-270	O
(	O
Army	O
)	O
,	O
and	O
the	O
Soviet	O
Union	O
.	O
</s>
<s>
Starting	O
in	O
the	O
1950s	O
,	O
a	O
number	O
of	O
over-the-horizon	B-Architecture
radars	I-Architecture
were	O
developed	O
that	O
greatly	O
extended	O
detection	O
ranges	O
,	O
generally	O
by	O
bouncing	O
the	O
signal	O
off	O
the	O
ionosphere	O
.	O
</s>
<s>
By	O
placing	O
the	O
radar	B-Application
on	O
an	O
aircraft	O
,	O
the	O
line-of-sight	O
to	O
the	O
horizon	O
is	O
greatly	O
extended	O
.	O
</s>
<s>
This	O
allows	O
the	O
radar	B-Application
to	O
use	O
high-frequency	O
signals	O
,	O
offering	O
high	O
resolution	O
,	O
while	O
still	O
offering	O
long	O
range	O
.	O
</s>
<s>
A	O
major	O
exception	O
to	O
this	O
rule	O
are	O
radars	B-Application
intended	O
to	O
warn	O
of	O
ballistic	O
missile	O
attacks	O
,	O
like	O
BMEWS	B-Architecture
,	O
as	O
the	O
high-altitude	O
exo-atmospheric	O
trajectory	O
of	O
these	O
weapons	O
allows	O
them	O
to	O
be	O
seen	O
at	O
great	O
ranges	O
even	O
from	O
ground-based	O
radars	B-Application
.	O
</s>
