<s>
Sugar	O
Tree	O
is	O
the	O
name	O
of	O
a	O
bistatic	B-Algorithm
over-the-horizon	B-Architecture
radar	I-Architecture
built	O
by	O
the	O
US	O
in	O
the	O
1960s	O
.	O
</s>
<s>
The	O
key	O
idea	O
in	O
Sugar	O
Tree	O
was	O
a	O
reinvention	O
of	O
the	O
Klein	B-Algorithm
Heidelberg	I-Algorithm
Nazi	O
German	O
passive	B-Algorithm
radar	I-Algorithm
system	O
developed	O
for	O
use	O
in	O
the	O
Second	O
World	O
War	O
.	O
</s>
<s>
Sugar	O
Tree	O
was	O
a	O
"	O
covert	O
hitchhiker	O
using	O
Soviet	O
,	O
surface-wave	O
HF	B-Algorithm
radio	I-Algorithm
broadcast	O
signals	O
and	O
a	O
remote	O
sky-wave	O
receiver	O
to	O
detect	O
Soviet	O
ballistic	O
missile	O
launches	O
"	O
.	O
</s>
