The GBU-39 Small Diameter Bomb (SDB) is a precision-guided glide bomb that is intended to provide aircraft with the ability to carry a higher number of bombs. Most US Air Force aircraft will be able to carry (using the BRU-61/A rack) a pack of four SDBs in place of a single bomb. The Small Diameter Bomb II (SDB-II) / GBU-53/B scheduled to enter production in January 2014 will add a tri-mode seeker (radar, infrared, and semiactive laser) to the INS and GPS guidance of the original SDB.