<s>
A	O
passive	B-Algorithm
electronically	I-Algorithm
scanned	I-Algorithm
array	I-Algorithm
(	O
PESA	O
)	O
,	O
also	O
known	O
as	O
passive	B-Algorithm
phased	I-Algorithm
array	I-Algorithm
,	O
is	O
an	O
antenna	O
in	O
which	O
the	O
beam	O
of	O
radio	O
waves	O
can	O
be	O
electronically	O
steered	O
to	O
point	O
in	O
different	O
directions	O
(	O
that	O
is	O
,	O
a	O
phased	O
array	O
antenna	O
)	O
,	O
in	O
which	O
all	O
the	O
antenna	O
elements	O
are	O
connected	O
to	O
a	O
single	O
transmitter	O
(	O
such	O
as	O
a	O
magnetron	B-Device
,	O
a	O
klystron	O
or	O
a	O
travelling	O
wave	O
tube	O
)	O
and/or	O
receiver	O
.	O
</s>
<s>
The	O
largest	O
use	O
of	O
phased	O
arrays	O
is	O
in	O
radars	B-Application
.	O
</s>
<s>
Most	O
phased	O
array	O
radars	B-Application
in	O
the	O
world	O
are	O
PESA	O
.	O
</s>
<s>
A	O
PESA	O
contrasts	O
with	O
an	O
active	B-Architecture
electronically	I-Architecture
scanned	I-Architecture
array	I-Architecture
(	O
AESA	O
)	O
antenna	O
,	O
which	O
has	O
a	O
separate	O
transmitter	O
and/or	O
receiver	O
unit	O
for	O
each	O
antenna	O
element	O
,	O
all	O
controlled	O
by	O
a	O
computer	O
;	O
AESA	O
is	O
a	O
more	O
advanced	O
,	O
sophisticated	O
versatile	O
second-generation	O
version	O
of	O
the	O
original	O
PESA	O
phased	O
array	O
technology	O
.	O
</s>
<s>
Pulsed	O
radar	B-Application
systems	I-Application
work	O
by	O
connecting	O
an	O
antenna	O
to	O
a	O
powerful	O
radio	O
transmitter	O
to	O
emit	O
a	O
short	O
pulse	O
of	O
signal	O
.	O
</s>
<s>
By	O
measuring	O
the	O
time	O
it	O
takes	O
for	O
the	O
signal	O
to	O
return	O
,	O
the	O
radar	B-Application
receiver	O
can	O
determine	O
the	O
distance	O
to	O
the	O
object	O
.	O
</s>
<s>
The	O
receiver	O
then	O
sends	O
the	O
resulting	O
output	O
to	O
a	O
display	B-Algorithm
of	I-Algorithm
some	I-Algorithm
sort	I-Algorithm
.	O
</s>
<s>
The	O
transmitter	O
elements	O
were	O
typically	O
klystron	O
tubes	O
or	O
magnetrons	B-Device
,	O
which	O
are	O
suitable	O
for	O
amplifying	O
or	O
generating	O
a	O
narrow	O
range	O
of	O
frequencies	O
to	O
high	O
power	O
levels	O
.	O
</s>
<s>
To	O
scan	O
a	O
portion	O
of	O
the	O
sky	O
,	O
a	O
non-PESA	O
radar	B-Application
antenna	O
must	O
be	O
physically	O
moved	O
to	O
point	O
in	O
different	O
directions	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
beam	O
of	O
a	O
PESA	O
radar	B-Application
can	O
rapidly	O
be	O
changed	O
to	O
point	O
in	O
a	O
different	O
direction	O
,	O
simply	O
by	O
electrically	O
adjusting	O
the	O
phase	O
differences	O
between	O
different	O
elements	O
of	O
the	O
passive	B-Algorithm
electronically	I-Algorithm
scanned	I-Algorithm
array	I-Algorithm
(	O
PESA	O
)	O
.	O
</s>
<s>
In	O
1959	O
,	O
DARPA	O
developed	O
an	O
experimental	O
phased	O
array	O
radar	B-Application
called	O
Electronically	O
Steered	O
Array	O
Radar	B-Application
ESAR	O
.	O
</s>
<s>
That	O
led	O
to	O
the	O
first	O
practical	O
large-scale	O
passive	B-Algorithm
electronically	I-Algorithm
scanned	I-Algorithm
array	I-Algorithm
,	O
or	O
simply	O
phased	O
array	O
radar	B-Application
.	O
</s>
<s>
Thanks	O
to	O
progress	O
in	O
electronics	O
,	O
PESAs	O
added	O
the	O
ability	O
to	O
produce	O
several	O
active	O
beams	O
,	O
allowing	O
them	O
to	O
continue	O
scanning	O
the	O
sky	O
while	O
at	O
the	O
same	O
time	O
focusing	O
smaller	O
beams	O
on	O
certain	O
targets	O
for	O
tracking	O
or	O
guiding	O
semi-active	O
radar	B-Application
homing	O
missiles	O
.	O
</s>
