Student or Learner
However, the satellite system only becomes cost competitive with microwave radio and optical fiber systems when the distance is large (e.g., greater than 500 km).
Is "cost" used as an adverb here? Is it correct?
*NOT A TEACHER*
It isn't an adverb in this context. It modifies "competetive" by telling us in what sense the satellite system becomes competitive. I call it an adjective, although some might choose to call "cost competitive" a compound noun.