Complex sentence: The system entered service in the early 1980s and served as the primary early-warning system into the 1990s, although it was intended that it would be replaced by the second-generation US-KMO, which proved to be completely unreliable.
Split into simpler sentences: The system entered service in the early 1980s and served as the primary early-warning system into the 1990s. It was originally intended that it would be replaced by the second-generation US-KMO, but these proved to be completely unreliable.