The Maritime Self-Defense Force began construction of the Aegis-equipped in FY1988 as the core ship for air defense of its fleet. With the commission of the Kongo class, each of the four escort flotillas became having one Tartar-equipped destroyer and one Aegis-equipped destroyer. In the 2000s, the steam-powered s were nearing the end of their life, and their Tarter Weapon Systems were becoming obsolete. As their replacements, two Aegis-equipped destroyers, Atago class, were built in the FY2002 and 2003 budget.
Design
The design is fundamentally an improved and scaled-up version of the Kongō-class destroyers with a 4-meter extension of the hull. Just as the Kongō class, the superstructure is larger than aboard the American counterparts, Flight IIA. One of the most obvious changes is an additional hangar to carry one SH-60Khelicopter. In comparison to the Kongō class which only had a helicopter deck, these ships have better helicopter handling facilities. Another external feature is the reduction of the radar cross-sectional area. For example, the Atago class use a new stealthier plain-structure mast, which was originally designed in Japan, rather than familiar sturdy truss mast. A new modified smokestack and other improvements were also introduced to make the Atagos stealthier. The propulsion systems are almost the same as those of the Kongō class and Arleigh Burke class, powered by four Ishikawajima-Harima LM2500 gas turbines giving them a top speed of. The installation of three generators is also retained, but the output per unit is increased to 2,800 kilowatts.
Equipment
The class is equipped with the Aegis Weapon System. The system version was Baseline 7.1 immediately after they were put into service; then both ships have been updated to Baseline 9C with modernization. As surface-to-air missiles, The SM-2MR Block IIIA/B has been used. Since FY 2016, they have also been equipped with a missile defense capability with an Aegis BMD 5.0CU system to launch SM-3 Block IA and IB anti-ballistic missiles. The missile launcher is the same Mark 41 Vertical Launching System as the Kongo class, but the missile reloading crane has been omitted, so the number of missile cells is increased by 3 cells each on the bow and stern sides. The Kongo class had 29 cells on the foredeck and 61 cells on the afterdeck, but the Atago class has a hangar, so 64 cells are placed on the foredeck and 32 cells on the afterdeck. These cells contain not only SM-2 and SM-3, but also VL-ASROCs. In addition, they are equipped with HOS-302, one of the Japanese variant of the Mark 32 Surface Vessel Torpedo Tubes, as anti-submarine weapons. While the Kongō class is equipped with domestic sonar and ASW combat system, the Atago class is equipped with American-made AN/SQQ-8915 with AN/SQS-53C sonar. Only the towing sonar was a domestic OQR-2D-1, which was later replaced with an American-made SQR-20 Multi-Function Towed Array. The gun caliber has extended from the 54 caliber of the Kongō class to the 62 caliber with strengthened powder charge enabling a firing range. As with other Japanese ships being refit, the American-made Harpoon missiles can be replaced with the Japanese-made Type 90 surface-to-surface missiles; those interfaces are compatible.
