Sea Technology

AUG 2012

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

Issue link: http://sea-technology.epubxp.com/i/77248

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Navigation

Page 21 of 79

studies in restricted areas. The first step was to calibrate the sensors with the manufacturer's standard procedures. Calibration for the fluorescence and turbidity sensors needed to be im- proved using in-situ samples. The second step was to calibrate the compass. The EcoMapper relies on a magnetic compass and a Doppler ve- locity log for navigation, which are usually enough for standard surveys. In this case, it was crucial to have them well-calibrated to minimize nav- igation errors. The third step was to calibrate the most accurate georefer- enced image of the area, i.e., the latest one. In this case, an error of a few me- ters was found in the image, which was critical due to the narrow naviga- tion margin. For restricted areas, it is important to know the exact position of the fixed potential risk, which, in this study, was the rafts. The mussel farm had up to 100 rafts, which, despite being an- chored to the seafloor, moved with wind and tidal currents and were not in the positions shown on the georef- erenced image. To account for this dynamic risk, a new map of the rafts' boundaries needed to be drawn every day. (Top) Seaweed blocks the AUV's propeller. (Bottom) Preliminary results from the AUV's temperature sensor, showing a transverse section (Y-Z plane) interpolated from the actual tracks. Each day, the first step of the survey was to per- form one tour around the rafts using a small boat, getting as close to them as possible to obtain GPS positioning, create safe passages and inform the AUV planning software for the day. The ideal way to carry out the study would have been to create safe AUV passages before deployment using wind and current historical information, as well as raft dimensions, anchor points and predicted raft drift- ing, to estimate raft movement without the GPS presurvey. This was not done because it would have been time-consuming. Survey Execution The raft positions were tracked after the systematic check of the AUV and all the calibrations. As determining the rafts' GIS location and modeling their movement take a considerable amount of time, the best option was to navi- gate around the platforms to esti- mate the positions for setting up the AUV paths within the rafts' border limits. This was valid only if the wind and current conditions remained the same. The final test before the planned mission was a surface track to check the depth of the navigation path and the presence of any obstacles. 22 st / AUGUST 2012 www.sea-technology.com

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