 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 kn, and the range is selectable at 2, 4, 8 and 16 km respectively.