Abstract: An underwater navigation system is provided, comprising: a transducer configured to emit a first and second acoustic pulses separated by a predetermined time period; a receiver array comprising a plurality of acoustic receivers each configured to receive first reflected portions of the first acoustic pulses and second reflected portions of the second acoustic pulses; the array including a plurality of neighbouring pairs of acoustic receivers wherein a distance between a first neighbouring pair is different from a distance between a second neighbouring pair; and a processor coupled to the receiver array, and configured to generate a velocity measurement based on the predetermined time period and signals from the receiver array representing the first and second reflected portions.