These boats were first equipped with Soviet Tamir-11 (MG-11, NATO reporting name: Stag Hoof) hull-mounted sonars which had a typical range of 3 kilometers , but in the 1970s, they were superseded by domestic Chinese SJD-3 sonar, which is modification of original MG-11: instead of being fixed to the hull, SJD-3 has a telescoping arm, so when not in use, the sonar is stored in the hull, and when deployed, the sonar is lowered into water several meter below the hull, thus increased detection range by avoiding baffles generated by the hull.