<s>
GNSS	B-General_Concept
reflectometry	I-General_Concept
(	O
or	O
GNSS-R	B-General_Concept
)	O
involves	O
making	O
measurements	O
from	O
the	O
reflections	O
from	O
the	O
Earth	O
of	O
navigation	O
signals	O
from	O
Global	O
Navigation	O
Satellite	O
Systems	O
such	O
as	O
GPS	O
.	O
</s>
<s>
The	O
idea	O
of	O
using	O
reflected	O
GNSS	O
signal	O
for	O
earth	O
observation	O
became	O
more	O
and	O
more	O
popular	O
in	O
the	O
mid-1990s	O
at	O
NASA	O
Langley	O
Research	O
Center	O
and	O
is	O
also	O
known	O
as	O
GPS	B-General_Concept
reflectometry	I-General_Concept
.	O
</s>
<s>
GNSS	B-General_Concept
reflectometry	I-General_Concept
is	O
passive	B-Algorithm
sensing	I-Algorithm
that	O
takes	O
advantage	O
of	O
and	O
relies	O
on	O
separate	O
active	O
sources	O
-	O
the	O
satellites	O
generating	O
the	O
navigation	O
signals	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
Delay	O
Doppler	O
Map	O
(	O
DDM	O
)	O
can	O
be	O
obtained	O
as	O
GNSS-R	B-General_Concept
observable	O
.	O
</s>
<s>
GNSS	B-General_Concept
reflectometry	I-General_Concept
works	O
as	O
a	O
bi-static	B-Algorithm
radar	I-Algorithm
,	O
where	O
transmitter	O
and	O
receiver	O
are	O
separated	O
by	O
a	O
significant	O
distance	O
.	O
</s>
<s>
Since	O
in	O
GNSS	B-General_Concept
reflectometry	I-General_Concept
one	O
receiver	O
simultaneously	O
can	O
track	O
multiple	O
transmitters	O
(	O
i.e.	O
</s>
<s>
GNSS	O
satellites	O
)	O
,	O
the	O
system	O
also	O
has	O
the	O
nature	O
of	O
multi-static	B-Algorithm
radar	I-Algorithm
.	I-Algorithm
</s>
