to allow them to coordinate their searches. Earlier S-75 versions used a targeting radar known as Knife Rest, which was replaced in Soviet use, but can still be found in older installations. Major variants Upgrades to anti-aircraft missile systems typically combine improved missiles, radars, and operator consoles. Usually missile upgrades drive changes to other components to take advantage of the missile's improved performance. Therefore, when the Soviets introduced a new S-75, it was paired with an improved radar to match the missile's greater range and altitude. SA-75 Dvina () (NATO codename SA-2) with Fan Song-A guidance radar and V-750 or V-750V missiles. Initial deployment began in 1957. The combined missile and booster was long, with a booster having a diameter of , and the missile a diameter of . Launch weight is . The missile has a maximum effective range of , a minimum range of , and an intercept altitude envelope of between . S-75M-2 Volkhov-M (Russian - Volkhov River) (NATO codename SA-N-2A): Naval version of the A model fitted to the Sverdlov Class cruiser Dzerzhinski. Generally considered unsuccessful and not fitted to any other ships. S-75 Desna (Russian - Desna River) (NATO codename SA-2B). This version featured upgraded Fan Song-B radars with V-750VK and V-750VN missiles. This second deployment version entered service in 1959. The missiles were slightly longer than the A versions, at , due to a more powerful booster. The Desna could engage targets at altitudes between and ranges up to . S-75M Volkhov (NATO codename SA-2C). Once again, the new model featured an upgraded radar, the Fan Song-C, mated to an improved V-750M missile. The improved Volkhov was deployed in 1961. The V-750M was externally identical to the V-750VK/V-750VN, but it had improved performance for range up to and reduced lower altitude limits of . S-75SM (NATO codename SA-2D); Fan Song-E radar and V-750SM missiles. The V-750SM differed significantly from previous versions in having new antennas and a longer barometric nose probe. Several other differences were associated with the sustainer motor casing. The missile is long and has the same body diameters and warhead as the V-75M, but the weight is increased to . The effective maximum range is , the minimum range is , and the intercept altitude envelope is between . Improved aircraft counter measures led to the development of the Fan Song-E with its better antennas which could cut through heavy jamming.