An aircraft acting as a platform (16) above the ocean surface (3), acts as the source location of an ocean penetrating green laser beam (15). The beam is directed to the surface of the ocean and through it to the ocean bottom and reflected back to the surface and to a receiver on the platform. At the commencement of one traverse, a reading is taken on a small field of view as indicated at (17), and at the other end of the traverse, a large field of view reading is taken, as indicated at (18). The device of the invention comprises a pair of shutters (20 and 21) in the return green beam aperture driven respectively by stepper motors (25 and 26) which may be closed to reduce the field of view to that given by the small circular aperture (22), or opened to give the full aperture of the iris.
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