November29, 2006: The U.S. Navy is spending some $200 million to upgrade and test the Mk 6 guidance systems in its 425 Trident II D5 SLBMs (sea launched ballistic missiles). The missiles are carried by American SSBNs (nuclear powered ballistic missile submarines.) The guidance systems are 1980s technology, and some of them are twenty years old as well (production of the Trident II D5 ended last year, although another hundred or so missiles may still be produced.) The upgraded guidance systems will be more accurate, reliable and easier to maintain.