submarines, which are difficult to track. Chinese specific variants A total of four variants of Type 039A have been developed by the first half of 2014, and they are listed here in chronological order based on their public debut: Type 039A: Original Yuan class. The most obvious external visual difference between Type 039A and its predecessor Type 039/039G Song-class submarine is in the conning tower. The conning tower of Type 039A is similar to that of Type 039G, but the tower lacks the fin-shaped extrusion on the rear section of the conning towers of both the Type 039 and the Type 039G. Type 039AG: This second member of Type 039A series is frequently but erroneously identified as its development, Type 039B, because externally, the two version looks the same above waterline. The only difference lies below the waterline, which is not readily observed, and it is believed that all Type 039AG have been converted to Type 039B. The most obvious external visual difference between Type 039AG along with its development Type 039B and earlier Type 039A submarine is also in the conning tower: the shape edge at the top of conning tower of Type 039A is replaced with smooth round shape transition. In addition, there is a bulge in the middle of the smooth transition in the forward portion of the conning tower housing some kind of sensor, and this is a new feature earlier Type 039A lacks. Type 039B: This third member of Type 039A series looks identical to earlier Type 039AG from which it is developed from. The biggest difference between Type 039B and Type 039AG is that Type 039B has incorporated flank sonar array, which was subsequently retrofitted on Type 039AG and some earlier boats. This difference, however, is not readily observable because the flank array is installed at the lower portion of the hull. It was only in the early 2010s when a Type 039B built by Shanghai Changxing (长兴) Shipyard exposed in the berth, when the flank array of Type 039B became publicized, thus distinguish the type from earlier Type 039AG that lacked the flank sonar. Type 039B (upgraded): The fourth variant of the Yuan has been first publicized in December 2013, with a modified hull and redesigned conning tower with extrusion at the root of the conning tower in both the bow and stern direction, similar to that of conning tower of . In