A system for determining the directional properties of surface waves or internal waves of a fluid medium includes a sonar system having a plurality of transducers for generating respective, separate acoustic beams and receiving echoes from one or more range cells located substantially within the beams; and a computer program executed by a processor for determining the directional properties associated with the surface waves or internal waves from the received echoes, wherein the computer program determines along-beam velocities along the separate acoustic beams and combines the along-beam velocities to form an equivalent orbital velocity vector.