The Kilo class, Soviet designation Project 877 Paltus (, meaning "halibut"), NATO reporting name Kilo, is a class of diesel-electric attack submarines originally designed in the 1970s and built in the Soviet Union for the Soviet Navy. The initial version of Kilo submarines entered operational service in 1980 and were built until the mid-1990s. Production was switched to the more advanced Project 636 Varshavyanka (, meaning "Varsovian (inhabitant of Warsaw; feminine)") variant in the mid-1990s, also known as Improved Kilo class in the West. The class was updated again by the Rubin Design Bureau in the mid-2010s and called the Project 636.3. Role The Project 877 attack submarines were mainly intended for anti-shipping and anti-submarine operations in relatively shallow waters. Original Project 877 boats are equipped with Rubikon MGK-400 sonar system (with NATO reporting name Shark Gill), which includes a mine detection and avoidance sonar MG-519 Arfa (with NATO reporting name Mouse Roar). Newer Project 636 boats are equipped with improved MGK-400EM, with MG-519 Arfa also upgraded to MG-519EM. MGK 400E can detect submarines with 0.05 Pa/Hz noisiness in 16 km and surface vessels with 10 Pa/Hz noisiness in 100 km. The improved sonar systems have reduced the number of operators needed by sharing the same console via automation. Anechoic tiles are fitted on casings and fins to absorb the sound waves of active sonar, which results in a reduction and distortion of the return signal. These tiles also help attenuate sounds that are emitted from the submarine, thus reducing the range at which the submarine may be detected by passive sonar. Lada-class successor The Kilo class was planned to have been succeeded by the . However, by November 2011 it was apparent that the Lada-class would be delayed because Sankt Peterburg (B-585), the lead boat of the class, had shown major deficiencies. On 27 July 2012, the Russian Navy commander-in-chief announced that construction of the Lada-class submarines would resume, having undergone design changes. Series production was reported to be underway in the latter 2010s. Project 636.3 The Russian Navy also moved forward in the late 2010s, with the construction of Project 636.3, an improved version of the Kilo class. The first-in-class was named Petropavlovsk-Kamchatsky (PPK) and was launched by the head of Admiralty Shipyard Alexander Buzakov on 28 March 2019, or some thirty months after commission. By November 2019, six units had been built for the Black Sea Fleet