up near Moscow and manned by recently moved LPTI personnel; it was first used on July 22, when it detected at night an incoming flight of about 200 German bombers while they were 100 km away. This was the first air attack on Moscow, and it immediately led to three rings of anti-aircraft batteries being built around the city, all connected to a central command post. Several transmitters and receivers built for RUS-2 systems were quickly adapted by the NIII-KA for fixed radio-location stations around Moscow. Designated as RUS-2S and also P2 Pegmatit, these had their Yagi antenna mounted on 20-meter steel towers and could scan a sector of 270 degrees. For building additional equipment, in January 1942, Factory 339 in Moscow became the first manufacturing facility in the Soviet Union devoted to radio-location sets (soon officially called radar). During 1942, this facility built and installed 53 RUS-2S sets around Moscow and other critical locations in the USSR. Factory 339 had an outstanding research and engineering staff; this had earlier been administratively separated and designated as the Scientific Institute of Radio Industry No. 20 (NII-20). Victor V. Tikhomirov, a pioneer in domestic aircraft radio engineering, was the Technical Director. (Later, the Tikhomirov Scientific Research Institute of Instrument Design was named in his honor.) Factory 339 and the associated NII-20 dominated radar equipment development and fabrication in the USSR throughout the war. Many sets of a number of different versions of the RUS-2 were built at Factory 339 during the war. While providing early warning, these sets suffered from the deficiency of not providing target height (elevation angle). Thus, they were mainly used in conjunction with visual-observation posts, with humans using optical devices for estimating altitude and identifying the type of aircraft. From the time of the first efforts in radio-location, the question had been raised as to how the aircraft identification could be made – was it friendly or an enemy? With the introduction of RUS-2, this problem required an immediate solution. The NII-20 developed a unit to be carried on an aircraft that would automatically respond as "friendly" to a radio illumination from a Soviet radar. A transponder, designated as SCH-3 and later called an Identification Friend or Foe (IFF) unit, was placed into production at Factory 339 in 1943. This unit initially responded only to the signal of RUS-2, and only a relatively small number of these and