<s>
Space-based	B-Algorithm
radar	I-Algorithm
or	O
spaceborne	O
radar	B-Application
is	O
a	O
radar	B-Application
operating	O
in	O
outer	O
space	O
;	O
</s>
<s>
Earth	O
orbiting	B-Algorithm
radar	I-Algorithm
is	O
a	O
radar	B-Application
in	O
geocentric	O
orbit	B-Application
.	O
</s>
<s>
A	O
number	O
of	O
Earth-observing	O
satellites	B-Application
,	O
such	O
as	O
RADARSAT	O
,	O
have	O
employed	O
synthetic	B-Application
aperture	I-Application
radar	I-Application
(	O
SAR	O
)	O
to	O
obtain	O
terrain	O
and	O
land-cover	O
information	O
about	O
the	O
Earth	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
Discoverer	O
II	O
was	O
a	O
proposed	O
military	O
space-based	B-Algorithm
radar	I-Algorithm
program	O
initiated	O
in	O
February	O
1998	O
as	O
a	O
joint	O
Air	O
Force	O
,	O
DARPA	O
,	O
and	O
NRO	O
program	O
.	O
</s>
<s>
Space-based	B-Algorithm
radar	I-Algorithm
(	O
SBR	O
)	O
is	O
a	O
proposed	O
constellation	O
of	O
active	O
radar	B-Application
satellites	B-Application
for	O
the	O
United	O
States	O
Department	O
of	O
Defense	O
.	O
</s>
<s>
Use	O
of	O
radar	B-Application
sensor	O
for	O
Earth	O
observation	O
purposes	O
was	O
started	O
by	O
NASA/JPL	O
'	O
s	O
Seasat	B-Algorithm
satellite	B-Application
,	O
which	O
carried	O
three	O
different	O
radar	B-Application
sensors	O
:	O
</s>
<s>
After	O
Seasat	B-Algorithm
,	O
SARs	O
,	O
altimeters	O
and	O
scatterometers	O
have	O
been	O
flown	O
on	O
several	O
other	O
space	O
missions	O
.	O
</s>
<s>
While	O
the	O
SAR	O
,	O
in	O
principle	O
,	O
is	O
similar	O
to	O
its	O
airborne	O
counterparts	O
(	O
with	O
the	O
advantage	O
of	O
the	O
increased	O
coverage	O
and	O
worldwide	O
access	O
offered	O
by	O
the	O
satellite	B-Application
platform	O
)	O
,	O
the	O
other	O
two	O
are	O
specific	O
to	O
satellite	B-Application
operations	O
.	O
</s>
<s>
A	O
satellite	B-Application
radar-altimeter	O
is	O
a	O
nadir-looking	O
radar	B-Application
with	O
very	O
high	O
range	O
resolution	O
,	O
which	O
measures	O
the	O
ocean	O
surface	O
topography	O
with	O
an	O
accuracy	O
in	O
the	O
order	O
of	O
few	O
centimeters	O
.	O
</s>
<s>
Some	O
radar-altimeters	O
(	O
like	O
CryoSat/SIRAL	O
)	O
employ	O
synthetic	B-Application
aperture	I-Application
and/or	O
interferometric	O
techniques	O
:	O
their	O
reduced	O
footprint	O
allows	O
mapping	O
of	O
rougher	O
surfaces	O
like	O
polar	O
ices	O
.	O
</s>
<s>
A	O
wind	O
scatterometer	O
observes	O
the	O
same	O
portion	O
of	O
the	O
ocean	O
surface	O
from	O
different	O
(	O
at	O
least	O
3	O
)	O
angles	O
of	O
view	O
as	O
the	O
satellite	B-Application
passes	O
by	O
,	O
measuring	O
the	O
echo	O
amplitude	O
and	O
the	O
corresponding	O
surface	O
reflectivity	O
.	O
</s>
<s>
These	O
three	O
types	O
of	O
radar	B-Application
are	O
currently	O
used	O
on	O
several	O
satellites	B-Application
.	O
</s>
<s>
Data	O
from	O
radar	B-Application
altimeters	O
are	O
used	O
for	O
the	O
accurate	O
determination	O
of	O
the	O
geoid	O
,	O
monitoring	O
of	O
tides	O
,	O
ocean	O
currents	O
and	O
other	O
large-scale	O
ocean	O
phenomena	O
such	O
as	O
El	O
Niño	O
.	O
</s>
<s>
SARs	O
applications	O
are	O
many	O
:	O
they	O
range	O
from	O
geology	O
to	O
crop	O
monitoring	O
,	O
from	O
measurement	O
of	O
sea	O
ice	O
to	O
disaster	O
monitoring	O
to	O
vessel	O
traffic	O
surveillance	O
,	O
not	O
to	O
forget	O
the	O
military	O
applications	O
(	O
many	O
civilian	O
SAR	B-Application
satellites	I-Application
are	O
,	O
in	O
fact	O
,	O
dual-use	O
systems	O
)	O
.	O
</s>
<s>
Additionally	O
,	O
SAR	O
interferometry	O
(	O
both	O
dual-pass	O
or	O
single-pass	O
,	O
as	O
used	O
in	O
the	O
SRTM	B-Algorithm
mission	O
)	O
allows	O
accurate	O
3-D	O
Reconstruction	O
.	O
</s>
<s>
Other	O
types	O
of	O
radars	B-Application
have	O
been	O
flown	O
for	O
Earth	O
observation	O
missions	O
:	O
precipitation	O
radars	B-Application
such	O
as	O
the	O
Tropical	O
Rainfall	O
Measuring	O
Mission	O
,	O
or	O
cloud	O
radars	B-Application
like	O
the	O
one	O
used	O
on	O
Cloudsat	O
.	O
</s>
<s>
Like	O
other	O
Earth	O
observation	O
satellites	B-Application
,	O
radar	B-Application
satellites	B-Application
often	O
use	O
Sun-synchronous	O
orbits	B-Application
so	O
that	O
diurnal	O
variations	O
of	O
vegetation	O
are	O
ignored	O
,	O
allowing	O
long-term	O
variations	O
to	O
be	O
more	O
accurately	O
measured	O
.	O
</s>
<s>
Earth-observing	O
radar	B-Application
satellites	B-Application
include	O
:	O
</s>
<s>
Most	O
of	O
the	O
radars	B-Application
flown	O
as	O
payload	O
in	O
planetary	O
missions	O
(	O
i.e.	O
,	O
not	O
considering	O
avionics	O
radar	B-Application
,	O
such	O
as	O
docking	O
and	O
landing	O
radars	B-Application
used	O
in	O
Apollo	O
and	O
LEM	O
)	O
belong	O
to	O
two	O
categories	O
:	O
imaging	B-Algorithm
radars	I-Algorithm
and	O
sounders	O
.	O
</s>
<s>
Imaging	B-Algorithm
radars	I-Algorithm
:	O
Synthetic	B-Application
aperture	I-Application
radars	I-Application
are	O
the	O
only	O
instruments	O
capable	O
of	O
penetrating	O
heavy	O
cloud	O
cover	O
around	O
planets	O
such	O
as	O
Venus	B-Application
,	O
which	O
was	O
the	O
first	O
target	O
for	O
such	O
missions	O
.	O
</s>
<s>
Two	O
Soviet	O
spacecraft	O
(	O
Venera	O
15	O
and	O
Venera	O
16	O
)	O
imaged	O
the	O
planet	O
in	O
1983	O
and	O
1984	O
using	O
SAR	O
and	O
Radar	B-Application
altimeters	O
.	O
</s>
<s>
The	O
Magellan	B-Architecture
probe	I-Architecture
also	O
imaged	O
Venus	B-Application
in	O
1990	O
and	O
1994	O
.	O
</s>
<s>
The	O
only	O
other	O
target	O
of	O
an	O
imaging	B-Algorithm
radar	I-Algorithm
mission	O
has	O
been	O
Titan	O
,	O
the	O
largest	O
moon	O
of	O
Saturn	B-Device
,	O
in	O
order	O
to	O
penetrate	O
its	O
opaque	O
atmosphere	O
.	O
</s>
<s>
The	O
radar	B-Application
of	O
the	O
Cassini	O
probe	O
,	O
which	O
orbited	O
Saturn	B-Device
between	O
2004	O
and	O
2017	O
,	O
provided	O
images	O
of	O
Titan	O
's	O
surface	O
during	O
each	O
fly-by	O
of	O
the	O
moon	O
.	O
</s>
<s>
The	O
Cassini	O
radar	B-Application
was	O
a	O
multimode	O
system	O
and	O
could	O
operate	O
as	O
Synthetic	B-Application
Aperture	I-Application
Radar	I-Application
,	O
radar	B-Application
altimeter	O
,	O
scatterometer	O
and	O
radiometer	O
.	O
</s>
<s>
Sounding	O
radars	B-Application
:	O
these	O
are	O
low-frequency	O
(	O
normally	O
,	O
HF	O
-	O
3	O
to	O
30MHz	O
-	O
or	O
lower	O
)	O
ground-penetrating	O
Radars	B-Application
,	O
used	O
to	O
acquire	O
data	O
about	O
the	O
planet	O
sub-surface	O
structure	O
.	O
</s>
<s>
Synthetic	B-Application
aperture	I-Application
techniques	O
are	O
normally	O
exploited	O
to	O
reduce	O
the	O
ground	O
footprint	O
(	O
due	O
to	O
the	O
low	O
operating	O
frequency	O
and	O
the	O
small	O
allowable	O
antenna	O
dimensions	O
,	O
the	O
beam	O
is	O
very	O
wide	O
)	O
and	O
,	O
thus	O
,	O
the	O
unwanted	O
echo	O
from	O
other	O
surface	O
objects	O
.	O
</s>
<s>
The	O
first	O
radar	B-Application
sounder	O
flown	O
was	O
ALSE	B-Algorithm
(	O
Apollo	B-Algorithm
Lunar	I-Algorithm
Sounder	I-Algorithm
Experiment	I-Algorithm
)	O
on	O
board	O
Apollo	O
17	O
in	O
1972	O
.	O
</s>
<s>
Other	O
sounder	O
instruments	O
flown	O
(	O
in	O
this	O
case	O
around	O
Mars	B-Application
)	O
,	O
are	O
MARSIS	O
(	O
Mars	B-Application
Advanced	O
Radar	B-Application
for	O
SubSurface	O
and	O
Ionosphere	O
Sounding	O
)	O
on	O
board	O
the	O
European	O
Space	O
Agency	O
's	O
Mars	B-Application
Express	O
probe	O
,	O
and	O
SHARAD	O
(	O
mars	B-Application
SHAllow	O
RADar	B-Application
sounder	O
)	O
on	O
JPL	O
's	O
Mars	B-Application
Reconnaissance	O
Orbiter	O
(	O
MRO	O
)	O
.	O
</s>
<s>
A	O
radar	B-Application
sounder	O
is	O
also	O
used	O
on	O
the	O
Japanese	O
Moon	O
probe	O
SELENE	B-Device
,	O
launched	O
September	O
14	O
,	O
2007	O
.	O
</s>
