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32 st / May 2016 www.sea-technology.com with a manually movable depres- sor or with an electric-actuated one. This allows determination of the towing speed and the ability to change it even during operation. Furthermore, the electric-actuat- ed depressor enables the system to operate in a closed-loop with the navigation devices. Autopilot functions such as stabilization of the horizontal and vertical move- ment or the automated following of predefned paths are possible. This closed-loop control makes for easier use of the towed system. The auto-depth function can be used to hold the system at a specif- ic depth, while the towing speed may vary. In conjunction with an installed altimeter, an auto-height function enables the system to keep a fxed height above the seabed. Since the system follows the ref- erence value for the depth/height automatically, a movement path may be defned by varying the ref- erence value with time. This allows, e.g., an undulated movement of the towed system without changing the towing speed or interaction of a pi- lot. With an undulated movement, data samples from specifc depths can be acquired easily. The data acquisition may be triggered auto- matically when reaching specifc depths. In this way, a wide depth range can be surveyed in a single run. Since the system is either attached to a trawl net or towed behind a vessel, its geographical position can be deter- mined easily. The depth measurement and the length of the deployed umbilical or trawl net is enough to calculate the offset to reference GPS data at the surface. No expensive baseline system is necessary, although one can be attached to the system. Frame Concept As usual for all the systems designed and built by Mari- scope, the frame is manufactured of stainless steel and hand-welded with TIG procedure. This allows customizing each unit according to the individual application and needs of the users. Moreover, the frame involves all the compo- nents of the system, including the depressor. This reduces the risk of damaging the system in case of a collision, when the special hook rail allows the system to overturn and not get entangled. Entanglement is the most common reason for the loss of a towed system. The modularity of the mechanical parts continues with the modularity of the electronic interfaces. A specially de- signed mainboard serves as the host for a wide range of daughter boards with interfaces to all subsystems. Available daughter boards include analog front ends for sensor data acquisition, analog video transmission, digital communica- tion interfaces (RS232, RS485), Ethernet communication in- cluding IP video, dimmable LED driver, and irDa interfaces for emulation of infrared remotes. The power supply is selected to ft the requirements of the installed equipment. Unlike typical AUVs, the system does be programmed in advance. A scheduler allows periodic recording of videos, switching of cameras, regulating of il- lumination, and control of additional equipment. The high- defnition videos are stored on SD cards within the system, and an internal data logger simultaneously records all sen- sor values annotated with RTC timestamps. After recovering the system, the data can be merged into the recorded videos for the purpose of documentation or processed separately. The system can easily be converted into an online towed system. The same frame and components are used but turned upside down in order to use the predisposed towing hook. In online operation mode, all functionality can be ac- cessed directly from the same software running on any PC or notebook. In this case, a live video (FBAS) is transmitted to provide direct feedback to a pilot. The live video, including a freely customizable overlay, can be recorded to a con- nected PC or notebook. The Observer III is equipped either (Top) Control software showing undulation of the system. Typi- cal trawl net used in Argentina. (Bottom) The bottom trawl.