A synthetic antenna sonar system, includes: a sonar provided with emission elements and reception elements, the reception elements including a composite physical antenna having N transducers, the sonar being located on a mobile carrier; an inertial unit arranged on the carrier and suitable for measuring instantaneous position and orientation informations of the carrier; a common clock supplying a temporal signal enabling the synchronization of both the inertial unit and the sonar; elements for determining the instantaneous speed of the carrier, the determined speed feeding the inertial unit to correct the drift thereof; and micronavigation elements enabling an image of the seafloor to be obtained through formation of the synthetic antenna channels from K sonar operation recurrences, considering that the physical antenna movement during the carrier displacement is fully known from the informations measured by the inertial unit.