The Haijiu class submarine chasers are two (possibly four) modified versions of the Hainan class submarine chasers (Type 037) of the People's Liberation Army Navy. The main difference is that four ASW rocket launchers have replaced the original two ASW mortars. The biggest improvement, however, it's in the sonar system. Original SJD-3 sonar on earlier Type 037 class submarine chaser is replaced by a 13 kHz (2 frequency ranges) French Thomson Sintra SS-12/DIP 12 hull mounted (HM)/variable depth sonar (VDS). DIP 12, the VDS version weigh 4 tons and has a 200 meter cable with towing speed of 16 kts, and the range is selectable at 2, 4, 8 and 16 km respectively. However, during deployment, it was discovered that the endurance of this class is limited and the capabilities of SS/DIP 12 sonar sys