Abstract: An acoustic beam having at least one preset frequency is generated and dited towards a preselected non-linear acoustic reradiator. The reradiator returns acoustic waves having a different frequency. These reradiated waves are monitored, and a preselected parameter is detected in response to the returned reradiated acoustic waves. In a first embodiment, the preset frequency acoustic beam is generated at one or more frequencies and radiated by at least one projector. Upon incidence of the preset frequency acoustic beam, a preselected reradiator generates acoustic waves having a different frequency. The reradiated acoustic waves are used to calibrate a hydrophone. The reradiated acoustic waves can be controlled by the acoustic projector without a wire link.In another embodiment, the non-linear reradiation is caused by the action of a projected beam on cavitation generated by a vessel remote from the projector.