The contract for the refit and modernisation of the INS Sindhurakshak (S63), a Project 877 EKM (NATO Kilo-class) submarine was signed in June 2010, RIA Novosti news agency had earlier reported.
Part of the refit involved installation of equipment for Klub-S (3M54E1 anti-ship and 3M14E land attack) cruise missiles and over 10 Indian and foreign-made systems, including the Ushus hydro-acoustic (sonar) system and CSS-MK-2 radio communications system.
In addition, the boat's cooling system was modified, a "Porpoise" radio-locater fitted and other work carried out "increasing the boat's military capacity and safety".
The Sindhurakshak was laid down in one of Russia's oldest shipyards, the Admiralty Wherf yard in St. Petersburg in 1995.
It was launched in 1997 and delivered in December that year.
The boat displaces 2,300 tonnes, carries 52 crew members, has a top speed of 19 knots (35 km per hour) and diving depth of 300 metres.
Zvezdochka, which specialises in repair and refit of nuclear-powered boats, has already refitted four of India's diesel-electric fleet -- Sindhuvir (S58), Sinduratna (S59), Sindhugosh (S55) and Sindhudvhaj (S56).