<s>
Over-the-horizon	B-Architecture
radar	I-Architecture
(	O
OTH	O
)	O
,	O
sometimes	O
called	O
beyond	O
the	O
horizon	B-Algorithm
radar	B-Application
(	O
BTH	O
)	O
,	O
is	O
a	O
type	O
of	O
radar	B-Application
system	I-Application
with	O
the	O
ability	O
to	O
detect	O
targets	O
at	O
very	O
long	O
ranges	O
,	O
typically	O
hundreds	O
to	O
thousands	O
of	O
kilometres	O
,	O
beyond	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
,	O
which	O
is	O
the	O
distance	O
limit	O
for	O
ordinary	O
radar	B-Application
.	O
</s>
<s>
Several	O
OTH	B-Architecture
radar	I-Architecture
systems	O
were	O
deployed	O
starting	O
in	O
the	O
1950s	O
and	O
1960s	O
as	O
part	O
of	O
early	B-Algorithm
warning	I-Algorithm
radar	I-Algorithm
systems	O
,	O
but	O
these	O
have	O
generally	O
been	O
replaced	O
by	O
airborne	O
early	O
warning	O
systems	O
.	O
</s>
<s>
OTH	B-Architecture
radars	I-Architecture
have	O
recently	O
been	O
making	O
a	O
comeback	O
,	O
as	O
the	O
need	O
for	O
accurate	O
long-range	O
tracking	O
becomes	O
less	O
important	O
with	O
the	O
ending	O
of	O
the	O
Cold	B-Application
War	I-Application
,	O
and	O
less-expensive	O
ground-based	O
radars	B-Application
are	O
once	O
again	O
being	O
considered	O
for	O
roles	O
such	O
as	O
maritime	O
reconnaissance	O
and	O
drug	O
enforcement	O
.	O
</s>
<s>
The	O
frequency	O
of	O
radio	O
waves	O
used	O
by	O
most	O
radars	B-Application
,	O
in	O
the	O
form	O
of	O
microwaves	O
,	O
travel	O
in	O
straight	O
lines	O
.	O
</s>
<s>
This	O
generally	O
limits	O
the	O
detection	O
range	O
of	O
radar	B-Application
systems	I-Application
to	O
objects	O
on	O
their	O
horizon	B-Algorithm
(	O
generally	O
referred	O
to	O
as	O
"	O
line	O
of	O
sight	O
"	O
since	O
the	O
aircraft	O
must	O
be	O
at	O
least	O
theoretically	O
visible	O
to	O
a	O
person	O
at	O
the	O
location	O
and	O
elevation	O
of	O
the	O
radar	B-Application
transmitter	O
)	O
due	O
to	O
the	O
curvature	O
of	O
the	O
Earth	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
radar	B-Application
mounted	O
on	O
top	O
of	O
a	O
mast	O
has	O
a	O
range	O
to	O
the	O
horizon	B-Algorithm
of	O
about	O
,	O
taking	O
into	O
account	O
atmospheric	O
refraction	O
effects	O
.	O
</s>
<s>
If	O
the	O
target	O
is	O
above	O
the	O
surface	O
,	O
this	O
range	O
will	O
be	O
increased	O
accordingly	O
,	O
so	O
a	O
target	O
high	O
can	O
be	O
detected	O
by	O
the	O
same	O
radar	B-Application
at	O
.	O
</s>
<s>
Siting	O
the	O
antenna	O
on	O
a	O
high	O
mountain	O
can	O
increase	O
the	O
range	O
somewhat	O
;	O
but	O
,	O
in	O
general	O
,	O
it	O
is	O
impractical	O
to	O
build	O
radar	B-Application
systems	I-Application
with	O
line-of-sight	O
ranges	O
beyond	O
a	O
few	O
hundred	O
kilometres	O
.	O
</s>
<s>
OTH	B-Architecture
radars	I-Architecture
use	O
various	O
techniques	O
to	O
see	O
beyond	O
that	O
limit	O
.	O
</s>
<s>
Two	O
techniques	O
are	O
most	O
commonly	O
used	O
;	O
shortwave	O
systems	O
that	O
refract	O
their	O
signals	O
off	O
the	O
ionosphere	O
for	O
very	O
long-range	O
detection	O
,	O
and	O
surface	O
wave	O
systems	O
,	O
which	O
use	O
low	O
frequency	O
radio	O
waves	O
that	O
,	O
due	O
to	O
diffraction	O
,	O
follow	O
the	O
curvature	O
of	O
the	O
Earth	O
to	O
reach	O
beyond	O
the	O
horizon	B-Algorithm
.	O
</s>
<s>
These	O
systems	O
achieve	O
detection	O
ranges	O
of	O
the	O
order	O
of	O
a	O
hundred	O
kilometres	O
from	O
small	O
,	O
conventional	O
radar	B-Application
installations	O
.	O
</s>
<s>
They	O
can	O
scan	O
a	O
series	O
of	O
high	B-Algorithm
frequencies	I-Algorithm
using	O
a	O
chirp	O
transmitter	O
.	O
</s>
<s>
The	O
most	O
common	O
type	O
of	O
OTH	B-Architecture
radar	I-Architecture
uses	O
skywave	O
or	O
"	O
skip	O
"	O
propagation	O
,	O
in	O
which	O
shortwave	O
radio	O
waves	O
are	O
refracted	O
off	O
an	O
ionized	O
layer	O
in	O
the	O
atmosphere	O
,	O
the	O
ionosphere	O
.	O
</s>
<s>
Given	O
certain	O
conditions	O
in	O
the	O
atmosphere	O
,	O
radio	O
signals	O
transmitted	O
at	O
an	O
angle	O
into	O
the	O
sky	O
will	O
be	O
refracted	O
towards	O
the	O
ground	O
by	O
the	O
ionosphere	O
,	O
allowing	O
them	O
to	O
return	O
to	O
earth	O
beyond	O
the	O
horizon	B-Algorithm
.	O
</s>
<s>
Only	O
one	O
range	O
of	O
frequencies	O
regularly	O
exhibits	O
this	O
behaviour	O
:	O
the	O
high	B-Algorithm
frequency	I-Algorithm
(	O
HF	O
)	O
or	O
shortwave	O
part	O
of	O
the	O
spectrum	O
from	O
3	O
–	O
30MHz	O
.	O
</s>
<s>
The	O
resolution	O
of	O
any	O
radar	B-Application
depends	O
on	O
the	O
width	O
of	O
the	O
beam	O
and	O
the	O
range	O
to	O
the	O
target	O
.	O
</s>
<s>
For	O
example	O
;	O
a	O
radar	B-Application
with	O
1degree	O
beam	O
width	O
and	O
a	O
target	O
at	O
range	O
will	O
show	O
the	O
target	O
as	O
wide	O
.	O
</s>
<s>
Another	O
problem	O
is	O
that	O
the	O
refraction	O
process	O
is	O
highly	O
dependent	O
on	O
the	O
angle	O
between	O
the	O
signal	O
and	O
the	O
ionosphere	O
,	O
and	O
is	O
generally	O
limited	O
to	O
about	O
2	O
–	O
4degrees	O
off	O
the	O
local	O
horizon	B-Algorithm
.	O
</s>
<s>
Given	O
the	O
losses	O
at	O
each	O
refraction	O
,	O
this	O
"	O
backscatter	O
"	O
signal	O
is	O
extremely	O
small	O
,	O
which	O
is	O
one	O
reason	O
why	O
OTH	B-Architecture
radars	I-Architecture
were	O
not	O
practical	O
until	O
the	O
1960s	O
,	O
when	O
extremely	O
low-noise	O
amplifiers	O
were	O
first	O
being	O
designed	O
.	O
</s>
<s>
This	O
basic	O
concept	O
is	O
used	O
in	O
almost	O
all	O
modern	O
radars	B-Application
,	O
but	O
in	O
the	O
case	O
of	O
OTH	O
systems	O
it	O
becomes	O
considerably	O
more	O
complex	O
due	O
to	O
similar	O
effects	O
introduced	O
by	O
movement	O
of	O
the	O
ionosphere	O
.	O
</s>
<s>
Most	O
systems	O
used	O
a	O
second	O
transmitter	O
broadcasting	O
directly	O
up	O
at	O
the	O
ionosphere	O
to	O
measure	O
its	O
movement	O
and	O
adjust	O
the	O
returns	O
of	O
the	O
main	O
radar	B-Application
in	O
real-time	O
.	O
</s>
<s>
A	O
second	O
type	O
of	O
OTH	B-Architecture
radar	I-Architecture
uses	O
much	O
lower	O
frequencies	O
,	O
in	O
the	O
longwave	B-Algorithm
bands	O
.	O
</s>
<s>
Radio	O
waves	O
at	O
these	O
frequencies	O
can	O
diffract	O
around	O
obstacles	O
and	O
follow	O
the	O
curving	O
contour	O
of	O
the	O
earth	O
,	O
traveling	O
beyond	O
the	O
horizon	B-Algorithm
.	O
</s>
<s>
Like	O
the	O
ionospheric	O
high-frequency	B-Algorithm
systems	O
,	O
the	O
received	O
signal	O
from	O
these	O
ground	O
wave	O
systems	O
is	O
very	O
low	O
,	O
and	O
demands	O
extremely	O
sensitive	O
electronics	O
.	O
</s>
<s>
However	O
,	O
the	O
use	O
of	O
bistatic	B-Algorithm
techniques	O
and	O
computer	O
processing	O
can	O
produce	O
higher	O
resolutions	O
,	O
and	O
has	O
been	O
used	O
beginning	O
in	O
the	O
1990s	O
.	O
</s>
<s>
A	O
greatly	O
improved	O
system	O
,	O
a	O
testbed	O
for	O
an	O
operational	O
radar	B-Application
,	O
was	O
built	O
in	O
1961	O
as	O
MADRE	O
(	O
Magnetic-Drum	O
Radar	B-Application
Equipment	O
)	O
at	O
Chesapeake	O
Bay	O
.	O
</s>
<s>
As	O
the	O
names	O
imply	O
,	O
both	O
of	O
the	O
NRL	O
systems	O
relied	O
on	O
the	O
comparison	O
of	O
returned	O
signals	O
stored	O
on	O
magnetic	B-General_Concept
drums	I-General_Concept
.	O
</s>
<s>
In	O
an	O
attempt	O
to	O
remove	O
clutter	O
from	O
radar	B-Application
displays	O
,	O
many	O
late-war	O
and	O
post-war	O
radar	B-Application
systems	I-Application
added	O
an	O
acoustic	O
delay	O
line	O
that	O
stored	O
the	O
received	O
signal	O
for	O
exactly	O
the	O
amount	O
of	O
time	O
needed	O
for	O
the	O
next	O
signal	O
pulse	O
to	O
arrive	O
.	O
</s>
<s>
This	O
basic	O
concept	O
would	O
work	O
for	O
a	O
long-range	O
radar	B-Application
as	O
well	O
,	O
but	O
had	O
the	O
problem	O
that	O
a	O
delay	O
line	O
has	O
to	O
be	O
mechanically	O
sized	O
to	O
the	O
pulse	O
repetition	O
frequency	O
of	O
the	O
radar	B-Application
,	O
or	O
PRF	O
.	O
</s>
<s>
For	O
this	O
role	O
,	O
the	O
delay	O
line	O
was	O
not	O
usable	O
,	O
and	O
the	O
magnetic	B-General_Concept
drum	I-General_Concept
,	O
recently	O
introduced	O
,	O
provided	O
a	O
convenient	O
and	O
easily	O
controlled	O
variable-delay	O
system	O
.	O
</s>
<s>
This	O
consisted	O
of	O
several	O
antennas	O
positioned	O
to	O
be	O
four	O
wavelengths	O
apart	O
,	O
allowing	O
the	O
system	O
to	O
use	O
phase-shift	O
beam	B-Algorithm
forming	I-Algorithm
to	O
steer	O
the	O
direction	O
of	O
sensitivity	O
and	O
adjust	O
it	O
to	O
cover	O
Singapore	O
,	O
Calcutta	O
and	O
the	O
UK	O
.	O
</s>
<s>
The	O
first	O
truly	O
operational	O
development	O
was	O
an	O
Anglo-American	O
system	O
known	O
as	O
Cobra	B-Algorithm
Mist	I-Algorithm
,	O
which	O
began	O
construction	O
in	O
the	O
late	O
1960s	O
.	O
</s>
<s>
Cobra	B-Algorithm
Mist	I-Algorithm
used	O
an	O
enormous	O
10MW	O
transmitter	O
and	O
could	O
detect	O
aircraft	O
over	O
the	O
western	O
Soviet	O
Union	O
from	O
its	O
location	O
in	O
Suffolk	O
.	O
</s>
<s>
Cobra	O
Shoe	O
was	O
a	O
reported	O
Over	O
The	O
Horizon	B-Algorithm
(	O
Backscatter	O
)	O
(	O
OTH-B	B-Architecture
)	O
radar	B-Application
designed	O
by	O
RCA	O
Corporation	O
,	O
designed	O
to	O
monitor	O
ballistic	O
missile	O
tests	O
in	O
the	O
interior	O
of	O
the	O
Soviet	O
Union	O
,	O
installed	O
in	O
the	O
Western	O
Sovereign	O
Base	O
Area	O
(	O
Akrotiri	O
)	O
,	O
Cyprus	O
.	O
</s>
<s>
the	O
Sugar	B-Algorithm
Tree	I-Algorithm
radar	I-Algorithm
system	O
.	O
</s>
<s>
The	O
United	O
States	O
Air	O
Force	O
Rome	O
Laboratory	O
had	O
the	O
first	O
complete	O
success	O
with	O
their	O
AN/FPS	O
-118	O
OTH-B	B-Architecture
.	O
</s>
<s>
Radar	B-Application
data	O
were	O
fed	O
to	O
the	O
U.S.	O
Customs/Coast	O
Guard	O
C3I	O
Center	O
,	O
Miami	O
;	O
Joint	O
Task	O
Force	O
4	O
Operations	O
Center	O
,	O
Key	O
West	O
;	O
U.S.	O
Southern	O
Command	O
Operations	O
Center	O
,	O
Key	O
West	O
;	O
and	O
U.S.	O
Southern	O
Command	O
Operations	O
Center	O
,	O
Panama	O
.	O
</s>
<s>
With	O
the	O
end	O
of	O
the	O
Cold	B-Application
War	I-Application
,	O
the	O
influence	O
of	O
the	O
two	O
senators	O
from	O
Maine	O
was	O
not	O
enough	O
to	O
save	O
the	O
operation	O
and	O
the	O
Alaska	O
and	O
southern-facing	O
sites	O
were	O
canceled	O
,	O
the	O
two	O
so-far	O
completed	O
western	O
sectors	O
and	O
the	O
eastern	O
ones	O
were	O
turned	O
off	O
and	O
placed	O
in	O
"	O
warm	O
storage	O
,	O
"	O
allowing	O
them	O
to	O
be	O
used	O
again	O
if	O
needed	O
.	O
</s>
<s>
After	O
a	O
period	O
of	O
public	O
input	O
and	O
environmental	O
studies	O
,	O
in	O
July	O
2005	O
the	O
U.S.	O
Air	O
Force	O
Air	O
Combat	O
Command	O
published	O
a	O
"	O
Final	O
Environmental	O
Assessment	O
for	O
Equipment	O
Removal	O
at	O
Over-the-Horizon	B-Architecture
Backscatter	O
Radar	B-Application
-	O
West	O
Coast	O
Facilities	O
"	O
.	O
</s>
<s>
A	O
final	O
decision	O
was	O
made	O
to	O
remove	O
all	O
radar	B-Application
equipment	O
at	O
the	O
west	O
coast	O
sector	O
's	O
transmitter	O
site	O
at	O
Christmas	O
Valley	O
Air	O
Force	O
Station	O
outside	O
Christmas	O
Valley	O
,	O
Oregon	O
and	O
its	O
receiver	O
site	O
near	O
Tulelake	O
,	O
California	O
.	O
</s>
<s>
In	O
2018	O
,	O
development	O
started	O
on	O
the	O
high-frequency	B-Algorithm
Tactical	O
Multi-Mission	O
Over	B-Architecture
the	I-Architecture
Horizon	I-Architecture
Radar	I-Architecture
(	O
TACMOR	O
)	O
,	O
a	O
technology	O
prototype	O
to	O
expand	O
air	O
and	O
maritime	O
awareness	O
over	O
the	O
Western	O
Pacific	O
.	O
</s>
<s>
In	O
2022	O
construction	O
of	O
a	O
TACMOR	O
radar	B-Application
station	I-Application
in	O
Palau	O
was	O
agreed	O
,	O
expected	O
to	O
be	O
operational	O
in	O
2026	O
.	O
</s>
<s>
The	O
United	O
States	O
Navy	O
created	O
their	O
own	O
system	O
,	O
the	O
AN/TPS	O
-71	O
ROTHR	O
(	O
Relocatable	O
Over-the-Horizon	B-Architecture
Radar	I-Architecture
)	O
,	O
which	O
covers	O
a	O
64-degree	O
wedge-shaped	O
area	O
at	O
ranges	O
from	O
500	O
to	O
1,600	O
nautical	O
miles	O
(	O
925	O
to	O
3,000	O
km	O
)	O
.	O
</s>
<s>
Podsolnukh	O
(	O
Sunflower	O
)	O
-	O
Coast-horizon	O
shortwave	O
station	O
short-range	O
radar	B-Application
.	O
</s>
<s>
"	O
Sunflower	O
"	O
allows	O
operators	O
to	O
automatically	O
and	O
simultaneously	O
detect	O
,	O
track	O
and	O
classify	O
up	O
to	O
300	O
offshore	O
and	O
100	O
air	O
objects	O
beyond	O
the	O
radio	O
horizon	B-Algorithm
,	O
and	O
provide	O
their	O
coordinates	O
to	O
the	O
targeting	O
systems	O
and	O
armaments	O
of	O
ships	O
and	O
air	O
defense	O
systems	O
.	O
</s>
<s>
The	O
radar	B-Application
passed	O
state	O
tests	O
in	O
2008	O
.	O
</s>
<s>
A	O
more	O
recent	O
addition	O
is	O
the	O
Jindalee	O
Operational	O
Radar	B-Application
Network	O
developed	O
by	O
the	O
Australian	O
Department	O
of	O
Defence	O
in	O
1998	O
and	O
completed	O
in	O
2000	O
.	O
</s>
<s>
1	O
Radar	B-Application
Surveillance	O
Unit	O
of	O
the	O
Royal	O
Australian	O
Air	O
Force	O
.	O
</s>
<s>
Jindalee	O
is	O
a	O
multistatic	B-Algorithm
radar	I-Algorithm
(	O
multiple-receiver	O
)	O
system	O
using	O
OTH-B	B-Architecture
,	O
allowing	O
it	O
to	O
have	O
both	O
long	O
range	O
as	O
well	O
as	O
anti-stealth	O
capabilities	O
.	O
</s>
<s>
Jindalee	O
uses	O
560kW	O
compared	O
to	O
the	O
United	O
States	O
 '	O
OTH-B	B-Architecture
'	O
s	O
1	O
MW	O
,	O
yet	O
offers	O
far	O
better	O
range	O
than	O
the	O
U.S.	O
1980s	O
system	O
,	O
due	O
to	O
the	O
considerably	O
improved	O
electronics	O
and	O
signal	O
processing	O
.	O
</s>
<s>
Canada	O
has	O
been	O
investigating	O
the	O
use	O
of	O
High	B-Algorithm
Frequency	I-Algorithm
Surface	O
Wave	O
Radar	B-Application
(	O
HFSWR	O
)	O
for	O
surveillance	O
of	O
the	O
200	O
nautical	O
mile	O
Exclusive	O
Economic	O
Zone	O
(	O
EEZ	O
)	O
for	O
more	O
than	O
30	O
years	O
.	O
</s>
<s>
International	O
sales	O
of	O
the	O
SWR503	O
radar	B-Application
followed	O
with	O
operational	O
systems	O
installed	O
in	O
Asia	O
(	O
2008	O
)	O
and	O
Europe	O
(	O
2009	O
)	O
.	O
</s>
<s>
The	O
French	O
developed	O
an	O
OTH	B-Architecture
radar	I-Architecture
called	O
NOSTRADAMUS	O
during	O
the	O
1990s	O
(	O
NOSTRADAMUS	O
stands	O
for	O
New	O
Transhorizon	O
Decametric	O
System	O
Applying	O
Studio	O
Methods	O
(	O
French	O
:	O
nouveau	O
système	O
transhorizon	O
décamétrique	O
appliquant	O
les	O
méthodes	O
utilisées	O
en	O
studio	O
)	O
.	O
)	O
</s>
<s>
In	O
March	O
1999	O
,	O
the	O
OTH	B-Architecture
radar	I-Architecture
NOSTRADAMUS	O
was	O
said	O
to	O
have	O
detected	O
two	O
Northrop	O
B2	O
Spirit	O
flying	O
to	O
Kosovo	O
.	O
</s>
<s>
Launched	O
officially	O
in	O
2009	O
,	O
the	O
French	O
developed	O
a	O
new	O
over-the-horizon	B-Architecture
radar	I-Architecture
(	O
High	B-Algorithm
Frequency	I-Algorithm
Surface	O
Wave	O
Radar	B-Application
–	O
HFSWR	O
)	O
capable	O
of	O
monitoring	O
maritime	O
traffic	O
up	O
to	O
200	O
nautical	O
miles	O
offshore	O
.	O
</s>
<s>
A	O
number	O
of	O
OTH-B	B-Architecture
and	O
OTH-SW	O
radars	B-Application
are	O
reportedly	O
in	O
operation	O
in	O
China	O
.	O
</s>
<s>
However	O
,	O
transmission	O
from	O
these	O
radars	B-Application
causes	O
much	O
interference	O
to	O
other	O
international	O
licensed	O
users	O
.	O
</s>
<s>
One	O
set	O
of	O
Chinese	O
OTH-B	B-Architecture
radars	I-Architecture
is	O
found	O
on	O
Google	O
Maps	O
for	O
the	O
and	O
.	O
</s>
<s>
Iran	O
is	O
working	O
on	O
an	O
OTH	B-Architecture
radar	I-Architecture
called	O
Sepehr	O
,	O
with	O
a	O
reported	O
range	O
of	O
3,000	O
kilometres	O
.	O
</s>
<s>
The	O
OTH	O
0100	O
Radar	B-Application
is	O
capable	O
of	O
monitoring	O
vessels	O
beyond	O
200	O
MN	O
(	O
370	O
KM	O
)	O
away	O
from	O
shore	O
,	O
exceeding	O
the	O
direct	O
line	O
of	O
sight	O
of	O
conventional	O
radars	B-Application
.	O
</s>
<s>
India	O
has	O
developed	O
a	O
variety	O
of	O
long	O
and	O
short	O
range	O
radars	B-Application
.	O
</s>
<s>
Although	O
it	O
does	O
n't	O
have	O
an	O
operational	O
over	B-Architecture
the	I-Architecture
horizon	I-Architecture
radar	I-Architecture
at	O
present	O
,	O
India	O
's	O
Swordfish	O
Long	O
Range	O
Tracking	O
Radar	B-Application
,	O
a	O
part	O
of	O
India	O
's	O
missile	O
defense	O
system	O
has	O
a	O
maximum	O
range	O
of	O
800	O
kilometers	O
currently	O
being	O
upgraded	O
for	O
1500	O
kilometers	O
.	O
</s>
<s>
DRDO	O
's	O
LRDE	O
is	O
working	O
on	O
a	O
prototype	O
OTH	B-Architecture
radar	I-Architecture
.	O
</s>
<s>
After	O
successful	O
trials	O
of	O
existing	O
system	O
,	O
India	O
is	O
expected	O
to	O
develop	O
a	O
large	O
OTH	B-Architecture
radar	I-Architecture
based	O
on	O
same	O
design	O
.	O
</s>
<s>
Another	O
common	O
application	O
of	O
over-horizon	O
radar	B-Application
uses	O
surface	O
waves	O
,	O
also	O
known	O
as	O
groundwaves	O
.	O
</s>
<s>
This	O
type	O
of	O
radar	B-Application
,	O
surface-wave	O
OTH	O
,	O
is	O
used	O
for	O
surveillance	O
,	O
and	O
operates	O
most	O
commonly	O
between	O
4	O
and	O
20MHz	O
.	O
</s>
<s>
Lower	O
frequencies	O
enjoy	O
better	O
propagation	O
but	O
poorer	O
radar	B-Application
reflection	O
from	O
small	O
targets	O
,	O
so	O
there	O
is	O
usually	O
an	O
optimum	O
frequency	O
that	O
depends	O
on	O
the	O
type	O
of	O
target	O
.	O
</s>
<s>
A	O
different	O
approach	O
to	O
over-the-horizon	B-Architecture
radar	I-Architecture
is	O
to	O
use	O
creeping	O
waves	O
or	O
electromagnetic	O
surface	O
waves	O
at	O
much	O
lower	O
frequencies	O
.	O
</s>
<s>
In	O
the	O
radar	B-Application
role	O
,	O
the	O
creeping	O
waves	O
in	O
question	O
are	O
diffracting	O
around	O
the	O
Earth	O
,	O
although	O
processing	O
the	O
returned	O
signal	O
is	O
difficult	O
.	O
</s>
<s>
A	O
newer	O
system	O
has	O
recently	O
been	O
used	O
for	O
coastal	O
surveillance	O
in	O
Canada	O
,	O
and	O
is	O
now	O
offered	O
for	O
sales	O
by	O
Maerospace	O
,	O
Australia	O
has	O
also	O
deployed	O
a	O
High	B-Algorithm
Frequency	I-Algorithm
Surface	O
Wave	O
Radar	B-Application
.	O
</s>
