(AN/APS-2 commonly known as "Dog"), a search and homing radar used by the Navy on smaller bombers; this was followed by several lighter versions, including the AIA-1 known as the "radar gunsight". The Alvarez antenna was also used in developing the Ground Control Approach (GCA), a combined S-Band and X-Band blind-landing system for bomber bases; this system was particularly used in assisting planes returning from missions in poor weather. The BTL also developed X-Band radars. The Mark 8 (FH) fire-control radar, was based on a new type of antenna developed by George Mueller. This was an end-fired array of 42 pipe-like waveguides that allowed electronic steering of the beam; for this the BTL developed the Mark 4 Fire Control Computer. The Mark 22 was a "nodding" system used for target height-finding with fire-control radars. With an antenna shaped like an orange slice, it gave a very narrow, horizontal beam to search the sky. The Army also adopted this as the AN/TPS-10, a land-version that was commonly called "Li'l Abner" after a popular comic strip character. Although not implemented into a full system until after the war, the monopulse technique was first demonstrated at the NRL in 1943 on an existing X-Band set. The concept is attributed to Robert Page at the NRL, and was developed to improve the tracking accuracy of radars. Following the war, essentially all new radar systems used this technology, including the AN/FPS-16, the most widely used tracking radar in history. Soviet Union The Soviet Union invaded Poland in September 1939 under the Molotov–Ribbentrop Pact with Germany; the Soviet Union invaded Finland in November 1939; in June 1941, Germany abrogated the non-aggression pact and invaded the Soviet Union. Although the USSR had outstanding scientists and engineers, began research on what would later become radar (radiolokatsiya, lit. radiolocation) as soon as anyone else, and made good progress with early magnetron development, it entered the war without a fielded, fully capable radar system. Pre-War Radio-Location Research The USSR military forces were the Raboche-Krest'yanskaya Krasnaya Armiya (RKKA, the Workers' and Peasants' Red Army), the Raboche-Krest'yansky Krasny Flot (RKKF, the Workers' and Peasants' Red Fleet), and the Voyenno-Vozdushnye Sily (VVS, Soviet Air Forces). By the mid 1930s, Germany's Luftwaffe had aircraft capable of penetrating deep into Soviet territory. Visual observation was used for detecting approaching aircraft. For nighttime detection, the Glavnoye artilleriyskoye upravleniye (GAU, Main Artillery Administration), of the Red Army,