<s>
Wave	B-Algorithm
radar	I-Algorithm
is	O
a	O
type	O
of	O
radar	B-Application
for	O
measuring	O
wind	O
waves	O
.	O
</s>
<s>
This	O
article	O
(	O
see	O
also	O
Grønlie	O
2004	O
)	O
,	O
gives	O
a	O
brief	O
description	O
of	O
the	O
most	O
common	O
ground-based	O
radar	B-Application
remote	B-Application
sensing	I-Application
techniques	O
.	O
</s>
<s>
Instruments	O
based	O
on	O
radar	B-Application
remote	B-Application
sensing	I-Application
techniques	O
have	O
become	O
of	O
particular	O
interest	O
in	O
applications	O
where	O
it	O
is	O
important	O
to	O
avoid	O
direct	O
contact	O
with	O
the	O
water	O
surface	O
and	O
avoid	O
structural	O
interference	O
.	O
</s>
<s>
Basically	O
there	O
are	O
two	O
different	O
classes	O
of	O
radar	B-Application
remote	B-Application
sensors	I-Application
for	O
ocean	O
waves	O
.	O
</s>
<s>
Indirect	O
sensors	O
observe	O
the	O
surface	O
waves	O
via	O
the	O
interaction	O
with	O
some	O
other	O
physical	O
process	O
as	O
for	O
example	O
the	O
radar	B-Application
cross	O
section	O
of	O
the	O
sea	O
surface	O
.	O
</s>
<s>
Microwave	B-Application
radars	I-Application
may	O
be	O
used	O
in	O
two	O
different	O
modes	O
;	O
</s>
<s>
The	O
radar	B-Application
echo	O
is	O
generated	O
by	O
specular	O
reflections	O
from	O
the	O
sea	O
surface	O
.	O
</s>
<s>
The	O
radar	B-Application
echo	O
is	O
generated	O
by	O
Bragg	O
scattering	O
,	O
hence	O
wind	O
generated	O
surface	O
ripple	O
(	O
capillary	O
waves	O
)	O
must	O
be	O
present	O
.	O
</s>
<s>
An	O
excellent	O
presentation	O
of	O
the	O
theories	O
of	O
microwave	O
remote	B-Application
sensing	I-Application
of	O
the	O
sea	O
surface	O
is	O
given	O
by	O
Plant	O
and	O
Shuler	O
(	O
1980	O
)	O
.	O
</s>
<s>
The	O
radar	B-Application
footprint	O
(	O
the	O
size	O
of	O
the	O
surface	O
area	O
which	O
is	O
illuminated	O
by	O
the	O
radar	B-Application
)	O
must	O
be	O
small	O
in	O
comparison	O
with	O
all	O
ocean	O
wavelengths	O
of	O
interest	O
.	O
</s>
<s>
The	O
radar	B-Application
spatial	O
resolution	O
is	O
determined	O
by	O
the	O
bandwidth	O
of	O
the	O
radar	B-Application
signal	O
(	O
see	O
radar	B-Algorithm
signal	I-Algorithm
characteristics	I-Algorithm
)	O
and	O
the	O
beamwidth	O
of	O
the	O
radar	B-Application
antenna	O
.	O
</s>
<s>
For	O
all	O
practical	O
purposes	O
,	O
the	O
beam	O
of	O
an	O
IR	O
radar	B-Application
(	O
laser	O
)	O
does	O
not	O
diverge	O
.	O
</s>
<s>
HF	B-Algorithm
radars	B-Application
utilize	O
the	O
Bragg	O
scattering	O
mechanism	O
and	O
do	O
always	O
operate	O
at	O
very	O
low	O
grazing	O
angles	O
.	O
</s>
<s>
Due	O
to	O
the	O
low	O
frequency	O
of	O
operation	O
the	O
radar	B-Application
waves	O
are	O
backscattered	O
directly	O
from	O
the	O
gravity	O
waves	O
and	O
surface	O
ripple	O
need	O
not	O
be	O
present	O
.	O
</s>
<s>
Radar	B-Application
transceivers	O
may	O
be	O
coherent	O
or	O
non-coherent	O
.	O
</s>
<s>
Coherent	O
radars	B-Application
measure	O
Doppler-modulation	O
as	O
well	O
as	O
amplitude	O
modulation	O
,	O
while	O
non-coherent	O
radars	B-Application
only	O
measure	O
amplitude	O
modulation	O
.	O
</s>
<s>
Consequently	O
,	O
a	O
non-coherent	O
radar	B-Application
echo	O
contains	O
less	O
information	O
about	O
the	O
sea	O
surface	O
properties	O
.	O
</s>
<s>
Examples	O
of	O
non-coherent	O
radars	B-Application
are	O
conventional	O
marine	O
navigation	O
radars	B-Application
.	O
</s>
<s>
The	O
radar	B-Application
transmitter	O
waveform	O
may	O
be	O
either	O
unmodulated	O
continuous	O
wave	O
,	O
modulated	O
or	O
pulsed	O
.	O
</s>
<s>
An	O
unmodulated	O
continuous	O
wave	B-Algorithm
radar	I-Algorithm
has	O
no	O
range	O
resolution	O
,	O
but	O
can	O
resolve	O
targets	O
on	O
the	O
basis	O
of	O
different	O
velocity	O
,	O
while	O
a	O
modulated	O
or	O
pulsed	O
radar	B-Application
can	O
resolve	O
echoes	O
from	O
different	O
ranges	O
.	O
</s>
<s>
The	O
radar	B-Application
waveform	O
plays	O
a	O
very	O
important	O
role	O
in	O
radar	B-Application
theory	O
(	O
Plant	O
and	O
Shuler	O
,	O
1980	O
)	O
.	O
</s>
<s>
An	O
excellent	O
survey	O
of	O
different	O
radar	B-Application
techniques	O
for	O
remote	B-Application
sensing	I-Application
of	O
waves	O
is	O
given	O
by	O
Tucker	O
(	O
1991	O
)	O
.	O
</s>
<s>
Microwave	O
rangefinders	B-Algorithm
also	O
operate	O
in	O
vertical	O
mode	O
at	O
GHz	O
frequencies	O
and	O
are	O
not	O
as	O
affected	O
by	O
fog	O
and	O
water	O
spray	O
as	O
the	O
laser	O
rangefinder	B-Algorithm
.	O
</s>
<s>
A	O
continuous	O
wave	O
frequency	O
modulated	O
(	O
CWFM	O
)	O
or	O
pulsed	O
radar	B-Application
waveform	O
is	O
normally	O
used	O
to	O
provide	O
range	O
resolution	O
.	O
</s>
<s>
Another	O
example	O
is	O
the	O
,	O
which	O
is	O
a	O
derivative	O
of	O
a	O
Rosemount	O
tank	O
radar	B-Application
.	O
</s>
<s>
An	O
array	O
of	O
three	O
vertical	O
radars	B-Application
,	O
having	O
footprints	O
at	O
the	O
vertices	O
of	O
a	O
horizontal	O
,	O
equilateral	O
triangle	O
,	O
can	O
provide	O
the	O
necessary	O
data	O
on	O
water	O
surface	O
elevation	O
.	O
</s>
<s>
“	O
Directional	O
WaveGuide	O
”	O
is	O
a	O
commercial	O
radar	B-Application
system	I-Application
based	O
on	O
this	O
technique	O
.	O
</s>
<s>
Marine	O
navigation	O
radars	B-Application
(	O
X	B-Algorithm
band	I-Algorithm
)	O
provide	O
sea	O
clutter	O
images	O
which	O
contain	O
a	O
pattern	O
resembling	O
a	O
sea	O
wave	O
pattern	O
.	O
</s>
<s>
By	O
digitizing	O
the	O
radar	B-Application
video	O
signal	O
it	O
can	O
be	O
processed	O
by	O
a	O
digital	O
computer	O
.	O
</s>
<s>
The	O
marine	O
navigation	O
radar	B-Application
operates	O
in	O
low	O
grazing	O
angle	O
mode	O
and	O
wind	O
generated	O
surface	O
ripple	O
must	O
be	O
present	O
.	O
</s>
<s>
The	O
marine	O
navigation	O
radar	B-Application
is	O
non-coherent	O
and	O
is	O
a	O
typical	O
example	O
of	O
an	O
indirect	O
wave	O
sensor	O
,	O
because	O
there	O
is	O
no	O
direct	O
relation	O
between	O
wave	O
height	O
and	O
radar	B-Application
back-scatter	O
modulation	O
amplitude	O
.	O
</s>
<s>
Marine	O
navigation	O
radar	B-Application
based	O
wave	O
sensors	O
are	O
excellent	O
tools	O
for	O
wave	O
direction	O
measurements	O
.	O
</s>
<s>
A	O
marine	O
navigation	O
radar	B-Application
may	O
also	O
be	O
a	O
tool	O
for	O
surface	O
current	O
measurements	O
.	O
</s>
<s>
Another	O
example	O
of	O
a	O
marine	O
radar	B-Application
based	O
system	O
is	O
OceanWaves	O
WaMoS	O
II	O
.	O
</s>
<s>
The	O
range	O
gated	O
pulsed	O
Doppler	O
microwave	B-Application
radar	I-Application
operates	O
in	O
low	O
grazing	O
angle	O
mode	O
.	O
</s>
<s>
is	O
the	O
only	O
available	O
wave	O
sensor	O
based	O
on	O
the	O
range	O
gated	O
pulsed	O
Doppler	O
radar	B-Application
technique	O
.	O
</s>
<s>
The	O
dual	O
frequency	O
microwave	B-Application
radar	I-Application
transmits	O
two	O
microwave	O
frequencies	O
simultaneously	O
.	O
</s>
<s>
The	O
dual	O
frequency	O
radar	B-Application
may	O
be	O
considered	O
a	O
microwave	O
equivalent	O
of	O
the	O
high	B-Algorithm
frequency	I-Algorithm
(	O
HF	B-Algorithm
)	O
radar	B-Application
(	O
see	O
below	O
)	O
.	O
</s>
<s>
The	O
dual	O
frequency	O
radar	B-Application
is	O
suitable	O
for	O
the	O
measurement	O
of	O
surface	O
current	O
.	O
</s>
<s>
The	O
HF	B-Algorithm
radar	B-Application
and	O
are	O
well	O
established	O
as	O
a	O
powerful	O
tool	O
for	O
sea	O
current	O
measurements	O
up	O
to	O
a	O
range	O
of	O
300km	O
.	O
</s>
<s>
It	O
operates	O
in	O
the	O
HF	B-Algorithm
and	O
low	O
VHF	O
frequencies	O
band	O
corresponding	O
to	O
a	O
radar	B-Application
wavelength	O
in	O
the	O
range	O
of	O
10	O
to	O
300m	O
.	O
</s>
<s>
The	O
Doppler	O
shift	O
of	O
the	O
first	O
order	O
Bragg	O
lines	O
of	O
the	O
radar	B-Application
echo	O
is	O
used	O
to	O
derive	O
sea	O
current	O
estimates	O
in	O
very	O
much	O
the	O
same	O
way	O
as	O
for	O
the	O
dual	O
frequency	O
microwave	B-Application
radar	I-Application
.	O
</s>
<s>
Two	O
radar	B-Application
installations	O
are	O
normally	O
required	O
,	O
looking	O
at	O
the	O
same	O
patch	O
of	O
the	O
sea	O
surface	O
from	O
different	O
angles	O
.	O
</s>
<s>
The	O
latest	O
generation	O
of	O
shore-based	O
ocean	O
radar	B-Application
can	O
reach	O
more	O
than	O
200km	O
for	O
ocean	O
current	O
mapping	O
and	O
more	O
than	O
100km	O
for	O
wave	O
measurements	O
.	O
</s>
<s>
For	O
all	O
ocean	O
radars	B-Application
,	O
the	O
accuracy	O
in	O
range	O
is	O
excellent	O
.	O
</s>
<s>
This	O
technology	O
differs	O
from	O
existing	O
wave	O
forecasting	O
systems	O
by	O
using	O
a	O
customized	O
wave	O
sensing	O
radar	B-Application
capable	O
of	O
measuring	O
backscatter	O
Doppler	O
out	O
to	O
ranges	O
of	O
approximately	O
5km	O
.	O
</s>
<s>
The	O
radar	B-Application
antenna	O
is	O
vertically	O
polarized	O
to	O
enhance	O
the	O
sea-surface	O
backscatter	O
signal	O
.	O
</s>
<s>
It	O
also	O
uses	O
an	O
innovative	O
radar	B-Application
signal	O
processing	O
scheme	O
that	O
addresses	O
the	O
inherently	O
noisy	O
backscatter	O
signals	O
through	O
a	O
mathematical	O
process	O
termed	O
least	O
squares	O
inversion	O
.	O
</s>
<s>
This	O
approach	O
applies	O
a	O
highly	O
over-determined	O
filter	O
to	O
the	O
radar	B-Application
data	O
,	O
and	O
rejects	O
radar	B-Application
scans	O
that	O
do	O
not	O
observe	O
incoming	O
waves	O
.	O
</s>
<s>
Finally	O
,	O
it	O
uses	O
the	O
Gravity	O
open	O
architecture	O
middleware	B-General_Concept
solution	O
to	O
integrate	O
the	O
sensor	O
feeds	O
,	O
processing	O
subroutines	O
and	O
user	O
displays	O
.	O
</s>
