very long seeker settling time on the order of 22 seconds, as opposed to 11 seconds for the original version. The R-3S was seen by the West in 1961 and given the NATO reporting name AA-2A 'Atoll'. Minimum engagement range for the R-3S is about one kilometre. All K-13 variants are physically similar to Sidewinder, sharing the 5 inch (127 mm) diameter. Subsequent examination of AA-2 missiles captured by NATO forces showed that parts from an AIM-9 could be interchanged with parts from an AA-2 and either combination would still work. While the R-3S was being introduced in 1961, work started on a semi-active radar homing (SARH) version for high-altitude use, the K-13R (R-3R or Object 320) with 8 km range, similar to the little-used US Navy AIM-9C Sidewinder (carried by the F-8 Crusader). This took longer to develop, and did not enter service until 1966. This version was designated AA-2B by NATO. Three training versions were also developed. The R-3U ("uchebnaya", training) was an empty missile body with a homing set, allowing pilots to get used to the aiming of the system. The R-3P ("prakticheskaya", practice) was a complete missile without an explosive warhead. The RM-3V ("raketa-mishen", target-missile) served as an aerial target. Later versions The Vympel team started working on a more ambitious upgrade in the late 1960s, emerging as the K-13M (R-13M, Object 380) for the IRH and K-13R (R-3R) for the SARH variant, were developed in the late 1960s. These were dubbed Advanced Atoll (AA-2C and AA-2D, respectively) in the west. The R-13M was roughly equivalent to the improved USN AIM-9G Sidewinder, with a new proximity fuse, more propellant for longer range, better maneuverability, and a more sensitive nitrogen-cooled seeker head. None were all-aspect missiles. The same electronics upgrades were also applied to the Kaliningrad K-5 (AA-1) to arm fighters that did not carry the K-13. The K-13 in different versions was widely exported to the Warsaw Pact and other air forces, and remains in service with a few smaller nations. A license-built version called A-91 was built in Romania, and the People's Republic of China copied the K-13 as the PL-2. Updated Chinese versions were the PL-3 and PL-5. Soviet Union provided China K-13 missile technology as a part of MiG-21 fighter jet deal in 1962. In 1967, China successfully completed locally produced K-13 (PL-2) missile tests, and started to deploy this missile to combat