Tupolev knew this area intimately. The water was almost perfectly isothermal; there was no thermal layer for a submarine to hide under. They were far enough from the freshwater rivers on the north coast of Russia not to have to worry about pools and walls of variable salinity interfering with their sonar searches. The Konovalov had been built with the best sonar systems the Soviet Union had yet produced, copied closely from the French DUUV-23 and a bit improved, the factory technicians said.
