A swath sounding system is under development which is capable of being towed from ships of opportunity. The principle on which it is based is an acoustic interferometer to which a vernier has been added to resolve the ambiguity of phase ramp identification. The ‘fish’ is equipped with both rotational and translational attitude sensors, though no attempt is made to stabilize it. This is in the interest of size and hence of both instrumental and deployment economy.The paper, which was presented at Oceanology International 1982 at Brighton, proposes a novel method to permit replication of the swath depth measurements, and also to provide an independent means of ground scale correction, which may obviate the need to deploy a positioning network for deep-water high-resolution swath surveys.