A system for monitoring a twin trawl being towed by a vessel, the twin trawl comprising two trawl bags, two trawl doors and a clump, comprises at least one sensor arrangement for measuring distance between the doors and between one door and the clump, and a processing unit for computing geometric indicators based on the measured values. Latches are provided to connect the open walls and provide support between the walls. The wireless network sends and receives transmissions to and from the mobile device. Each chip 1 has a plurality of pads 9 which are disposed thereon into an almost linear array substantially along the center line in the direction of short side edges, which pads include addressing pads 9a that are located on the side of central part on a specified plane of the substrate 2, and control-use pads 9b of control signals that are also placed on the center side of the substrate 2.