<s>
Frequency	B-Algorithm
agility	I-Algorithm
is	O
the	O
ability	O
of	O
a	O
radar	B-Application
system	I-Application
to	O
quickly	O
shift	O
its	O
operating	O
frequency	O
to	O
account	O
for	O
atmospheric	O
effects	O
,	O
jamming	O
,	O
mutual	O
interference	O
with	O
friendly	O
sources	O
,	O
or	O
to	O
make	O
it	O
more	O
difficult	O
to	O
locate	O
the	O
radar	B-Application
broadcaster	O
through	O
radio	B-Device
direction	I-Device
finding	I-Device
.	O
</s>
<s>
The	O
term	O
can	O
also	O
be	O
applied	O
to	O
other	O
fields	O
,	O
including	O
lasers	O
or	O
traditional	O
radio	O
transceivers	O
using	O
frequency-division	B-Protocol
multiplexing	I-Protocol
,	O
but	O
it	O
remains	O
most	O
closely	O
associated	O
with	O
the	O
radar	B-Application
field	O
and	O
these	O
other	O
roles	O
generally	O
use	O
the	O
more	O
generic	O
term	O
"	O
frequency	B-Protocol
hopping	I-Protocol
"	O
.	O
</s>
<s>
Radar	B-Application
systems	I-Application
generally	O
operate	O
by	O
sending	O
out	O
short	O
pulses	O
of	O
radio	O
energy	O
and	O
then	O
turning	O
off	O
the	O
broadcaster	O
and	O
listening	O
for	O
the	O
returning	O
echoes	O
from	O
various	O
objects	O
.	O
</s>
<s>
Due	O
to	O
the	O
size	O
of	O
the	O
tube-based	O
electronics	O
used	O
to	O
construct	O
the	O
transceivers	O
,	O
early	O
radar	B-Application
systems	I-Application
,	O
like	O
those	O
deployed	O
in	O
World	O
War	O
II	O
,	O
were	O
generally	O
limited	O
to	O
operating	O
on	O
a	O
single	O
frequency	O
.	O
</s>
<s>
Knowing	O
this	O
operating	O
frequency	O
gives	O
an	O
adversary	O
enormous	O
power	O
to	O
interfere	O
with	O
radar	B-Application
operation	O
or	O
gather	O
further	O
intelligence	O
.	O
</s>
<s>
The	O
British	O
used	O
the	O
frequency	O
information	O
about	O
the	O
Würzburg	O
radar	B-Application
gathered	O
in	O
Operation	O
Biting	O
to	O
produce	O
"	O
Window	O
"	O
,	O
aluminum	O
foil	O
strips	O
cut	O
to	O
1/2	O
the	O
length	O
of	O
the	O
wavelength	O
of	O
the	O
Würzburg	O
,	O
rendering	O
it	O
almost	O
useless	O
.	O
</s>
<s>
Knowing	O
the	O
frequency	O
of	O
the	O
Würzburg	O
also	O
helped	O
the	O
British	O
in	O
their	O
attempts	O
to	O
locate	O
the	O
systems	O
using	O
radio	B-Device
direction	I-Device
finders	I-Device
,	O
allowing	O
aircraft	O
to	O
be	O
routed	O
around	O
the	O
radars	B-Application
,	O
or	O
at	O
least	O
be	O
kept	O
at	O
longer	O
distances	O
from	O
them	O
.	O
</s>
<s>
A	O
radar	B-Application
system	I-Application
that	O
can	O
operate	O
on	O
several	O
different	O
frequencies	O
makes	O
these	O
countermeasures	O
more	O
difficult	O
to	O
implement	O
.	O
</s>
<s>
To	O
counter	O
this	O
,	O
the	O
jammer	O
has	O
to	O
listen	O
on	O
both	O
frequencies	O
,	O
and	O
broadcast	O
on	O
the	O
one	O
that	O
particular	O
radar	B-Application
is	O
using	O
.	O
</s>
<s>
To	O
further	O
frustrate	O
these	O
efforts	O
,	O
a	O
radar	B-Application
can	O
rapidly	O
switch	O
between	O
the	O
two	O
frequencies	O
.	O
</s>
<s>
In	O
its	O
ultimate	O
incarnation	O
,	O
each	O
radar	B-Application
pulse	O
is	O
sent	O
out	O
on	O
a	O
different	O
frequency	O
and	O
therefore	O
renders	O
single-frequency	O
jamming	O
almost	O
impossible	O
.	O
</s>
<s>
This	O
was	O
the	O
idea	O
behind	O
the	O
AMES	B-Algorithm
Type	O
85	O
radar	B-Application
in	O
the	O
Linesman/Mediator	O
network	O
in	O
the	O
United	O
Kingdom	O
.	O
</s>
<s>
One	O
of	O
the	O
primary	O
reasons	O
that	O
early	O
radars	B-Application
did	O
not	O
use	O
more	O
than	O
one	O
frequency	O
was	O
the	O
size	O
of	O
their	O
tube	O
based	O
electronics	O
.	O
</s>
<s>
"	O
Brute	O
force	O
"	O
frequency	B-Algorithm
agility	I-Algorithm
,	O
like	O
the	O
Linesman	O
,	O
was	O
common	O
on	O
large	O
early	B-Algorithm
warning	I-Algorithm
radars	I-Algorithm
but	O
less	O
common	O
on	O
smaller	O
units	O
where	O
the	O
size	O
of	O
klystrons	O
remained	O
a	O
problem	O
.	O
</s>
<s>
Passive	B-Algorithm
electronically	I-Algorithm
scanned	I-Algorithm
array	I-Algorithm
(	O
PESA	O
)	O
radars	B-Application
,	O
introduced	O
in	O
the	O
1960s	O
,	O
used	O
a	O
single	O
microwave	O
source	O
and	O
a	O
series	O
of	O
delays	O
to	O
drive	O
a	O
large	O
number	O
of	O
antenna	O
elements	O
(	O
the	O
array	O
)	O
and	O
electronically	O
steer	O
the	O
radar	B-Application
beam	O
by	O
changing	O
the	O
delay	O
times	O
slightly	O
.	O
</s>
<s>
Solid-state	O
amplifiers	O
can	O
operate	O
at	O
a	O
wide	O
range	O
of	O
frequencies	O
,	O
unlike	O
a	O
klystron	O
,	O
so	O
solid-state	O
PESAs	O
offered	O
much	O
greater	O
frequency	B-Algorithm
agility	I-Algorithm
,	O
and	O
were	O
much	O
more	O
resistant	O
to	O
jamming	O
.	O
</s>
<s>
The	O
introduction	O
of	O
active	B-Architecture
electronically	I-Architecture
scanned	I-Architecture
arrays	I-Architecture
(	O
AESAs	O
)	O
further	O
evolved	O
this	O
process	O
.	O
</s>
<s>
The	O
radar	B-Application
unit	O
knows	O
which	O
frequencies	O
were	O
broadcast	O
,	O
and	O
amplifies	O
and	O
combines	O
only	O
those	O
return	O
signals	O
,	O
thereby	O
reconstructing	O
a	O
single	O
powerful	O
echo	O
on	O
reception	O
.	O
</s>
<s>
An	O
adversary	O
,	O
unaware	O
of	O
which	O
frequencies	O
are	O
active	O
,	O
has	O
no	O
signal	O
to	O
see	O
,	O
making	O
detection	O
on	O
radar	B-Application
warning	O
receivers	O
extremely	O
difficult	O
.	O
</s>
<s>
Modern	O
radars	B-Application
like	O
the	O
F-35	O
'	O
s	O
AN/APG	B-Device
-81	I-Device
use	O
thousands	O
of	O
broadcaster/receiver	O
modules	O
,	O
one	O
for	O
each	O
antenna	O
element	O
.	O
</s>
<s>
The	O
reason	O
that	O
several	O
cell	O
phones	O
can	O
be	O
used	O
at	O
the	O
same	O
time	O
in	O
the	O
same	O
location	O
is	O
due	O
to	O
the	O
use	O
of	O
frequency	B-Protocol
hopping	I-Protocol
.	O
</s>
<s>
Frequency	B-Algorithm
agile	I-Algorithm
radars	B-Application
can	O
offer	O
the	O
same	O
advantages	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
several	O
aircraft	O
operating	O
in	O
the	O
same	O
location	O
,	O
the	O
radars	B-Application
can	O
select	O
frequencies	O
that	O
are	O
not	O
being	O
used	O
in	O
order	O
to	O
avoid	O
interference	O
.	O
</s>
<s>
This	O
is	O
not	O
as	O
simple	O
as	O
the	O
case	O
of	O
a	O
cell	O
phone	O
,	O
however	O
,	O
because	O
ideally	O
the	O
radars	B-Application
would	O
change	O
their	O
operating	O
frequencies	O
with	O
every	O
pulse	O
.	O
</s>
<s>
Another	O
reason	O
for	O
adding	O
frequency	B-Algorithm
agility	I-Algorithm
has	O
nothing	O
to	O
do	O
with	O
military	O
use	O
;	O
weather	B-Application
radars	I-Application
often	O
have	O
limited	O
agility	O
to	O
allow	O
them	O
to	O
strongly	O
reflect	O
off	O
rain	O
,	O
or	O
alternately	O
,	O
to	O
see	O
through	O
it	O
.	O
</s>
