sentences:  The attrition was stemmed to some extent by extensive upgrade programs on the MiG-21 ("bis" upgrade), MiG-27, Jaguar ("DARIN I" upgrade) and other aircraft in the mid-1990s.An aggressive program of upgrades was initiated.The MiG-21bis aircraft have been upgraded to MiG-21 Bison levels (125 aircraft will be upgraded to that standard), Jaguars to "DARIN III" standards, and Mirage 2000s and MiG-27s were also upgraded.