In 1895, Alexander Stepanovich Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes. The next year, he added a spark-gap transmitter and demonstrated the first radio communication set in Russia. During 1897, while testing this in communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation. In a few years following the 1917 Russian Revolution and the establishment the Union of Soviet Socialist Republics (USSR or Soviet Union) in 1924, Germany's "Luftwaffe" had aircraft capable of penetrating deep into Soviet territory. Thus, the detection of aircraft at night or above clouds was of great interest to the Soviet Air Defense Forces (PVO). The PVO depended on optical devices for locating targets, and had physicist Pavel K. Oshchepkov conducting research in possible improvement of these devices. In June 1933, Oshchepkov changed his research from optics to radio techniques and started the development of a "razvedyvlatl’naya elektromagnitnaya stantsiya" (reconnaissance electromagnetic station). In a short time, Oshchepkov was made responsible for a technical expertise sector of PVO devoted to "radiolokatory" (radio-location) techniques as well as heading a Special Design Bureau (SKB, spetsialnoe konstruktorskoe byuro) in Leningrad. The "Glavnoe Artilleriyskoe Upravlenie" (GAU, Main Artillery Administration) was considered the “brains” of the Red Army. It not only had competent engineers and physicists on its central staff, but also had a number of scientific research institutes. Thus, the GAU was also assigned the aircraft detection problem, and Lt. Gen. M. M. Lobanov was placed in charge. After examining existing optical and acoustical equipment, Lobanov also turned to radio-location techniques. For this he approached the "Tsentral’naya Radiolaboratoriya" (TsRL, Central Radio Laboratory) in Leningrad. Here, Yu. K. Korovin was conducting research on VHF communications, and had built a 50 cm (600 MHz), 0.2 W transmitter using a Barkhausen-Kurz tube.