At the same time, the ROK Navy steadily upgraded its naval forces: In order to strengthen the surface combatants, the ROK Navy launched the lead ship (DDH 971) of the "Kwanggaeto the Great"-class destroyer in 1996 to replace the former USN destroyers. For building up a submarine force, the ROK Navy acquired its first submarine (other than midget submarines), ROKS "Chang Bogo" (SS 061), from Howaldtswerke-Deutsche Werft of Germany in 1992. In order to replace its antiquated S-2 Tracker anti-submarine warfare aircraft, the ROK Navy purchased a squadron of P-3C Orion maritime patrol aircraft, which were delivered starting in 1995.