Complex sentence: The newest models (TR-1, U- 2R, U- 2S) entered service in the 1980s with the latest model, the U- 2S, receiving its technical upgrade in 2012.
Split into simpler sentences: The newest models (TR-1, U- 2R, U- 2S) entered service in the 1980s. The current model, the U- 2S, received its most recent technical upgrade in 2012.