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. 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

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... vessel, but this device is not always available or suitable. 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, 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). The different SSS mosaics results are compared to the digital terrain models (DTM) of the same area, showing an error positioning of about one meter.