The M1 Abrams first entered US Army service in 1980. An improved version of the M1, the M1A1, was introduced in 1985. The M1A1 has the M256 120 mm smoothbore cannon, improved armor, and a CBRN protection system. The M1A2 is a further improvement of the M1A1 with a commander's independent thermal viewer and weapon station, position navigation equipment, digital data bus and a radio interface unit. Further upgrades include depleted uranium armor for all variants, a system overhaul that returns all A1s to like-new condition (M1A1 AIM), a digital enhancement package for the A1 (M1A1D), a commonality program to standardize parts between the U.S. Army and the Marine Corps (M1A1HC) and an electronic upgrade for the A2 (M1A2 SEP). During Operations Desert Shield and Desert Storm and for Bosnia, some M1A1s were modified with armor upgrades. The M1 can be equipped with mine plow and mine roller attachments if needed.