<s>
Radar	B-Application
is	O
a	O
radiolocation	B-Algorithm
system	O
that	O
uses	O
radio	B-Algorithm
waves	O
to	O
determine	O
the	O
distance	O
(	O
ranging	O
)	O
,	O
angle	O
(	O
azimuth	O
)	O
,	O
and	O
radial	O
velocity	O
of	O
objects	O
relative	O
to	O
the	O
site	O
.	O
</s>
<s>
It	O
is	O
used	O
to	O
detect	O
and	O
track	O
aircraft	O
,	O
ships	B-Algorithm
,	O
spacecraft	O
,	O
guided	O
missiles	O
,	O
and	O
motor	O
vehicles	O
,	O
and	O
map	O
weather	B-Application
formations	I-Application
,	O
and	O
terrain	O
.	O
</s>
<s>
A	O
radar	B-Application
system	I-Application
consists	O
of	O
a	O
transmitter	O
producing	O
electromagnetic	O
waves	O
in	O
the	O
radio	B-Algorithm
or	O
microwaves	O
domain	O
,	O
a	O
transmitting	O
antenna	O
,	O
a	O
receiving	O
antenna	O
(	O
often	O
the	O
same	O
antenna	O
is	O
used	O
for	O
transmitting	O
and	O
receiving	O
)	O
and	O
a	O
receiver	O
and	O
processor	O
to	O
determine	O
properties	O
of	O
the	O
objects	O
.	O
</s>
<s>
Radio	B-Algorithm
waves	O
(	O
pulsed	O
or	O
continuous	O
)	O
from	O
the	O
transmitter	O
reflect	O
off	O
the	O
objects	O
and	O
return	O
to	O
the	O
receiver	O
,	O
giving	O
information	O
about	O
the	O
objects	O
 '	O
locations	O
and	O
speeds	O
.	O
</s>
<s>
Radar	B-Application
was	O
developed	O
secretly	O
for	O
military	O
use	O
by	O
several	O
countries	O
in	O
the	O
period	O
before	O
and	O
during	O
World	O
War	O
II	O
.	O
</s>
<s>
A	O
key	O
development	O
was	O
the	O
cavity	B-Architecture
magnetron	I-Architecture
in	O
the	O
United	O
Kingdom	O
,	O
which	O
allowed	O
the	O
creation	O
of	O
relatively	O
small	O
systems	O
with	O
sub-meter	O
resolution	O
.	O
</s>
<s>
The	O
term	O
RADAR	B-Application
was	O
coined	O
in	O
1940	O
by	O
the	O
United	O
States	O
Navy	O
as	O
an	O
acronym	O
for	O
radio	B-Application
detection	I-Application
and	I-Application
ranging	I-Application
.	O
</s>
<s>
The	O
term	O
radar	B-Application
has	O
since	O
entered	O
English	O
and	O
other	O
languages	O
as	O
a	O
common	O
noun	O
,	O
losing	O
all	O
capitalization	O
.	O
</s>
<s>
The	O
modern	O
uses	O
of	O
radar	B-Application
are	O
highly	O
diverse	O
,	O
including	O
air	O
and	O
terrestrial	O
traffic	O
control	O
,	O
radar	B-Algorithm
astronomy	I-Algorithm
,	O
air-defense	O
systems	O
,	O
anti-missile	O
systems	O
,	O
marine	B-Algorithm
radars	I-Algorithm
to	O
locate	O
landmarks	O
and	O
other	O
ships	B-Algorithm
,	O
aircraft	O
anti-collision	O
systems	O
,	O
ocean	O
surveillance	O
systems	O
,	O
outer	O
space	O
surveillance	O
and	O
rendezvous	O
systems	O
,	O
meteorological	O
precipitation	O
monitoring	O
,	O
altimetry	O
and	O
flight	O
control	O
systems	O
,	O
guided	O
missile	O
target	O
locating	O
systems	O
,	O
self-driving	O
cars	O
,	O
and	O
ground-penetrating	B-Algorithm
radar	I-Algorithm
for	O
geological	O
observations	O
.	O
</s>
<s>
Modern	O
high	O
tech	O
radar	B-Application
systems	I-Application
use	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
and	O
machine	O
learning	O
and	O
are	O
capable	O
of	O
extracting	O
useful	O
information	O
from	O
very	O
high	O
noise	O
levels	O
.	O
</s>
<s>
Other	O
systems	O
similar	O
to	O
radar	B-Application
make	O
use	O
of	O
other	O
parts	O
of	O
the	O
electromagnetic	O
spectrum	O
.	O
</s>
<s>
One	O
example	O
is	O
lidar	O
,	O
which	O
uses	O
predominantly	O
infrared	O
light	O
from	O
lasers	O
rather	O
than	O
radio	B-Algorithm
waves	O
.	O
</s>
<s>
With	O
the	O
emergence	O
of	O
driver-less	O
vehicles	O
,	O
radar	B-Application
is	O
expected	O
to	O
assist	O
the	O
automated	O
platform	O
to	O
monitor	O
its	O
environment	O
,	O
thus	O
preventing	O
unwanted	O
incidents	O
.	O
</s>
<s>
As	O
early	O
as	O
1886	O
,	O
German	O
physicist	O
Heinrich	O
Hertz	O
showed	O
that	O
radio	B-Algorithm
waves	O
could	O
be	O
reflected	O
from	O
solid	O
objects	O
.	O
</s>
<s>
In	O
1895	O
,	O
Alexander	O
Popov	O
,	O
a	O
physics	O
instructor	O
at	O
the	O
Imperial	O
Russian	O
Navy	O
school	O
in	O
Kronstadt	O
,	O
developed	O
an	O
apparatus	O
using	O
a	O
coherer	O
tube	O
for	O
detecting	O
distant	O
lightning	B-Architecture
strikes	O
.	O
</s>
<s>
In	O
1897	O
,	O
while	O
testing	O
this	O
equipment	O
for	O
communicating	O
between	O
two	O
ships	B-Algorithm
in	O
the	O
Baltic	O
Sea	O
,	O
he	O
took	O
note	O
of	O
an	O
interference	B-Application
beat	I-Application
caused	O
by	O
the	O
passage	O
of	O
a	O
third	O
vessel	O
.	O
</s>
<s>
The	O
German	O
inventor	O
Christian	O
Hülsmeyer	O
was	O
the	O
first	O
to	O
use	O
radio	B-Algorithm
waves	O
to	O
detect	O
"	O
the	O
presence	O
of	O
distant	O
metallic	O
objects	O
"	O
.	O
</s>
<s>
He	O
also	O
obtained	O
a	O
British	O
patent	O
on	O
23	O
September	O
1904	O
for	O
a	O
full	O
radar	B-Application
system	I-Application
,	O
that	O
he	O
called	O
a	O
telemobiloscope	O
.	O
</s>
<s>
It	O
operated	O
on	O
a	O
50cm	O
wavelength	O
and	O
the	O
pulsed	B-Algorithm
radar	I-Algorithm
signal	O
was	O
created	O
via	O
a	O
spark-gap	O
.	O
</s>
<s>
In	O
1915	O
,	O
Robert	O
Watson-Watt	O
used	O
radio	B-Algorithm
technology	O
to	O
provide	O
advance	O
warning	O
to	O
airmen	O
and	O
during	O
the	O
1920s	O
went	O
on	O
to	O
lead	O
the	O
U.K.	O
research	O
establishment	O
to	O
make	O
many	O
advances	O
using	O
radio	B-Algorithm
techniques	O
,	O
including	O
the	O
probing	O
of	O
the	O
ionosphere	O
and	O
the	O
detection	O
of	O
lightning	B-Architecture
at	O
long	O
distances	O
.	O
</s>
<s>
Through	O
his	O
lightning	B-Architecture
experiments	O
,	O
Watson-Watt	O
became	O
an	O
expert	O
on	O
the	O
use	O
of	O
radio	B-Device
direction	I-Device
finding	I-Device
before	O
turning	O
his	O
inquiry	O
to	O
shortwave	O
transmission	O
.	O
</s>
<s>
Wilkins	O
would	O
select	O
a	O
General	O
Post	O
Office	O
model	O
after	O
noting	O
its	O
manual	O
's	O
description	O
of	O
a	O
"	O
fading	O
"	O
effect	O
(	O
the	O
common	O
term	O
for	O
interference	O
at	O
the	O
time	O
)	O
when	O
aircraft	O
flew	O
overhead	O
.	O
</s>
<s>
Across	O
the	O
Atlantic	O
in	O
1922	O
,	O
after	O
placing	O
a	O
transmitter	O
and	O
receiver	O
on	O
opposite	O
sides	O
of	O
the	O
Potomac	O
River	O
,	O
U.S.	O
Navy	O
researchers	O
A	O
.	O
Hoyt	O
Taylor	O
and	O
Leo	O
C	B-Algorithm
.	O
Young	O
discovered	O
that	O
ships	B-Algorithm
passing	O
through	O
the	O
beam	O
path	O
caused	O
the	O
received	O
signal	O
to	O
fade	O
in	O
and	O
out	O
.	O
</s>
<s>
Taylor	O
submitted	O
a	O
report	O
,	O
suggesting	O
that	O
this	O
phenomenon	O
might	O
be	O
used	O
to	O
detect	O
the	O
presence	O
of	O
ships	B-Algorithm
in	O
low	O
visibility	O
,	O
but	O
the	O
Navy	O
did	O
not	O
immediately	O
continue	O
the	O
work	O
.	O
</s>
<s>
Eight	O
years	O
later	O
,	O
Lawrence	O
A	O
.	O
Hyland	O
at	O
the	O
Naval	O
Research	O
Laboratory	O
(	O
NRL	O
)	O
observed	O
similar	O
fading	O
effects	O
from	O
passing	O
aircraft	O
;	O
this	O
revelation	O
led	O
to	O
a	O
patent	O
application	O
as	O
well	O
as	O
a	O
proposal	O
for	O
further	O
intensive	O
research	O
on	O
radio-echo	O
signals	O
from	O
moving	O
targets	O
to	O
take	O
place	O
at	O
NRL	O
,	O
where	O
Taylor	O
and	O
Young	O
were	O
based	O
at	O
the	O
time	O
.	O
</s>
<s>
Similarly	O
,	O
in	O
the	O
UK	O
,	O
L	B-Algorithm
.	O
S	B-Algorithm
.	O
Alder	O
took	O
out	O
a	O
secret	O
provisional	O
patent	O
for	O
Naval	O
radar	B-Application
in	O
1928	O
.	O
</s>
<s>
W.A.S.	O
</s>
<s>
Butement	O
and	O
P	O
.	O
E	O
.	O
Pollard	O
developed	O
a	O
breadboard	O
test	O
unit	O
,	O
operating	O
at	O
50	O
cm	O
(	O
600	O
MHz	O
)	O
and	O
using	O
pulsed	O
modulation	B-Application
which	O
gave	O
successful	O
laboratory	O
results	O
.	O
</s>
<s>
Before	O
the	O
Second	O
World	O
War	O
,	O
researchers	O
in	O
the	O
United	O
Kingdom	O
,	O
France	O
,	O
Germany	O
,	O
Italy	O
,	O
Japan	O
,	O
the	O
Netherlands	O
,	O
the	O
Soviet	O
Union	O
,	O
and	O
the	O
United	O
States	O
,	O
independently	O
and	O
in	O
great	O
secrecy	O
,	O
developed	O
technologies	O
that	O
led	O
to	O
the	O
modern	O
version	O
of	O
radar	B-Application
.	O
</s>
<s>
Australia	O
,	O
Canada	O
,	O
New	O
Zealand	O
,	O
and	O
South	O
Africa	O
followed	O
prewar	O
Great	O
Britain	O
's	O
radar	B-Application
development	O
,	O
and	O
Hungary	O
generated	O
its	O
radar	B-Application
technology	O
during	O
the	O
war	O
.	O
</s>
<s>
In	O
France	O
in	O
1934	O
,	O
following	O
systematic	O
studies	O
on	O
the	O
split-anode	O
magnetron	B-Architecture
,	O
the	O
research	O
branch	O
of	O
the	O
Compagnie	O
générale	O
de	O
la	O
télégraphie	O
sans	O
fil	O
(	O
CSF	O
)	O
headed	O
by	O
Maurice	O
Ponte	O
with	O
Henri	O
Gutton	O
,	O
Sylvain	O
Berline	O
and	O
M	O
.	O
Hugon	O
,	O
began	O
developing	O
an	O
obstacle-locating	O
radio	B-Algorithm
apparatus	O
,	O
aspects	O
of	O
which	O
were	O
installed	O
on	O
the	O
ocean	O
liner	O
Normandie	O
in	O
1935	O
.	O
</s>
<s>
During	O
the	O
same	O
period	O
,	O
Soviet	O
military	O
engineer	O
P.K.	O
</s>
<s>
The	O
Soviets	O
produced	O
their	O
first	O
mass	O
production	O
radars	B-Application
RUS-1	O
and	O
RUS-2	O
Redut	O
in	O
1939	O
but	O
further	O
development	O
was	O
slowed	O
following	O
the	O
arrest	O
of	O
Oshchepkov	O
and	O
his	O
subsequent	O
gulag	O
sentence	O
.	O
</s>
<s>
The	O
first	O
Russian	O
airborne	O
radar	B-Application
,	O
Gneiss-2	O
,	O
entered	O
into	O
service	O
in	O
June	O
1943	O
on	O
Pe-2	O
dive	O
bombers	O
.	O
</s>
<s>
The	O
French	O
and	O
Soviet	O
systems	O
,	O
however	O
,	O
featured	O
continuous-wave	O
operation	O
that	O
did	O
not	O
provide	O
the	O
full	O
performance	O
ultimately	O
synonymous	O
with	O
modern	O
radar	B-Application
systems	I-Application
.	O
</s>
<s>
Full	O
radar	B-Application
evolved	O
as	O
a	O
pulsed	O
system	O
,	O
and	O
the	O
first	O
such	O
elementary	O
apparatus	O
was	O
demonstrated	O
in	O
December	O
1934	O
by	O
the	O
American	O
Robert	O
M	O
.	O
Page	O
,	O
working	O
at	O
the	O
Naval	O
Research	O
Laboratory	O
.	O
</s>
<s>
The	O
following	O
year	O
,	O
the	O
United	O
States	O
Army	O
successfully	O
tested	O
a	O
primitive	O
surface-to-surface	O
radar	B-Application
to	O
aim	O
coastal	O
battery	O
searchlights	O
at	O
night	O
.	O
</s>
<s>
This	O
design	O
was	O
followed	O
by	O
a	O
pulsed	O
system	O
demonstrated	O
in	O
May	O
1935	O
by	O
Rudolf	O
Kühnhold	O
and	O
the	O
firm	O
in	O
Germany	O
and	O
then	O
another	O
in	O
June	O
1935	O
by	O
an	O
Air	O
Ministry	O
team	O
led	O
by	O
Robert	O
Watson-Watt	O
in	O
Great	O
Britain	O
.	O
</s>
<s>
In	O
1935	O
,	O
Watson-Watt	O
was	O
asked	O
to	O
judge	O
recent	O
reports	O
of	O
a	O
German	O
radio-based	O
death	O
ray	O
and	O
turned	O
the	O
request	O
over	O
to	O
Wilkins	O
.	O
</s>
<s>
When	O
Watson-Watt	O
then	O
asked	O
what	O
such	O
a	O
system	O
might	O
do	O
,	O
Wilkins	O
recalled	O
the	O
earlier	O
report	O
about	O
aircraft	O
causing	O
radio	B-Algorithm
interference	O
.	O
</s>
<s>
When	O
the	O
plane	O
was	O
clearly	O
detected	O
,	O
Hugh	O
Dowding	O
,	O
the	O
Air	O
Member	O
for	O
Supply	O
and	O
Research	O
was	O
very	O
impressed	O
with	O
their	O
system	O
's	O
potential	O
and	O
funds	O
were	O
immediately	O
provided	O
for	O
further	O
operational	O
development	O
.	O
</s>
<s>
Watson-Watt	O
'	O
s	B-Algorithm
team	O
patented	O
the	O
device	O
in	O
GB593017	O
.	O
</s>
<s>
Development	O
of	O
radar	B-Application
greatly	O
expanded	O
on	O
1	O
September	O
1936	O
,	O
when	O
Watson-Watt	O
became	O
superintendent	O
of	O
a	O
new	O
establishment	O
under	O
the	O
British	O
Air	O
Ministry	O
,	O
Bawdsey	O
Research	O
Station	O
located	O
in	O
Bawdsey	O
Manor	O
,	O
near	O
Felixstowe	O
,	O
Suffolk	O
.	O
</s>
<s>
This	O
system	O
provided	O
the	O
vital	O
advance	O
information	O
that	O
helped	O
the	O
Royal	O
Air	O
Force	O
win	O
the	O
Battle	B-Application
of	I-Application
Britain	I-Application
;	O
without	O
it	O
,	O
significant	O
numbers	O
of	O
fighter	O
aircraft	O
,	O
which	O
Great	O
Britain	O
did	O
not	O
have	O
available	O
,	O
would	O
always	O
have	O
needed	O
to	O
be	O
in	O
the	O
air	O
to	O
respond	O
quickly	O
.	O
</s>
<s>
If	O
German-aircraft	O
detection	O
had	O
relied	O
solely	O
on	O
the	O
observations	O
of	O
ground-based	O
individuals	O
,	O
Great	O
Britain	O
might	O
have	O
lost	O
the	O
Battle	B-Application
of	I-Application
Britain	I-Application
.	O
</s>
<s>
The	O
radar	B-Application
formed	O
part	O
of	O
the	O
"	O
Dowding	O
system	O
"	O
for	O
collecting	O
reports	O
of	O
enemy	O
aircraft	O
and	O
coordinating	O
the	O
response	O
.	O
</s>
<s>
Given	O
all	O
required	O
funding	O
and	O
development	O
support	O
,	O
the	O
team	O
produced	O
working	O
radar	B-Application
systems	I-Application
in	O
1935	O
and	O
began	O
deployment	O
.	O
</s>
<s>
Even	O
by	O
standards	O
of	O
the	O
era	O
,	O
CH	O
was	O
crude	O
;	O
instead	O
of	O
broadcasting	B-Operating_System
and	O
receiving	O
from	O
an	O
aimed	O
antenna	O
,	O
CH	O
broadcast	B-Operating_System
a	O
signal	O
floodlighting	O
the	O
entire	O
area	O
in	O
front	O
of	O
it	O
,	O
and	O
then	O
used	O
one	O
of	O
Watson-Watt	O
'	O
s	B-Algorithm
own	O
radio	B-Device
direction	I-Device
finders	I-Device
to	O
determine	O
the	O
direction	O
of	O
the	O
returned	O
echoes	O
.	O
</s>
<s>
A	O
key	O
development	O
was	O
the	O
cavity	B-Architecture
magnetron	I-Architecture
in	O
the	O
UK	O
,	O
which	O
allowed	O
the	O
creation	O
of	O
relatively	O
small	O
systems	O
with	O
sub-meter	O
resolution	O
.	O
</s>
<s>
Britain	O
shared	O
the	O
technology	O
with	O
the	O
U.S.	O
during	O
the	O
1940	O
Tizard	O
Mission	O
.	O
</s>
<s>
In	O
April	O
1940	O
,	O
Popular	O
Science	O
showed	O
an	O
example	O
of	O
a	O
radar	B-Application
unit	O
using	O
the	O
Watson-Watt	O
patent	O
in	O
an	O
article	O
on	O
air	O
defence	O
.	O
</s>
<s>
Also	O
,	O
in	O
late	O
1941	O
Popular	O
Mechanics	O
had	O
an	O
article	O
in	O
which	O
a	O
U.S.	O
scientist	O
speculated	O
about	O
the	O
British	O
early	O
warning	O
system	O
on	O
the	O
English	O
east	O
coast	O
and	O
came	O
close	O
to	O
what	O
it	O
was	O
and	O
how	O
it	O
worked	O
.	O
</s>
<s>
Watson-Watt	O
was	O
sent	O
to	O
the	O
U.S.	O
in	O
1941	O
to	O
advise	O
on	O
air	O
defense	O
after	O
Japan	O
's	O
attack	B-Application
on	I-Application
Pearl	I-Application
Harbor	I-Application
.	O
</s>
<s>
Alfred	O
Lee	O
Loomis	O
organized	O
the	O
secret	O
MIT	O
Radiation	O
Laboratory	O
at	O
Massachusetts	O
Institute	O
of	O
Technology	O
,	O
Cambridge	O
,	O
Massachusetts	O
which	O
developed	O
microwave	B-Application
radar	I-Application
technology	O
in	O
the	O
years	O
1941	O
–	O
45	O
.	O
</s>
<s>
Later	O
,	O
in	O
1943	O
,	O
Page	O
greatly	O
improved	O
radar	B-Application
with	O
the	O
monopulse	B-Algorithm
technique	I-Algorithm
that	O
was	O
used	O
for	O
many	O
years	O
in	O
most	O
radar	B-Application
applications	O
.	O
</s>
<s>
The	O
war	O
precipitated	O
research	O
to	O
find	O
better	O
resolution	O
,	O
more	O
portability	O
,	O
and	O
more	O
features	O
for	O
radar	B-Application
,	O
including	O
complementary	O
navigation	O
systems	O
like	O
Oboe	O
used	O
by	O
the	O
RAF	O
's	O
Pathfinder	O
.	O
</s>
<s>
The	O
information	O
provided	O
by	O
radar	B-Application
includes	O
the	O
bearing	O
and	O
range	O
(	O
and	O
therefore	O
position	O
)	O
of	O
the	O
object	O
from	O
the	O
radar	B-Application
scanner	O
.	O
</s>
<s>
The	O
first	O
use	O
of	O
radar	B-Application
was	O
for	O
military	O
purposes	O
:	O
to	O
locate	O
air	O
,	O
ground	O
and	O
sea	O
targets	O
.	O
</s>
<s>
This	O
evolved	O
in	O
the	O
civilian	O
field	O
into	O
applications	O
for	O
aircraft	O
,	O
ships	B-Algorithm
,	O
and	O
automobiles	O
.	O
</s>
<s>
In	O
aviation	O
,	O
aircraft	O
can	O
be	O
equipped	O
with	O
radar	B-Application
devices	O
that	O
warn	O
of	O
aircraft	O
or	O
other	O
obstacles	O
in	O
or	O
approaching	O
their	O
path	O
,	O
display	O
weather	O
information	O
,	O
and	O
give	O
accurate	O
altitude	O
readings	O
.	O
</s>
<s>
Aircraft	O
can	O
land	O
in	O
fog	O
at	O
airports	O
equipped	O
with	O
radar-assisted	O
ground-controlled	O
approach	O
systems	O
in	O
which	O
the	O
plane	O
's	O
position	O
is	O
observed	O
on	O
precision	B-Algorithm
approach	I-Algorithm
radar	I-Algorithm
screens	O
by	O
operators	O
who	O
thereby	O
give	O
radio	B-Algorithm
landing	O
instructions	O
to	O
the	O
pilot	O
,	O
maintaining	O
the	O
aircraft	O
on	O
a	O
defined	O
approach	O
path	O
to	O
the	O
runway	O
.	O
</s>
<s>
Military	O
fighter	O
aircraft	O
are	O
usually	O
fitted	O
with	O
air-to-air	O
targeting	O
radars	B-Application
,	O
to	O
detect	O
and	O
target	O
enemy	O
aircraft	O
.	O
</s>
<s>
In	O
addition	O
,	O
larger	O
specialized	O
military	O
aircraft	O
carry	O
powerful	O
airborne	O
radars	B-Application
to	O
observe	O
air	B-Application
traffic	I-Application
over	O
a	O
wide	O
region	O
and	O
direct	O
fighter	O
aircraft	O
towards	O
targets	O
.	O
</s>
<s>
Marine	B-Algorithm
radars	I-Algorithm
are	O
used	O
to	O
measure	O
the	O
bearing	O
and	O
distance	O
of	O
ships	B-Algorithm
to	O
prevent	O
collision	O
with	O
other	O
ships	B-Algorithm
,	O
to	O
navigate	O
,	O
and	O
to	O
fix	O
their	O
position	O
at	O
sea	O
when	O
within	O
range	O
of	O
shore	O
or	O
other	O
fixed	O
references	O
such	O
as	O
islands	O
,	O
buoys	O
,	O
and	O
lightships	O
.	O
</s>
<s>
In	O
port	O
or	O
in	O
harbour	O
,	O
vessel	O
traffic	O
service	O
radar	B-Application
systems	I-Application
are	O
used	O
to	O
monitor	O
and	O
regulate	O
ship	O
movements	O
in	O
busy	O
waters	O
.	O
</s>
<s>
Meteorologists	O
use	O
radar	B-Application
to	O
monitor	O
precipitation	O
and	O
wind	O
.	O
</s>
<s>
It	O
has	O
become	O
the	O
primary	O
tool	O
for	O
short-term	O
weather	O
forecasting	O
and	O
watching	O
for	O
severe	O
weather	O
such	O
as	O
thunderstorms	O
,	O
tornadoes	B-Language
,	O
winter	O
storms	O
,	O
precipitation	O
types	O
,	O
etc	O
.	O
</s>
<s>
Geologists	O
use	O
specialized	O
ground-penetrating	B-Algorithm
radars	I-Algorithm
to	O
map	O
the	O
composition	O
of	O
Earth	O
's	O
crust	O
.	O
</s>
<s>
Police	O
forces	O
use	O
radar	B-Application
guns	O
to	O
monitor	O
vehicle	O
speeds	O
on	O
the	O
roads	O
.	O
</s>
<s>
Smaller	O
radar	B-Application
systems	I-Application
are	O
used	O
to	O
detect	O
human	O
movement	O
.	O
</s>
<s>
Examples	O
are	O
breathing	O
pattern	O
detection	O
for	O
sleep	O
monitoring	O
and	O
hand	O
and	O
finger	O
gesture	B-General_Concept
detection	I-General_Concept
for	O
computer	O
interaction	O
.	O
</s>
<s>
A	O
radar	B-Application
system	I-Application
has	O
a	O
transmitter	O
that	O
emits	O
radio	B-Algorithm
waves	O
known	O
as	O
radar	B-Application
signals	O
in	O
predetermined	O
directions	O
.	O
</s>
<s>
Radar	B-Application
signals	O
are	O
reflected	O
especially	O
well	O
by	O
materials	O
of	O
considerable	O
electrical	O
conductivity	O
—	O
such	O
as	O
most	O
metals	O
,	O
seawater	O
,	O
and	O
wet	O
ground	O
.	O
</s>
<s>
This	O
makes	O
the	O
use	O
of	O
radar	B-Application
altimeters	O
possible	O
in	O
certain	O
cases	O
.	O
</s>
<s>
The	O
radar	B-Application
signals	O
that	O
are	O
reflected	O
back	O
towards	O
the	O
radar	B-Application
receiver	O
are	O
the	O
desirable	O
ones	O
that	O
make	O
radar	B-Application
detection	O
work	O
.	O
</s>
<s>
If	O
the	O
object	O
is	O
moving	O
either	O
toward	O
or	O
away	O
from	O
the	O
transmitter	O
,	O
there	O
will	O
be	O
a	O
slight	O
change	O
in	O
the	O
frequency	O
of	O
the	O
radio	B-Algorithm
waves	O
due	O
to	O
the	O
Doppler	O
effect	O
.	O
</s>
<s>
Radar	B-Application
receivers	O
are	O
usually	O
,	O
but	O
not	O
always	O
,	O
in	O
the	O
same	O
location	O
as	O
the	O
transmitter	O
.	O
</s>
<s>
The	O
reflected	O
radar	B-Application
signals	O
captured	O
by	O
the	O
receiving	O
antenna	O
are	O
usually	O
very	O
weak	O
.	O
</s>
<s>
More	O
sophisticated	O
methods	O
of	O
signal	O
processing	O
are	O
also	O
used	O
in	O
order	O
to	O
recover	O
useful	O
radar	B-Application
signals	O
.	O
</s>
<s>
The	O
weak	O
absorption	O
of	O
radio	B-Algorithm
waves	O
by	O
the	O
medium	O
through	O
which	O
they	O
pass	O
is	O
what	O
enables	O
radar	B-Application
sets	O
to	O
detect	O
objects	O
at	O
relatively	O
long	O
ranges	O
—	O
ranges	O
at	O
which	O
other	O
electromagnetic	O
wavelengths	O
,	O
such	O
as	O
visible	O
light	O
,	O
infrared	O
light	O
,	O
and	O
ultraviolet	O
light	O
,	O
are	O
too	O
strongly	O
attenuated	O
.	O
</s>
<s>
Weather	O
phenomena	O
,	O
such	O
as	O
fog	O
,	O
clouds	O
,	O
rain	O
,	O
falling	O
snow	O
,	O
and	O
sleet	O
,	O
that	O
block	O
visible	O
light	O
are	O
usually	O
transparent	O
to	O
radio	B-Algorithm
waves	O
.	O
</s>
<s>
Certain	O
radio	B-Algorithm
frequencies	O
that	O
are	O
absorbed	O
or	O
scattered	O
by	O
water	O
vapour	O
,	O
raindrops	O
,	O
or	O
atmospheric	O
gases	O
(	O
especially	O
oxygen	O
)	O
are	O
avoided	O
when	O
designing	O
radars	B-Application
,	O
except	O
when	O
their	O
detection	O
is	O
intended	O
.	O
</s>
<s>
Radar	B-Application
relies	O
on	O
its	O
own	O
transmissions	O
rather	O
than	O
light	O
from	O
the	O
Sun	O
or	O
the	O
Moon	O
,	O
or	O
from	O
electromagnetic	O
waves	O
emitted	O
by	O
the	O
target	O
objects	O
themselves	O
,	O
such	O
as	O
infrared	O
radiation	O
(	O
heat	O
)	O
.	O
</s>
<s>
This	O
process	O
of	O
directing	O
artificial	O
radio	B-Algorithm
waves	O
towards	O
objects	O
is	O
called	O
illumination	O
,	O
although	O
radio	B-Algorithm
waves	O
are	O
invisible	O
to	O
the	O
human	O
eye	O
as	O
well	O
as	O
optical	O
cameras	O
.	O
</s>
<s>
This	O
means	O
that	O
a	O
solid	O
object	O
in	O
air	O
or	O
in	O
a	O
vacuum	B-General_Concept
,	O
or	O
a	O
significant	O
change	O
in	O
atomic	O
density	O
between	O
the	O
object	O
and	O
what	O
is	O
surrounding	O
it	O
,	O
will	O
usually	O
scatter	O
radar	B-Application
(	O
radio	B-Algorithm
)	O
waves	O
from	O
its	O
surface	O
.	O
</s>
<s>
This	O
is	O
particularly	O
true	O
for	O
electrically	O
conductive	O
materials	O
such	O
as	O
metal	O
and	O
carbon	O
fibre	O
,	O
making	O
radar	B-Application
well-suited	O
to	O
the	O
detection	O
of	O
aircraft	O
and	O
ships	B-Algorithm
.	O
</s>
<s>
Radar	B-Application
absorbing	O
material	O
,	O
containing	O
resistive	O
and	O
sometimes	O
magnetic	O
substances	O
,	O
is	O
used	O
on	O
military	O
vehicles	O
to	O
reduce	O
radar	B-Application
reflection	O
.	O
</s>
<s>
This	O
is	O
the	O
radio	B-Algorithm
equivalent	O
of	O
painting	O
something	O
a	O
dark	O
colour	O
so	O
that	O
it	O
cannot	O
be	O
seen	O
by	O
the	O
eye	O
at	O
night	O
.	O
</s>
<s>
Radar	B-Application
waves	O
scatter	O
in	O
a	O
variety	O
of	O
ways	O
depending	O
on	O
the	O
size	O
(	O
wavelength	O
)	O
of	O
the	O
radio	B-Algorithm
wave	O
and	O
the	O
shape	O
of	O
the	O
target	O
.	O
</s>
<s>
If	O
the	O
wavelength	O
is	O
much	O
shorter	O
than	O
the	O
target	O
's	O
size	O
,	O
the	O
wave	O
will	O
bounce	O
off	O
in	O
a	O
way	O
similar	O
to	O
the	O
way	O
light	O
is	O
reflected	O
by	O
a	O
mirror	O
.	O
</s>
<s>
Low-frequency	O
radar	B-Application
technology	O
is	O
dependent	O
on	O
resonances	B-Application
for	O
detection	O
,	O
but	O
not	O
identification	O
,	O
of	O
targets	O
.	O
</s>
<s>
This	O
is	O
described	O
by	O
Rayleigh	O
scattering	O
,	O
an	O
effect	O
that	O
creates	O
Earth	O
's	O
blue	O
sky	O
and	O
red	O
sunsets	O
.	O
</s>
<s>
When	O
the	O
two	O
length	O
scales	O
are	O
comparable	O
,	O
there	O
may	O
be	O
resonances	B-Application
.	O
</s>
<s>
Early	O
radars	B-Application
used	O
very	O
long	O
wavelengths	O
that	O
were	O
larger	O
than	O
the	O
targets	O
and	O
thus	O
received	O
a	O
vague	O
signal	O
,	O
whereas	O
many	O
modern	O
systems	O
use	O
shorter	O
wavelengths	O
(	O
a	O
few	O
centimetres	O
or	O
less	O
)	O
that	O
can	O
image	O
objects	O
as	O
small	O
as	O
a	O
loaf	O
of	O
bread	O
.	O
</s>
<s>
Short	O
radio	B-Algorithm
waves	O
reflect	O
from	O
curves	O
and	O
corners	O
in	O
a	O
way	O
similar	O
to	O
glint	O
from	O
a	O
rounded	O
piece	O
of	O
glass	O
.	O
</s>
<s>
A	O
corner	B-Algorithm
reflector	I-Algorithm
consists	O
of	O
three	O
flat	O
surfaces	O
meeting	O
like	O
the	O
inside	O
corner	O
of	O
a	O
cube	O
.	O
</s>
<s>
They	O
are	O
commonly	O
used	O
as	O
radar	B-Algorithm
reflectors	I-Algorithm
to	O
make	O
otherwise	O
difficult-to-detect	O
objects	O
easier	O
to	O
detect	O
.	O
</s>
<s>
Corner	B-Algorithm
reflectors	I-Algorithm
on	O
boats	O
,	O
for	O
example	O
,	O
make	O
them	O
more	O
detectable	O
to	O
avoid	O
collision	O
or	O
during	O
a	O
rescue	O
.	O
</s>
<s>
The	O
extent	O
to	O
which	O
an	O
object	O
reflects	O
or	O
scatters	O
radio	B-Algorithm
waves	O
is	O
called	O
its	O
radar	B-Application
cross	O
section	O
.	O
</s>
<s>
Additional	O
filtering	O
and	O
pulse	O
integration	O
modifies	O
the	O
radar	B-Application
equation	O
slightly	O
for	O
pulse-Doppler	B-Algorithm
radar	I-Algorithm
performance	O
,	O
which	O
can	O
be	O
used	O
to	O
increase	O
detection	O
range	O
and	O
reduce	O
transmit	O
power	O
.	O
</s>
<s>
The	O
equation	O
above	O
with	O
F	O
=	O
1	O
is	O
a	O
simplification	O
for	O
transmission	O
in	O
a	O
vacuum	B-General_Concept
without	O
interference	O
.	O
</s>
<s>
Frequency	O
shift	O
is	O
caused	O
by	O
motion	O
that	O
changes	O
the	O
number	O
of	O
wavelengths	O
between	O
the	O
reflector	O
and	O
the	O
radar	B-Application
.	O
</s>
<s>
This	O
can	O
degrade	O
or	O
enhance	O
radar	B-Application
performance	O
depending	O
upon	O
how	O
it	O
affects	O
the	O
detection	O
process	O
.	O
</s>
<s>
Sea-based	O
radar	B-Application
systems	I-Application
,	O
semi-active	O
radar	B-Application
homing	O
,	O
active	O
radar	B-Application
homing	O
,	O
weather	B-Application
radar	I-Application
,	O
military	O
aircraft	O
,	O
and	O
radar	B-Algorithm
astronomy	I-Algorithm
rely	O
on	O
the	O
Doppler	O
effect	O
to	O
enhance	O
performance	O
.	O
</s>
<s>
Doppler	O
shift	O
depends	O
upon	O
whether	O
the	O
radar	B-Application
configuration	O
is	O
active	O
or	O
passive	O
.	O
</s>
<s>
Active	O
radar	B-Application
transmits	O
a	O
signal	O
that	O
is	O
reflected	O
back	O
to	O
the	O
receiver	O
.	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
depends	O
upon	O
the	O
object	O
sending	O
a	O
signal	O
to	O
the	O
receiver	O
.	O
</s>
<s>
The	O
Doppler	O
frequency	O
shift	O
for	O
active	O
radar	B-Application
is	O
as	O
follows	O
,	O
where	O
is	O
Doppler	O
frequency	O
,	O
is	O
transmit	O
frequency	O
,	O
is	O
radial	O
velocity	O
,	O
and	O
is	O
the	O
speed	O
of	O
light	O
:	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
is	O
applicable	O
to	O
electronic	O
countermeasures	O
and	O
radio	B-Algorithm
astronomy	O
as	O
follows	O
:	O
</s>
<s>
When	O
the	O
reflector	O
is	O
moving	O
at	O
right	O
angle	O
to	O
the	O
radar	B-Application
beam	O
,	O
it	O
has	O
no	O
relative	O
velocity	O
.	O
</s>
<s>
Vehicles	O
and	O
weather	O
moving	O
parallel	O
to	O
the	O
radar	B-Application
beam	O
produce	O
the	O
maximum	O
Doppler	O
frequency	O
shift	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
a	O
Doppler	B-Application
weather	I-Application
radar	I-Application
with	O
a	O
pulse	O
rate	O
of	O
2kHz	O
and	O
transmit	O
frequency	O
of	O
1GHz	O
can	O
reliably	O
measure	O
weather	O
speed	O
up	O
to	O
at	O
most	O
,	O
thus	O
cannot	O
reliably	O
determine	O
radial	O
velocity	O
of	O
aircraft	O
moving	O
.	O
</s>
<s>
For	O
a	O
transmitted	O
radar	B-Application
signal	O
,	O
the	O
polarization	O
can	O
be	O
controlled	O
to	O
yield	O
different	O
effects	O
.	O
</s>
<s>
Radars	B-Application
use	O
horizontal	O
,	O
vertical	O
,	O
linear	O
,	O
and	O
circular	O
polarization	O
to	O
detect	O
different	O
types	O
of	O
reflections	O
.	O
</s>
<s>
Random	O
polarization	O
returns	O
usually	O
indicate	O
a	O
fractal	B-Application
surface	O
,	O
such	O
as	O
rocks	O
or	O
soil	O
,	O
and	O
are	O
used	O
by	O
navigation	O
radars	B-Application
.	O
</s>
<s>
A	O
radar	B-Application
beam	O
follows	O
a	O
linear	O
path	O
in	O
vacuum	B-General_Concept
but	O
follows	O
a	O
somewhat	O
curved	O
path	O
in	O
atmosphere	O
due	O
to	O
variation	O
in	O
the	O
refractive	O
index	O
of	O
air	O
,	O
which	O
is	O
called	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
.	O
</s>
<s>
The	O
maximum	O
range	O
of	O
conventional	O
radar	B-Application
can	O
be	O
limited	O
by	O
a	O
number	O
of	O
factors	O
:	O
</s>
<s>
Radar	B-Application
sensitivity	O
and	O
the	O
power	O
of	O
the	O
return	O
signal	O
as	O
computed	O
in	O
the	O
radar	B-Application
equation	O
.	O
</s>
<s>
This	O
component	O
includes	O
factors	O
such	O
as	O
the	O
environmental	O
conditions	O
and	O
the	O
size	O
(	O
or	O
radar	B-Application
cross	O
section	O
)	O
of	O
the	O
target	O
.	O
</s>
<s>
Reflected	O
signals	O
decline	O
rapidly	O
as	O
distance	O
increases	O
,	O
so	O
noise	O
introduces	O
a	O
radar	B-Application
range	O
limitation	O
.	O
</s>
<s>
Noise	O
typically	O
appears	O
as	O
random	O
variations	O
superimposed	O
on	O
the	O
desired	O
echo	O
signal	O
received	O
in	O
the	O
radar	B-Application
receiver	O
.	O
</s>
<s>
There	O
will	O
also	O
be	O
flicker	O
noise	O
caused	O
by	O
electron	O
transit	O
through	O
amplification	O
devices	O
,	O
which	O
is	O
reduced	O
using	O
heterodyne	B-Algorithm
amplification	O
.	O
</s>
<s>
Another	O
reason	O
for	O
heterodyne	B-Algorithm
processing	O
is	O
that	O
for	O
fixed	O
fractional	O
bandwidth	O
,	O
the	O
instantaneous	O
bandwidth	O
increases	O
linearly	O
in	O
frequency	O
.	O
</s>
<s>
The	O
one	O
notable	O
exception	O
to	O
heterodyne	B-Algorithm
(	O
downconversion	O
)	O
radar	B-Application
systems	I-Application
is	O
ultra-wideband	O
radar	B-Application
.	O
</s>
<s>
In	O
modern	O
radar	B-Application
systems	I-Application
,	O
the	O
internal	O
noise	O
is	O
typically	O
about	O
equal	O
to	O
or	O
lower	O
than	O
the	O
external	O
noise	O
.	O
</s>
<s>
An	O
exception	O
is	O
if	O
the	O
radar	B-Application
is	O
aimed	O
upwards	O
at	O
clear	O
sky	O
,	O
where	O
the	O
scene	O
is	O
so	O
"	O
cold	O
"	O
that	O
it	O
generates	O
very	O
little	O
thermal	O
noise	O
.	O
</s>
<s>
There	O
is	O
an	O
appealing	O
intuitive	O
interpretation	O
of	O
this	O
relationship	O
in	O
a	O
radar	B-Application
.	O
</s>
<s>
Ultimately	O
,	O
radar	B-Application
,	O
like	O
all	O
macro-scale	O
entities	O
,	O
is	O
profoundly	O
impacted	O
by	O
quantum	O
theory	O
.	O
</s>
<s>
The	O
signal	O
can	O
also	O
be	O
split	O
among	O
multiple	O
filters	O
for	O
pulse-Doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
,	O
which	O
reduces	O
the	O
noise	O
floor	O
by	O
the	O
number	O
of	O
filters	O
.	O
</s>
<s>
Radar	B-Application
systems	I-Application
must	O
overcome	O
unwanted	O
signals	O
in	O
order	O
to	O
focus	O
on	O
the	O
targets	O
of	O
interest	O
.	O
</s>
<s>
The	O
ability	O
of	O
the	O
radar	B-Application
system	I-Application
to	O
overcome	O
these	O
unwanted	O
signals	O
defines	O
its	O
signal-to-noise	O
ratio	O
(	O
SNR	O
)	O
.	O
</s>
<s>
The	O
higher	O
a	O
system	O
's	O
SNR	O
the	O
better	O
it	O
is	O
at	O
discriminating	O
actual	O
targets	O
from	O
noise	O
signals	O
.	O
</s>
<s>
Clutter	O
refers	O
to	O
radio	B-Algorithm
frequency	O
(	O
RF	O
)	O
echoes	O
returned	O
from	O
targets	O
which	O
are	O
uninteresting	O
to	O
the	O
radar	B-Application
operators	O
.	O
</s>
<s>
Such	O
targets	O
include	O
natural	O
objects	O
such	O
as	O
ground	O
,	O
sea	O
,	O
and	O
when	O
not	O
being	O
tasked	O
for	O
meteorological	O
purposes	O
,	O
precipitation	O
(	O
such	O
as	O
rain	O
,	O
snow	O
or	O
hail	O
)	O
,	O
sand	B-Application
storms	I-Application
,	O
animals	O
(	O
especially	O
birds	O
)	O
,	O
atmospheric	O
turbulence	O
,	O
and	O
other	O
atmospheric	O
effects	O
,	O
such	O
as	O
ionosphere	O
reflections	O
,	O
meteor	B-Language
trails	O
,	O
and	O
hail	B-Algorithm
spike	I-Algorithm
.	O
</s>
<s>
Clutter	O
may	O
also	O
be	O
returned	O
from	O
man-made	O
objects	O
such	O
as	O
buildings	O
and	O
,	O
intentionally	O
,	O
by	O
radar	B-Application
countermeasures	O
such	O
as	O
chaff	O
.	O
</s>
<s>
Some	O
clutter	O
may	O
also	O
be	O
caused	O
by	O
a	O
long	O
radar	B-Application
waveguide	O
between	O
the	O
radar	B-Algorithm
transceiver	I-Algorithm
and	O
the	O
antenna	O
.	O
</s>
<s>
In	O
a	O
typical	O
plan	B-Algorithm
position	I-Algorithm
indicator	I-Algorithm
(	O
PPI	O
)	O
radar	B-Application
with	O
a	O
rotating	O
antenna	O
,	O
this	O
will	O
usually	O
be	O
seen	O
as	O
a	O
"	O
sun	O
"	O
or	O
"	O
sunburst	O
"	O
in	O
the	O
center	O
of	O
the	O
display	O
as	O
the	O
receiver	O
responds	O
to	O
echoes	O
from	O
dust	O
particles	O
and	O
misguided	O
RF	O
in	O
the	O
waveguide	O
.	O
</s>
<s>
Clutter	O
is	O
considered	O
a	O
passive	O
interference	O
source	O
,	O
since	O
it	O
only	O
appears	O
in	O
response	O
to	O
radar	B-Application
signals	O
sent	O
by	O
the	O
radar	B-Application
.	O
</s>
<s>
Clutter	O
tends	O
to	O
appear	O
static	O
between	O
radar	B-Application
scans	O
;	O
on	O
subsequent	O
scan	O
echoes	O
,	O
desirable	O
targets	O
will	O
appear	O
to	O
move	O
,	O
and	O
all	O
stationary	O
echoes	O
can	O
be	O
eliminated	O
.	O
</s>
<s>
Sea	O
clutter	O
can	O
be	O
reduced	O
by	O
using	O
horizontal	O
polarization	O
,	O
while	O
rain	O
is	O
reduced	O
with	O
circular	O
polarization	O
(	O
meteorological	B-Application
radars	I-Application
wish	O
for	O
the	O
opposite	O
effect	O
,	O
and	O
therefore	O
use	O
linear	O
polarization	O
to	O
detect	O
precipitation	O
)	O
.	O
</s>
<s>
The	O
most	O
effective	O
clutter	O
reduction	O
technique	O
is	O
pulse-Doppler	B-Algorithm
radar	I-Algorithm
.	O
</s>
<s>
This	O
can	O
be	O
adapted	O
for	O
systems	O
that	O
lack	O
a	O
coherent	O
transmitter	O
,	O
such	O
as	O
time-domain	O
pulse-amplitude	O
radar	B-Application
.	O
</s>
<s>
Constant	B-Algorithm
false	I-Algorithm
alarm	I-Algorithm
rate	I-Algorithm
,	O
a	O
form	O
of	O
automatic	O
gain	O
control	O
(	O
AGC	O
)	O
,	O
is	O
a	O
method	O
that	O
relies	O
on	O
clutter	O
returns	O
far	O
outnumbering	O
echoes	O
from	O
targets	O
of	O
interest	O
.	O
</s>
<s>
The	O
receiver	O
's	O
gain	O
is	O
automatically	O
adjusted	O
to	O
maintain	O
a	O
constant	O
level	O
of	O
overall	O
visible	O
clutter	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
radar	B-Application
AGC	O
was	O
electronically	O
controlled	O
and	O
affected	O
the	O
gain	O
of	O
the	O
entire	O
radar	B-Application
receiver	O
.	O
</s>
<s>
As	O
radars	B-Application
evolved	O
,	O
AGC	O
became	O
computer-software	O
controlled	O
and	O
affected	O
the	O
gain	O
with	O
greater	O
granularity	O
in	O
specific	O
detection	O
cells	O
.	O
</s>
<s>
Clutter	O
may	O
also	O
originate	O
from	O
multipath	O
echoes	O
from	O
valid	O
targets	O
caused	O
by	O
ground	O
reflection	O
,	O
atmospheric	O
ducting	O
or	O
ionospheric	O
reflection/refraction	O
(	O
e.g.	O
,	O
anomalous	B-Algorithm
propagation	I-Algorithm
)	O
.	O
</s>
<s>
The	O
radar	B-Application
may	O
try	O
to	O
unify	O
the	O
targets	O
,	O
reporting	O
the	O
target	O
at	O
an	O
incorrect	O
height	O
,	O
or	O
eliminating	O
it	O
on	O
the	O
basis	O
of	O
jitter	O
or	O
a	O
physical	O
impossibility	O
.	O
</s>
<s>
Terrain	O
bounce	O
jamming	O
exploits	O
this	O
response	O
by	O
amplifying	O
the	O
radar	B-Application
signal	O
and	O
directing	O
it	O
downward	O
.	O
</s>
<s>
These	O
problems	O
can	O
be	O
overcome	O
by	O
incorporating	O
a	O
ground	O
map	O
of	O
the	O
radar	B-Application
's	O
surroundings	O
and	O
eliminating	O
all	O
echoes	O
which	O
appear	O
to	O
originate	O
below	O
ground	O
or	O
above	O
a	O
certain	O
height	O
.	O
</s>
<s>
Monopulse	B-Algorithm
can	O
be	O
improved	O
by	O
altering	O
the	O
elevation	O
algorithm	O
used	O
at	O
low	O
elevation	O
.	O
</s>
<s>
In	O
newer	O
air	B-Application
traffic	I-Application
control	I-Application
radar	B-Application
equipment	O
,	O
algorithms	O
are	O
used	O
to	O
identify	O
the	O
false	O
targets	O
by	O
comparing	O
the	O
current	O
pulse	O
returns	O
to	O
those	O
adjacent	O
,	O
as	O
well	O
as	O
calculating	O
return	O
improbabilities	O
.	O
</s>
<s>
Radar	B-Application
jamming	O
refers	O
to	O
radio	B-Algorithm
frequency	O
signals	O
originating	O
from	O
sources	O
outside	O
the	O
radar	B-Application
,	O
transmitting	O
in	O
the	O
radar	B-Application
's	O
frequency	O
and	O
thereby	O
masking	O
targets	O
of	O
interest	O
.	O
</s>
<s>
Jamming	O
is	O
considered	O
an	O
active	O
interference	O
source	O
,	O
since	O
it	O
is	O
initiated	O
by	O
elements	O
outside	O
the	O
radar	B-Application
and	O
in	O
general	O
unrelated	O
to	O
the	O
radar	B-Application
signals	O
.	O
</s>
<s>
Jamming	O
is	O
problematic	O
to	O
radar	B-Application
since	O
the	O
jamming	O
signal	O
only	O
needs	O
to	O
travel	O
one	O
way	O
(	O
from	O
the	O
jammer	O
to	O
the	O
radar	B-Application
receiver	O
)	O
whereas	O
the	O
radar	B-Application
echoes	O
travel	O
two	O
ways	O
(	O
radar-target-radar	O
)	O
and	O
are	O
therefore	O
significantly	O
reduced	O
in	O
power	O
by	O
the	O
time	O
they	O
return	O
to	O
the	O
radar	B-Application
receiver	O
in	O
accordance	O
with	O
inverse-square	O
law	O
.	O
</s>
<s>
Jammers	O
therefore	O
can	O
be	O
much	O
less	O
powerful	O
than	O
their	O
jammed	O
radars	B-Application
and	O
still	O
effectively	O
mask	O
targets	O
along	O
the	O
line	O
of	O
sight	O
from	O
the	O
jammer	O
to	O
the	O
radar	B-Application
(	O
mainlobe	O
jamming	O
)	O
.	O
</s>
<s>
Jammers	O
have	O
an	O
added	O
effect	O
of	O
affecting	O
radars	B-Application
along	O
other	O
lines	O
of	O
sight	O
through	O
the	O
radar	B-Application
receiver	O
's	O
sidelobes	O
(	O
sidelobe	O
jamming	O
)	O
.	O
</s>
<s>
Mainlobe	O
jamming	O
can	O
generally	O
only	O
be	O
reduced	O
by	O
narrowing	O
the	O
mainlobe	O
solid	O
angle	O
and	O
cannot	O
fully	O
be	O
eliminated	O
when	O
directly	O
facing	O
a	O
jammer	O
which	O
uses	O
the	O
same	O
frequency	O
and	O
polarization	O
as	O
the	O
radar	B-Application
.	O
</s>
<s>
Sidelobe	O
jamming	O
can	O
be	O
overcome	O
by	O
reducing	O
receiving	O
sidelobes	O
in	O
the	O
radar	B-Application
antenna	O
design	O
and	O
by	O
using	O
an	O
omnidirectional	O
antenna	O
to	O
detect	O
and	O
disregard	O
non-mainlobe	O
signals	O
.	O
</s>
<s>
Other	O
anti-jamming	O
techniques	O
are	O
frequency	B-Protocol
hopping	I-Protocol
and	O
polarization	O
.	O
</s>
<s>
One	O
way	O
to	O
obtain	O
a	O
distance	O
measurement	O
(	O
ranging	O
)	O
is	O
based	O
on	O
the	O
time-of-flight	O
:	O
transmit	O
a	O
short	O
pulse	O
of	O
radio	B-Algorithm
signal	O
(	O
electromagnetic	O
radiation	O
)	O
and	O
measure	O
the	O
time	O
it	O
takes	O
for	O
the	O
reflection	O
to	O
return	O
.	O
</s>
<s>
Since	O
radio	B-Algorithm
waves	O
travel	O
at	O
the	O
speed	O
of	O
light	O
,	O
accurate	O
distance	O
measurement	O
requires	O
high-speed	O
electronics	O
.	O
</s>
<s>
Through	O
the	O
use	O
of	O
a	O
duplexer	O
,	O
the	O
radar	B-Application
switches	O
between	O
transmitting	O
and	O
receiving	O
at	O
a	O
predetermined	O
rate	O
.	O
</s>
<s>
These	O
two	O
effects	O
tend	O
to	O
be	O
at	O
odds	O
with	O
each	O
other	O
,	O
and	O
it	O
is	O
not	O
easy	O
to	O
combine	O
both	O
good	O
short	O
range	O
and	O
good	O
long	O
range	O
in	O
a	O
single	O
radar	B-Application
.	O
</s>
<s>
This	O
is	O
because	O
the	O
short	O
pulses	O
needed	O
for	O
a	O
good	O
minimum	O
range	O
broadcast	B-Operating_System
have	O
less	O
total	O
energy	O
,	O
making	O
the	O
returns	O
much	O
smaller	O
and	O
the	O
target	O
harder	O
to	O
detect	O
.	O
</s>
<s>
So	O
each	O
radar	B-Application
uses	O
a	O
particular	O
type	O
of	O
signal	O
.	O
</s>
<s>
Long-range	O
radars	B-Application
tend	O
to	O
use	O
long	O
pulses	O
with	O
long	O
delays	O
between	O
them	O
,	O
and	O
short	O
range	O
radars	B-Application
use	O
smaller	O
pulses	O
with	O
less	O
time	O
between	O
them	O
.	O
</s>
<s>
As	O
electronics	O
have	O
improved	O
many	O
radars	B-Application
now	O
can	O
change	O
their	O
pulse	O
repetition	O
frequency	O
,	O
thereby	O
changing	O
their	O
range	O
.	O
</s>
<s>
The	O
newest	O
radars	B-Application
fire	O
two	O
pulses	O
during	O
one	O
cell	O
,	O
one	O
for	O
short	O
range	O
(	O
about	O
)	O
and	O
a	O
separate	O
signal	O
for	O
longer	O
ranges	O
(	O
about	O
)	O
.	O
</s>
<s>
The	O
radar	B-Application
mile	O
is	O
the	O
time	O
it	O
takes	O
for	O
a	O
radar	B-Application
pulse	O
to	O
travel	O
one	O
nautical	O
mile	O
,	O
reflect	O
off	O
a	O
target	O
,	O
and	O
return	O
to	O
the	O
radar	B-Application
antenna	O
.	O
</s>
<s>
Since	O
a	O
nautical	O
mile	O
is	O
defined	O
as	O
1,852	O
m	O
,	O
then	O
dividing	O
this	O
distance	O
by	O
the	O
speed	O
of	O
light	O
(	O
299,792,458	O
m/s	O
)	O
,	O
and	O
then	O
multiplying	O
the	O
result	O
by	O
2	O
yields	O
a	O
result	O
of	O
12.36μs	O
in	O
duration	O
.	O
</s>
<s>
Another	O
form	O
of	O
distance	O
measuring	O
radar	B-Application
is	O
based	O
on	O
frequency	O
modulation	B-Application
.	O
</s>
<s>
Since	O
the	O
signal	O
takes	O
a	O
finite	O
time	O
to	O
travel	O
to	O
and	O
from	O
the	O
target	O
,	O
the	O
received	O
signal	O
is	O
a	O
different	O
frequency	O
than	O
what	O
the	O
transmitter	O
is	O
broadcasting	B-Operating_System
at	O
the	O
time	O
the	O
reflected	O
signal	O
arrives	O
back	O
at	O
the	O
radar	B-Application
.	O
</s>
<s>
A	O
further	O
advantage	O
is	O
that	O
the	O
radar	B-Application
can	O
operate	O
effectively	O
at	O
relatively	O
low	O
frequencies	O
.	O
</s>
<s>
This	O
was	O
important	O
in	O
the	O
early	O
development	O
of	O
this	O
type	O
when	O
high-frequency	B-Algorithm
signal	O
generation	O
was	O
difficult	O
or	O
expensive	O
.	O
</s>
<s>
This	O
technique	O
can	O
be	O
used	O
in	O
continuous	B-Algorithm
wave	I-Algorithm
radar	I-Algorithm
and	O
is	O
often	O
found	O
in	O
aircraft	O
radar	B-Application
altimeters	O
.	O
</s>
<s>
In	O
these	O
systems	O
a	O
"	O
carrier	O
"	O
radar	B-Application
signal	O
is	O
frequency	O
modulated	B-Application
in	O
a	O
predictable	O
way	O
,	O
typically	O
varying	O
up	O
and	O
down	O
with	O
a	O
sine	O
wave	O
or	O
sawtooth	O
pattern	O
at	O
audio	O
frequencies	O
.	O
</s>
<s>
The	O
signal	O
is	O
then	O
sent	O
out	O
from	O
one	O
antenna	O
and	O
received	O
on	O
another	O
,	O
typically	O
located	O
on	O
the	O
bottom	O
of	O
the	O
aircraft	O
,	O
and	O
the	O
signal	O
can	O
be	O
continuously	O
compared	O
using	O
a	O
simple	O
beat	B-Application
frequency	I-Application
modulator	B-Application
that	O
produces	O
an	O
audio	O
frequency	O
tone	O
from	O
the	O
returned	O
signal	O
and	O
a	O
portion	O
of	O
the	O
transmitted	O
signal	O
.	O
</s>
<s>
The	O
modulation	B-Application
index	O
riding	O
on	O
the	O
receive	O
signal	O
is	O
proportional	O
to	O
the	O
time	O
delay	O
between	O
the	O
radar	B-Application
and	O
the	O
reflector	O
.	O
</s>
<s>
This	O
signal	O
processing	O
is	O
similar	O
to	O
that	O
used	O
in	O
speed	O
detecting	O
Doppler	B-Algorithm
radar	I-Algorithm
.	O
</s>
<s>
Example	O
systems	O
using	O
this	O
approach	O
are	O
AZUSA	B-Application
,	O
MISTRAM	O
,	O
and	O
UDOP	B-Algorithm
.	O
</s>
<s>
Terrestrial	O
radar	B-Application
uses	O
low-power	O
FM	O
signals	O
that	O
cover	O
a	O
larger	O
frequency	O
range	O
.	O
</s>
<s>
Increasing	O
power	O
for	O
longer	O
range	O
while	O
maintaining	O
accuracy	O
demands	O
extremely	O
high	O
peak	O
power	O
,	O
with	O
1960s	O
early	B-Algorithm
warning	I-Algorithm
radars	I-Algorithm
often	O
operating	O
in	O
the	O
tens	O
of	O
megawatts	O
.	O
</s>
<s>
It	O
starts	O
with	O
a	O
longer	O
pulse	O
that	O
is	O
also	O
frequency	O
modulated	B-Application
.	O
</s>
<s>
Spreading	O
the	O
broadcast	B-Operating_System
energy	O
out	O
in	O
time	O
means	O
lower	O
peak	O
energies	O
can	O
be	O
used	O
,	O
with	O
modern	O
examples	O
typically	O
on	O
the	O
order	O
of	O
tens	O
of	O
kilowatts	O
.	O
</s>
<s>
The	O
technique	O
is	O
largely	O
universal	O
on	O
modern	O
large	O
radars	B-Application
.	O
</s>
<s>
At	O
one	O
time	O
the	O
memory	O
consisted	O
of	O
a	O
user	O
making	O
grease	O
pencil	O
marks	O
on	O
the	O
radar	B-Application
screen	O
and	O
then	O
calculating	O
the	O
speed	O
using	O
a	O
slide	O
rule	O
.	O
</s>
<s>
Modern	O
radar	B-Application
systems	I-Application
perform	O
the	O
equivalent	O
operation	O
faster	O
and	O
more	O
accurately	O
using	O
computers	O
.	O
</s>
<s>
If	O
the	O
transmitter	O
's	O
output	O
is	O
coherent	O
(	O
phase	O
synchronized	O
)	O
,	O
there	O
is	O
another	O
effect	O
that	O
can	O
be	O
used	O
to	O
make	O
almost	O
instant	O
speed	O
measurements	O
(	O
no	O
memory	O
is	O
required	O
)	O
,	O
known	O
as	O
the	O
Doppler	O
effect	O
.	O
</s>
<s>
Most	O
modern	O
radar	B-Application
systems	I-Application
use	O
this	O
principle	O
into	O
Doppler	B-Algorithm
radar	I-Algorithm
and	O
pulse-Doppler	B-Algorithm
radar	I-Algorithm
systems	I-Application
(	O
weather	B-Application
radar	I-Application
,	O
military	O
radar	B-Application
)	O
.	O
</s>
<s>
The	O
Doppler	O
effect	O
is	O
only	O
able	O
to	O
determine	O
the	O
relative	O
speed	O
of	O
the	O
target	O
along	O
the	O
line	O
of	O
sight	O
from	O
the	O
radar	B-Application
to	O
the	O
target	O
.	O
</s>
<s>
Any	O
component	O
of	O
target	O
velocity	O
perpendicular	O
to	O
the	O
line	O
of	O
sight	O
cannot	O
be	O
determined	O
by	O
using	O
the	O
Doppler	O
effect	O
alone	O
,	O
but	O
it	O
can	O
be	O
determined	O
by	O
tracking	O
the	O
target	O
's	O
azimuth	O
over	O
time	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
make	O
a	O
Doppler	B-Algorithm
radar	I-Algorithm
without	O
any	O
pulsing	O
,	O
known	O
as	O
a	O
continuous-wave	B-Algorithm
radar	I-Algorithm
(	O
CW	B-Algorithm
radar	I-Algorithm
)	O
,	O
by	O
sending	O
out	O
a	O
very	O
pure	O
signal	O
of	O
a	O
known	O
frequency	O
.	O
</s>
<s>
CW	B-Algorithm
radar	I-Algorithm
is	O
ideal	O
for	O
determining	O
the	O
radial	O
component	O
of	O
a	O
target	O
's	O
velocity	O
.	O
</s>
<s>
CW	B-Algorithm
radar	I-Algorithm
is	O
typically	O
used	O
by	O
traffic	O
enforcement	O
to	O
measure	O
vehicle	O
speed	O
quickly	O
and	O
accurately	O
where	O
the	O
range	O
is	O
not	O
important	O
.	O
</s>
<s>
When	O
using	O
a	O
pulsed	B-Algorithm
radar	I-Algorithm
,	O
the	O
variation	O
between	O
the	O
phase	O
of	O
successive	O
returns	O
gives	O
the	O
distance	O
the	O
target	O
has	O
moved	O
between	O
pulses	O
,	O
and	O
thus	O
its	O
speed	O
can	O
be	O
calculated	O
.	O
</s>
<s>
Other	O
mathematical	O
developments	O
in	O
radar	B-Application
signal	O
processing	O
include	O
time-frequency	B-Algorithm
analysis	I-Algorithm
(	O
Weyl	O
Heisenberg	O
or	O
wavelet	O
)	O
,	O
as	O
well	O
as	O
the	O
chirplet	B-Algorithm
transform	I-Algorithm
which	O
makes	O
use	O
of	O
the	O
change	O
of	O
frequency	O
of	O
returns	O
from	O
moving	O
targets	O
(	O
"	O
chirp	O
"	O
)	O
.	O
</s>
<s>
Pulse-Doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
includes	O
frequency	O
filtering	O
in	O
the	O
detection	O
process	O
.	O
</s>
<s>
This	O
is	O
used	O
with	O
weather	B-Application
radar	I-Application
to	O
measure	O
radial	O
wind	O
velocity	O
and	O
precipitation	O
rate	O
in	O
each	O
different	O
volume	O
of	O
air	O
.	O
</s>
<s>
Aircraft	O
safety	O
depends	O
upon	O
continuous	O
access	O
to	O
accurate	O
weather	B-Application
radar	I-Application
information	O
that	O
is	O
used	O
to	O
prevent	O
injuries	O
and	O
accidents	O
.	O
</s>
<s>
Weather	B-Application
radar	I-Application
uses	O
a	O
low	O
PRF	O
.	O
</s>
<s>
Less	O
sophisticated	O
filtering	O
is	O
required	O
,	O
and	O
range	O
ambiguity	O
processing	O
is	O
not	O
normally	O
needed	O
with	O
weather	B-Application
radar	I-Application
in	O
comparison	O
with	O
military	O
radar	B-Application
intended	O
to	O
track	O
air	O
vehicles	O
.	O
</s>
<s>
Pulse-Doppler	B-Algorithm
is	O
also	O
used	O
for	O
ground	O
based	O
surveillance	O
radar	B-Application
required	O
to	O
defend	O
personnel	O
and	O
vehicles	O
.	O
</s>
<s>
Pulse-doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
increases	O
the	O
maximum	O
detection	O
distance	O
using	O
less	O
radiation	O
close	O
to	O
aircraft	O
pilots	O
,	O
shipboard	O
personnel	O
,	O
infantry	O
,	O
and	O
artillery	O
.	O
</s>
<s>
Pulse-Doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
incorporates	O
more	O
sophisticated	O
electronic	O
filtering	O
that	O
safely	O
eliminates	O
this	O
kind	O
of	O
weakness	O
.	O
</s>
<s>
Military	O
applications	O
require	O
medium	O
PRF	O
which	O
prevents	O
range	O
from	O
being	O
determined	O
directly	O
,	O
and	O
range	B-Algorithm
ambiguity	I-Algorithm
resolution	I-Algorithm
processing	O
is	O
required	O
to	O
identify	O
the	O
true	O
range	O
of	O
all	O
reflected	O
signals	O
.	O
</s>
<s>
Radial	O
movement	O
is	O
usually	O
linked	O
with	O
Doppler	O
frequency	O
to	O
produce	O
a	O
lock	O
signal	O
that	O
cannot	O
be	O
produced	O
by	O
radar	B-Application
jamming	O
signals	O
.	O
</s>
<s>
Pulse-Doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
also	O
produces	O
audible	O
signals	O
that	O
can	O
be	O
used	O
for	O
threat	O
identification	O
.	O
</s>
<s>
Signal	O
processing	O
is	O
employed	O
in	O
radar	B-Application
systems	I-Application
to	O
reduce	O
the	O
radar	B-Application
interference	O
effects	O
.	O
</s>
<s>
Signal	O
processing	O
techniques	O
include	O
moving	O
target	O
indication	O
,	O
Pulse-Doppler	B-Algorithm
signal	I-Algorithm
processing	I-Algorithm
,	O
moving	O
target	O
detection	O
processors	O
,	O
correlation	O
with	O
secondary	O
surveillance	O
radar	B-Application
targets	O
,	O
space-time	B-Algorithm
adaptive	I-Algorithm
processing	I-Algorithm
,	O
and	O
track-before-detect	B-Algorithm
.	O
</s>
<s>
Constant	B-Algorithm
false	I-Algorithm
alarm	I-Algorithm
rate	I-Algorithm
and	O
digital	B-Application
terrain	I-Application
model	I-Application
processing	O
are	O
also	O
used	O
in	O
clutter	O
environments	O
.	O
</s>
<s>
A	O
Track	O
algorithm	O
is	O
a	O
radar	B-Application
performance	O
enhancement	O
strategy	O
.	O
</s>
<s>
Historical	O
information	O
is	O
accumulated	O
and	O
used	O
to	O
predict	O
future	O
position	O
for	O
use	O
with	O
air	B-Application
traffic	I-Application
control	I-Application
,	O
threat	O
estimation	O
,	O
combat	O
system	O
doctrine	O
,	O
gun	O
aiming	O
,	O
and	O
missile	O
guidance	O
.	O
</s>
<s>
Position	O
data	O
is	O
accumulated	O
by	O
radar	B-Application
sensors	O
over	O
the	O
span	O
of	O
a	O
few	O
minutes	O
.	O
</s>
<s>
Radar	B-Application
video	O
returns	O
from	O
aircraft	O
can	O
be	O
subjected	O
to	O
a	O
plot	O
extraction	O
process	O
whereby	O
spurious	O
and	O
interfering	O
signals	O
are	O
discarded	O
.	O
</s>
<s>
In	O
some	O
radar	B-Application
systems	I-Application
,	O
or	O
alternatively	O
in	O
the	O
command	O
and	O
control	O
system	O
to	O
which	O
the	O
radar	B-Application
is	O
connected	O
,	O
a	O
radar	B-Algorithm
tracker	I-Algorithm
is	O
used	O
to	O
associate	O
the	O
sequence	O
of	O
plots	O
belonging	O
to	O
individual	O
targets	O
and	O
estimate	O
the	O
targets	O
 '	O
headings	O
and	O
speeds	O
.	O
</s>
<s>
A	O
radar	B-Application
's	O
components	O
are	O
:	O
</s>
<s>
A	O
transmitter	O
that	O
generates	O
the	O
radio	B-Algorithm
signal	O
with	O
an	O
oscillator	O
such	O
as	O
a	O
klystron	O
or	O
a	O
magnetron	B-Architecture
and	O
controls	O
its	O
duration	O
by	O
a	O
modulator	B-Application
.	O
</s>
<s>
A	O
display	O
processor	O
to	O
produce	O
signals	O
for	O
human	O
readable	O
output	B-Algorithm
devices	I-Algorithm
.	O
</s>
<s>
An	O
electronic	O
section	O
that	O
controls	O
all	O
those	O
devices	O
and	O
the	O
antenna	O
to	O
perform	O
the	O
radar	B-Application
scan	O
ordered	O
by	O
software	O
.	O
</s>
<s>
Radio	B-Algorithm
signals	O
broadcast	B-Operating_System
from	O
a	O
single	O
antenna	O
will	O
spread	O
out	O
in	O
all	O
directions	O
,	O
and	O
likewise	O
a	O
single	O
antenna	O
will	O
receive	O
signals	O
equally	O
from	O
all	O
directions	O
.	O
</s>
<s>
This	O
leaves	O
the	O
radar	B-Application
with	O
the	O
problem	O
of	O
deciding	O
where	O
the	O
target	O
object	O
is	O
located	O
.	O
</s>
<s>
Early	O
systems	O
tended	O
to	O
use	O
omnidirectional	O
broadcast	B-Operating_System
antennas	O
,	O
with	O
directional	O
receiver	O
antennas	O
which	O
were	O
pointed	O
in	O
various	O
directions	O
.	O
</s>
<s>
One	O
serious	O
limitation	O
with	O
this	O
type	O
of	O
solution	O
is	O
that	O
the	O
broadcast	B-Operating_System
is	O
sent	O
out	O
in	O
all	O
directions	O
,	O
so	O
the	O
amount	O
of	O
energy	O
in	O
the	O
direction	O
being	O
examined	O
is	O
a	O
small	O
part	O
of	O
that	O
transmitted	O
.	O
</s>
<s>
More	O
modern	O
systems	O
use	O
a	O
steerable	O
parabolic	O
"	O
dish	O
"	O
to	O
create	O
a	O
tight	O
broadcast	B-Operating_System
beam	O
,	O
typically	O
using	O
the	O
same	O
dish	O
as	O
the	O
receiver	O
.	O
</s>
<s>
Such	O
systems	O
often	O
combine	O
two	O
radar	B-Application
frequencies	O
in	O
the	O
same	O
antenna	O
in	O
order	O
to	O
allow	O
automatic	O
steering	O
,	O
or	O
radar	B-Application
lock	O
.	O
</s>
<s>
Symmetric	O
parabolic	O
antennas	O
produce	O
a	O
narrow	O
"	O
pencil	O
"	O
beam	O
in	O
both	O
the	O
X	B-Algorithm
and	O
Y	O
dimensions	O
and	O
consequently	O
have	O
a	O
higher	O
gain	O
.	O
</s>
<s>
The	O
NEXRAD	B-Algorithm
Pulse-Doppler	B-Algorithm
weather	B-Application
radar	I-Application
uses	O
a	O
symmetric	O
antenna	O
to	O
perform	O
detailed	O
volumetric	O
scans	O
of	O
the	O
atmosphere	O
.	O
</s>
<s>
Most	O
2D	O
surveillance	O
radars	B-Application
use	O
a	O
spoiled	O
parabolic	O
antenna	O
with	O
a	O
narrow	O
azimuthal	O
beamwidth	O
and	O
wide	O
vertical	O
beamwidth	O
.	O
</s>
<s>
This	O
beam	O
configuration	O
allows	O
the	O
radar	B-Application
operator	O
to	O
detect	O
an	O
aircraft	O
at	O
a	O
specific	O
azimuth	O
but	O
at	O
an	O
indeterminate	O
height	O
.	O
</s>
<s>
Conversely	O
,	O
so-called	O
"	O
nodder	O
"	O
height	O
finding	O
radars	B-Application
use	O
a	O
dish	O
with	O
a	O
narrow	O
vertical	O
beamwidth	O
and	O
wide	O
azimuthal	O
beamwidth	O
to	O
detect	O
an	O
aircraft	O
at	O
a	O
specific	O
height	O
but	O
with	O
low	O
azimuthal	O
precision	O
.	O
</s>
<s>
Conical	B-Algorithm
scanning	I-Algorithm
:	O
The	O
radar	B-Application
beam	O
is	O
rotated	O
in	O
a	O
small	O
circle	O
around	O
the	O
"	O
boresight	O
"	O
axis	O
,	O
which	O
is	O
pointed	O
at	O
the	O
target	O
.	O
</s>
<s>
Owing	O
to	O
its	O
lower	O
cost	O
and	O
less	O
wind	O
exposure	O
,	O
shipboard	O
,	O
airport	O
surface	O
,	O
and	O
harbour	O
surveillance	O
radars	B-Application
now	O
use	O
this	O
approach	O
in	O
preference	O
to	O
a	O
parabolic	O
antenna	O
.	O
</s>
<s>
Another	O
method	O
of	O
steering	O
is	O
used	O
in	O
a	O
phased	O
array	O
radar	B-Application
.	O
</s>
<s>
Phased	O
array	O
radars	B-Application
have	O
been	O
in	O
use	O
since	O
the	O
earliest	O
years	O
of	O
radar	B-Application
in	O
World	O
War	O
II	O
(	O
Mammut	O
radar	B-Application
)	O
,	O
but	O
electronic	O
device	O
limitations	O
led	O
to	O
poor	O
performance	O
.	O
</s>
<s>
Phased	O
array	O
radars	B-Application
were	O
originally	O
used	O
for	O
missile	O
defence	O
(	O
see	O
for	O
example	O
Safeguard	O
Program	O
)	O
.	O
</s>
<s>
They	O
are	O
the	O
heart	O
of	O
the	O
ship-borne	O
Aegis	B-Application
Combat	I-Application
System	I-Application
and	O
the	O
Patriot	O
Missile	O
System	O
.	O
</s>
<s>
To	O
a	O
lesser	O
extent	O
,	O
Phased	O
array	O
radars	B-Application
have	O
been	O
used	O
in	O
weather	O
surveillance	O
.	O
</s>
<s>
As	O
of	O
2017	O
,	O
NOAA	O
plans	O
to	O
implement	O
a	O
national	O
network	O
of	O
Multi-Function	O
Phased	O
array	O
radars	B-Application
throughout	O
the	O
United	O
States	O
within	O
10	O
years	O
,	O
for	O
meteorological	O
studies	O
and	O
flight	O
monitoring	O
.	O
</s>
<s>
Phased	O
array	O
antennas	O
can	O
be	O
built	O
to	O
conform	O
to	O
specific	O
shapes	O
,	O
like	O
missiles	O
,	O
infantry	O
support	O
vehicles	O
,	O
ships	B-Algorithm
,	O
and	O
aircraft	O
.	O
</s>
<s>
As	O
the	O
price	O
of	O
electronics	O
has	O
fallen	O
,	O
phased	O
array	O
radars	B-Application
have	O
become	O
more	O
common	O
.	O
</s>
<s>
Almost	O
all	O
modern	O
military	O
radar	B-Application
systems	I-Application
are	O
based	O
on	O
phased	O
arrays	O
,	O
where	O
the	O
small	O
additional	O
cost	O
is	O
offset	O
by	O
the	O
improved	O
reliability	O
of	O
a	O
system	O
with	O
no	O
moving	O
parts	O
.	O
</s>
<s>
Traditional	O
moving-antenna	O
designs	O
are	O
still	O
widely	O
used	O
in	O
roles	O
where	O
cost	O
is	O
a	O
significant	O
factor	O
such	O
as	O
air	B-Application
traffic	I-Application
surveillance	O
and	O
similar	O
systems	O
.	O
</s>
<s>
Phased	O
array	O
radars	B-Application
are	O
valued	O
for	O
use	O
in	O
aircraft	O
since	O
they	O
can	O
track	O
multiple	O
targets	O
.	O
</s>
<s>
The	O
first	O
aircraft	O
to	O
use	O
a	O
phased	O
array	O
radar	B-Application
was	O
the	O
B-1B	O
Lancer	O
.	O
</s>
<s>
The	O
first	O
fighter	O
aircraft	O
to	O
use	O
phased	O
array	O
radar	B-Application
was	O
the	O
Mikoyan	O
MiG-31	O
.	O
</s>
<s>
The	O
MiG-31M	O
'	O
s	B-Algorithm
SBI-16	O
Zaslon	O
Passive	B-Algorithm
electronically	I-Algorithm
scanned	I-Algorithm
array	I-Algorithm
radar	B-Application
was	O
considered	O
to	O
be	O
the	O
world	O
's	O
most	O
powerful	O
fighter	O
radar	B-Application
,	O
until	O
the	O
AN/APG	B-Algorithm
-77	I-Algorithm
Active	B-Architecture
electronically	I-Architecture
scanned	I-Architecture
array	I-Architecture
was	O
introduced	O
on	O
the	O
Lockheed	O
Martin	O
F-22	O
Raptor	O
.	O
</s>
<s>
Phased-array	O
interferometry	O
or	O
aperture	B-Application
synthesis	I-Application
techniques	O
,	O
using	O
an	O
array	O
of	O
separate	O
dishes	O
that	O
are	O
phased	O
into	O
a	O
single	O
effective	O
aperture	O
,	O
are	O
not	O
typical	O
for	O
radar	B-Application
applications	O
,	O
although	O
they	O
are	O
widely	O
used	O
in	O
radio	B-Algorithm
astronomy	O
.	O
</s>
<s>
Aperture	B-Application
synthesis	I-Application
by	O
post-processing	O
motion	O
data	O
from	O
a	O
single	O
moving	O
source	O
,	O
on	O
the	O
other	O
hand	O
,	O
is	O
widely	O
used	O
in	O
space	O
and	O
airborne	O
radar	B-Application
systems	I-Application
.	O
</s>
<s>
Shorter	O
wavelengths	O
also	O
result	O
in	O
higher	O
resolution	O
due	O
to	O
diffraction	O
,	O
meaning	O
the	O
shaped	O
reflector	O
seen	O
on	O
most	O
radars	B-Application
can	O
also	O
be	O
made	O
smaller	O
for	O
any	O
desired	O
beamwidth	O
.	O
</s>
<s>
Another	O
issue	O
is	O
that	O
the	O
radar	B-Application
equation	O
's	O
effective	O
aperture	O
figure	O
means	O
that	O
for	O
any	O
given	O
antenna	O
(	O
or	O
reflector	O
)	O
size	O
will	O
be	O
more	O
efficient	O
at	O
longer	O
wavelengths	O
.	O
</s>
<s>
Very	O
long	O
wavelengths	O
also	O
have	O
additional	O
diffraction	O
effects	O
that	O
make	O
them	O
suitable	O
for	O
over	B-Architecture
the	I-Architecture
horizon	I-Architecture
radars	I-Architecture
.	O
</s>
<s>
Other	O
users	O
of	O
the	O
radio	B-Algorithm
spectrum	I-Algorithm
,	O
such	O
as	O
the	O
broadcasting	B-Operating_System
and	O
electronic	O
countermeasures	O
industries	O
,	O
have	O
replaced	O
the	O
traditional	O
military	O
designations	O
with	O
their	O
own	O
systems	O
.	O
</s>
<s>
+	O
Radar	B-Application
frequency	O
bandsBand	O
nameFrequency	O
rangeWavelength	O
rangeNotesHF3	O
–	O
30	O
MHz10	O
–	O
100	O
mCoastal	O
radar	B-Application
systems	I-Application
,	O
over-the-horizon	B-Architecture
(	O
OTH	O
)	O
radars	B-Application
;	O
'	O
high	O
frequency'VHF30–300	O
MHz1	O
–	O
10	O
mVery	O
long	O
range	O
,	O
ground	O
penetrating	O
;	O
'	O
very	B-Algorithm
high	I-Algorithm
frequency	I-Algorithm
 '	O
.	O
</s>
<s>
Early	O
radar	B-Application
systems	I-Application
generally	O
operated	O
in	O
VHF	B-Algorithm
as	O
suitable	O
electronics	O
had	O
already	O
been	O
developed	O
for	O
broadcast	B-Operating_System
radio	B-Algorithm
.	O
</s>
<s>
Today	O
this	O
band	O
is	O
heavily	O
congested	O
and	O
no	O
longer	O
suitable	O
for	O
radar	B-Application
due	O
to	O
interference.Pxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1	O
1	O
m'P'	O
for	O
'	O
previous	O
 '	O
,	O
applied	O
retrospectively	O
to	O
early	O
radar	B-Application
systems	I-Application
;	O
essentially	O
HF	B-Algorithm
+	O
VHF	B-Algorithm
.	O
</s>
<s>
ballistic	B-Architecture
missile	I-Architecture
early	I-Architecture
warning	I-Architecture
)	O
,	O
ground	O
penetrating	O
,	O
foliage	O
penetrating	O
;	O
'	O
ultra	O
high	B-Algorithm
frequency	I-Algorithm
 '	O
.	O
</s>
<s>
Efficiently	O
produced	O
and	O
received	O
at	O
very	O
high	O
energy	O
levels	O
,	O
and	O
also	O
reduces	O
the	O
effects	O
of	O
nuclear	O
blackout	O
,	O
making	O
them	O
useful	O
in	O
the	O
missile	O
detection	O
role.L1	O
–	O
2	O
GHz15	O
–	O
30	O
cmLong	O
range	O
air	B-Application
traffic	I-Application
control	I-Application
and	O
surveillance	O
;	O
'	O
L	B-Algorithm
 '	O
for	O
'	O
long	O
 '	O
.	O
</s>
<s>
Widely	O
used	O
for	O
long	O
range	O
early	B-Algorithm
warning	I-Algorithm
radars	I-Algorithm
as	O
they	O
combine	O
good	O
reception	O
qualities	O
with	O
reasonable	O
resolution.S2	O
–	O
4	O
GHz7.5	O
–	O
15	O
cmModerate	O
range	O
surveillance	O
,	O
Terminal	O
air	B-Application
traffic	I-Application
control	I-Application
,	O
long-range	O
weather	O
,	O
marine	B-Algorithm
radar	I-Algorithm
;	O
'	O
S	B-Algorithm
 '	O
for	O
'	O
sentimetric	O
 '	O
,	O
its	O
code-name	O
during	O
WWII	O
.	O
</s>
<s>
Less	O
efficient	O
than	O
L	B-Algorithm
,	O
but	O
offering	O
higher	O
resolution	O
,	O
making	O
them	O
especially	O
suitable	O
for	O
long-range	O
ground	O
controlled	O
interception	O
tasks.C4	O
–	O
8	O
GHz3.75	O
–	O
7.5	O
cmSatellite	O
transponders	O
;	O
a	O
compromise	O
(	O
hence	O
'	O
C	B-Algorithm
 '	O
)	O
between	O
X	B-Algorithm
and	O
S	B-Algorithm
bands	I-Algorithm
;	O
weather	O
;	O
long	O
range	O
trackingX8	O
–	O
12	O
GHz2.5	O
–	O
3.75	O
cmMissile	O
guidance	O
,	O
marine	B-Algorithm
radar	I-Algorithm
,	O
weather	O
,	O
medium-resolution	O
mapping	O
and	O
ground	O
surveillance	O
;	O
in	O
the	O
United	O
States	O
the	O
narrow	O
range	O
10.525	O
GHz	O
±25	O
MHz	O
is	O
used	O
for	O
airport	B-Application
radar	I-Application
;	O
short-range	O
tracking	O
.	O
</s>
<s>
Named	O
X	B-Algorithm
band	I-Algorithm
because	O
the	O
frequency	O
was	O
a	O
secret	O
during	O
WW2	O
.	O
</s>
<s>
Diffraction	O
off	O
raindrops	O
during	O
heavy	O
rain	O
limits	O
the	O
range	O
in	O
the	O
detection	O
role	O
and	O
makes	O
this	O
suitable	O
only	O
for	O
short-range	O
roles	O
or	O
those	O
that	O
deliberately	O
detect	O
rain.Ku12	O
–	O
18	O
GHz1.67	O
–	O
2.5	O
cmHigh-resolution	O
,	O
also	O
used	O
for	O
satellite	O
transponders	O
,	O
frequency	O
under	O
K	B-Algorithm
band	I-Algorithm
(	O
hence	O
'	O
u	O
 '	O
)	O
K18	O
–	O
24	O
GHz1.11	O
–	O
1.67	O
cmFrom	O
German	O
kurz	O
,	O
meaning	O
'	O
short	O
 '	O
.	O
</s>
<s>
K-band	B-Algorithm
is	O
used	O
for	O
detecting	O
clouds	O
by	O
meteorologists	O
,	O
and	O
by	O
police	O
for	O
detecting	O
speeding	O
motorists	O
.	O
</s>
<s>
K-band	B-Algorithm
radar	B-Application
guns	O
operate	O
at	O
24.150	O
±	O
0.100	O
GHz.Ka24	O
–	O
40	O
GHz0.75	O
–	O
1.11	O
cmMapping	O
,	O
short	O
range	O
,	O
airport	O
surveillance	O
;	O
frequency	O
just	O
above	O
K	B-Algorithm
band	I-Algorithm
(	O
hence	O
'	O
a	O
 '	O
)	O
Photo	O
radar	B-Application
,	O
used	O
to	O
trigger	O
cameras	O
which	O
take	O
pictures	O
of	O
license	O
plates	O
of	O
cars	O
running	O
red	O
lights	O
,	O
operates	O
at	O
34.300	O
±	O
0.100	O
GHz.mm40	O
–	O
300	O
GHz1.0	O
–	O
7.5	O
mm	O
Millimetre	O
band	O
,	O
subdivided	O
as	O
below	O
.	O
</s>
<s>
This	O
makes	O
these	O
frequencies	O
generally	O
useful	O
only	O
for	O
short-range	O
highly	O
specific	O
radars	B-Application
,	O
like	O
power	O
line	O
avoidance	O
systems	O
for	O
helicopters	B-Application
or	O
use	O
in	O
space	O
where	O
attenuation	O
is	O
not	O
a	O
problem	O
.	O
</s>
<s>
Modulators	B-Application
act	O
to	O
provide	O
the	O
waveform	O
of	O
the	O
RF-pulse	O
.	O
</s>
<s>
There	O
are	O
two	O
different	O
radar	B-Application
modulator	B-Application
designs	O
:	O
</s>
<s>
High	O
voltage	O
switch	O
for	O
non-coherent	O
keyed	O
power-oscillators	O
These	O
modulators	B-Application
consist	O
of	O
a	O
high	O
voltage	O
pulse	O
generator	O
formed	O
from	O
a	O
high	O
voltage	O
supply	O
,	O
a	O
pulse	O
forming	O
network	O
,	O
and	O
a	O
high	O
voltage	O
switch	O
such	O
as	O
a	O
thyratron	O
.	O
</s>
<s>
They	O
generate	O
short	O
pulses	O
of	O
power	O
to	O
feed	O
,	O
e.g.	O
,	O
the	O
magnetron	B-Architecture
,	O
a	O
special	O
type	O
of	O
vacuum	B-General_Concept
tube	O
that	O
converts	O
DC	O
(	O
usually	O
pulsed	O
)	O
into	O
microwaves	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
radar	B-Application
transmitter	O
must	O
be	O
a	O
power-amplifier	O
,	O
e.g.	O
,	O
a	O
klystron	O
or	O
a	O
solid	O
state	O
transmitter	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
the	O
transmitted	O
pulse	O
is	O
intrapulse-modulated	O
and	O
the	O
radar	B-Application
receiver	O
must	O
use	O
pulse	B-Algorithm
compression	I-Algorithm
techniques	O
.	O
</s>
<s>
This	O
introduces	O
FM	O
modulation	B-Application
which	O
degrades	O
Doppler	O
performance	O
.	O
</s>
<s>
To	O
prevent	O
this	O
,	O
liquid	O
coolant	O
with	O
minimum	O
pressure	O
and	O
flow	O
rate	O
is	O
required	O
,	O
and	O
deionized	O
water	O
is	O
normally	O
used	O
in	O
most	O
high	O
power	O
surface	O
radar	B-Application
systems	I-Application
that	O
use	O
Doppler	O
processing	O
.	O
</s>
<s>
Coolanol	O
(	O
silicate	O
ester	O
)	O
was	O
used	O
in	O
several	O
military	O
radars	B-Application
in	O
the	O
1970s	O
.	O
</s>
<s>
The	O
loss	O
of	O
a	O
U.S.	O
Navy	O
aircraft	O
in	O
1978	O
was	O
attributed	O
to	O
a	O
silicate	O
ester	O
fire	O
.	O
</s>
<s>
The	O
U.S.	O
Navy	O
has	O
instituted	O
a	O
program	O
named	O
Pollution	O
Prevention	O
(	O
P2	O
)	O
to	O
eliminate	O
or	O
reduce	O
the	O
volume	O
and	O
toxicity	O
of	O
waste	O
,	O
air	O
emissions	O
,	O
and	O
effluent	O
discharges	O
.	O
</s>
<s>
Radar	B-Application
(	O
also	O
:	O
RADAR	B-Application
)	O
is	O
defined	O
by	O
article	O
1.100	O
of	O
the	O
International	O
Telecommunication	O
Union	O
's	O
(	O
ITU	O
)	O
ITU	O
Radio	B-Algorithm
Regulations	O
(	O
RR	O
)	O
as	O
:	O
</s>
<s>
Radar	B-Application
come	O
in	O
a	O
variety	O
of	O
configurations	O
in	O
the	O
emitter	O
,	O
the	O
receiver	O
,	O
the	O
antenna	O
,	O
wavelength	O
,	O
scan	O
strategies	O
,	O
etc	O
.	O
</s>
