Complex sentence: Its headquarters are located 30 km. north of Elverum in the south east Norway, at "Rena leir" military base, which received its first active units in 1997 after being built in 1993--96.
Split into simpler sentences: Its headquarters are located 30 km. north of Elverum in the south east Norway, at "Rena leir" military base. Headquarters are at "Rena leir" military base, which received its first active units in 1997 after the base had been constructed in 1993--96.