The M1A2 Abrams program incorporated improved frontal armor protection, improved mission electronics on a digital data bus, and implementation of the independent thermal sight (ITS) (M1A1s had the mount in place but was never used until the M1A2 upgrade). While 77 new-build M1A2s rolled off the production line, most were upgrades for re-manufactured Abrams. The next round of improvements came as the System Enhancement Package (SEP) which added navigation and situational awareness. The additional mission electronics required improvements to the cooling system, both of which increased the electrical load on the tank. In order to operate the tank without the main engine operating constantly, an auxiliary power unit was added to the rear turret basket.