<s>
A	O
radio	B-Application
science	I-Application
subsystem	I-Application
(	O
RSS	O
)	O
is	O
a	O
subsystem	O
placed	O
on	O
board	O
a	O
spacecraft	O
for	O
radio	O
science	O
purposes	O
.	O
</s>
<s>
This	O
requires	O
a	O
highly	O
stable	O
oscillator	O
on	O
the	O
spacecraft	O
,	O
or	O
more	O
commonly	O
a	O
"	O
2-way	O
coherent	O
"	O
transponder	B-Architecture
that	O
phase	B-Algorithm
locks	I-Algorithm
the	O
transmitted	O
signal	O
frequency	O
to	O
a	O
rational	O
multiple	O
of	O
a	O
received	O
uplink	B-Application
signal	O
that	O
usually	O
also	O
carries	O
spacecraft	O
commands	O
.	O
</s>
<s>
Estimate	O
masses	O
of	O
celestial	O
satellites	B-Application
that	O
do	O
not	O
have	O
satellites	B-Application
of	O
their	O
own	O
.	O
</s>
