Towed Systems This class represents an overwhelming number of systems that have been towed behind ships and boats to perform many different types of work. The primary method of operation for towed systems is to launch the usually heavy vehicle (very heavy for deep applications) and then tow it at the desired depth by varying the length of the strong electromechanical cable. Whereas Kevlar has provided the breakthrough for long length cables for free flying ROVs, where the tether needs to remain essentially neutral in the water column, steel cables are quite acceptable for towed systems. Modern tow cables now include fiber optic communications that provide excellent bandwidth for the transmission of data from multiple sensors and TVs. One application for towed vehicles is oceanographic data collection. Many of the smaller vehicles are designed to undulate through the water column in order to provide profiles (e.g. plankton, etc.). Typical sensors used aboard these vehicles are CTDs, transmissometers, flourometers, nephelometers, bioluminescence and irradiance meters, optical plankton recorders, dissolved oxygen, pH, chlorophyll and others. Many towed vehicles are specifically designed to locate cables or pipeline either buried or unburied on the seabed. The vehicles are normally either a conventional towed body, or a sled, which can carry either a magnetometer or fluxgate gradiometer for locating metallic objects. A very unique design by Seatec (see figure at right) incorporates spinning rotors on the tow body that allow the vehicle to be steered along a pipeline. One of the most prominent uses of towed vehicles is for search and survey. These systems range in size and weight from very small, shallow water bodies to large full ocean depth systems. Such systems can survey the sea floor for many purposes including mapping, search and salvage, route survey, pipeline survey, environmental survey, etc. They can carry a variety of survey sensors including TV cameras, film cameras, digital cameras, laser imaging systems, side scan sonars, swath bathymetry sonars, multibeam sonars, sub-bottom profilers and magnetometers. Underwater search vehicles, such as Scripps Institution of Oceanography’s Deep Tow–one of the first such systems–have been used to locate everything from lost torpedoes and aircraft up to long lost ships. One of the most famous finds was by the Woods Hold Oceanographic Institution’s ARGO vehicle which lays claim to the discovery of the HMS Titanic.