I know its semantics, but the "no-op" _doesn't_ do anything.  The
Command Loss Timer is simply looking for a command, any command.  A
"no-op" is simply a spacecraft command that drops bits into the big
bit bucket in the sky.  "No-op" also get used as timekeepers to provide
millisecond delays between command sequences (used on the thruster preps
on GRO, er, Compton) and to verify command links at the beginning of
TDRS events.  All in all, a rather useful command.  And, an intelligent
FDC test on Galileo (the Command Loss Timer).