1. Apparatus for detection and classification of submerged swimming object from a passive signal generated by the object and an active signal reflected from the object, the apparatus comprising an active mode branch having a first input for receiving a first signal representative of the active signal, and a passive mode branch having an first second for receiving a second signal representative of the passive signal, wherein the active mode branch includes first signal processing means adapted to generate a first classification estimate output, with first clues, of the most probable class of target for the object, and the passive mode branch includes second signal processing means adapted to generate a second classification estimate output, with first clues, of the most probable class of target for the object, a merger adapted to receive and merge said first and second classification estimate outputs, and to output a merged classification estimate output with clues, and a joint classifier arranged to receive said a merged classification estimate output with clues, and adapted to classify the object by performing joint classification based on said merged classification estimate output with clues.