<s>
Radar	B-Algorithm
calibration	I-Algorithm
satellites	I-Algorithm
are	O
orbital	O
satellites	B-Application
used	O
to	O
calibrate	O
ground-based	O
space	O
surveillance	O
radars	O
.	O
</s>
<s>
Passive	O
calibration	O
satellites	B-Application
are	O
objects	O
of	O
known	O
shape	O
and	O
size	O
.	O
</s>
<s>
These	O
are	O
generally	O
solid	O
or	O
hollow	O
spheres	O
,	O
since	O
that	O
shape	O
will	O
have	O
the	O
same	O
cross-section	O
regardless	O
of	O
viewing	O
angle	O
,	O
though	O
later	O
passive	O
satellites	B-Application
used	O
wire	O
grid	O
designs	O
.	O
</s>
<s>
A	O
ground	O
radar	O
calculates	O
the	O
satellite	B-Application
's	O
position	O
using	O
knowledge	O
of	O
the	O
satellite	B-Application
's	O
radar	O
cross	O
section	O
,	O
and	O
this	O
is	O
compared	O
with	O
the	O
satellite	B-Application
's	O
known	O
position	O
and	O
velocity	O
.	O
</s>
<s>
Active	O
calibration	O
satellites	B-Application
are	O
equipped	O
with	O
transponders	B-Architecture
that	O
emit	O
a	O
signal	O
on	O
command	O
.	O
</s>
<s>
The	O
ground	O
radar	O
station	O
submits	O
a	O
transmit	O
command	O
and	O
takes	O
a	O
measurement	O
of	O
the	O
satellite	B-Application
's	O
location	O
.	O
</s>
<s>
The	O
transponder	B-Architecture
's	O
signal	O
is	O
received	O
by	O
a	O
radar	O
receiver	O
and	O
combined	O
with	O
the	O
satellite	B-Application
's	O
ephemeris	O
data	O
to	O
calculate	O
the	O
satellite	B-Application
's	O
actual	O
position	O
,	O
and	O
this	O
is	O
then	O
compared	O
with	O
the	O
position	O
calculated	O
by	O
the	O
radar	O
measurement	O
in	O
order	O
to	O
measure	O
the	O
radar	O
's	O
accuracy	O
.	O
</s>
<s>
Examples	O
include	O
the	O
SURCAL	O
series	O
developed	O
by	O
the	O
Naval	O
Research	O
Laboratory	O
and	O
RADCAL	B-Algorithm
,	O
.	O
</s>
