Towed sensors positioning with forward looking sonars: Application on a Side Scan Sonar
Résumé
In order to have good data quality, sensors like sidescan sonars (SSS) or magnetometers have to be deployed near the seabed. So, they are often carried by a towfish and linked via an umbilical cable to the ship. Unfortunately, because the cable is frequently not tensioned or not in the axis of the vessel, their ship relative position is difficult to estimate, even if cable and sea models are used. A first solution to solve this problem is to use an ultra-short baseline (USBL) to obtain a positioning of the sensor with respect to the vessel, but this device is not always available or suitable (case of magnetometers for example, where USBL causes noise in the magnetic signal). The solution proposed here is to use a forward looking sonar (FLS) to track the positioning of the towfish. A first part of this work is dedicated to show the experimental conditions. A second section is devoted to the tracking of the fish reflector in the FLS image. This is performed by an image processing module. This work is applied here on SSS data. So, the third part of this article is expended to carry out the image sonar mosaicing in a geographical information system (GIS), with the help of the tracking results and the GPS positions and heading of the ship. The different SSS mosaics results (with or without layback corrections) can be compared to the digital terrain models (DTM) of the same area, in order to estimate the benefits of these corrections.
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