An American firm (Arete Associates) has developed a "periscope" that
doesn't have to be raised. Basically it's an underwater camera, which uses
software to eliminate the distortion created low light conditions, waves and
looking through water. Thus the hulls of ships on the surface be clearly seen.
This would not replace a periscope, but a relatively flat device, floated to
the surface from a sub, while attacked to a power/communications cable, would
use high resolution video cameras, and even radars and sound detectors, to look
around. That's what the new "periscope" will be. The U.S. Navy is
building such a device. The latest class of American subs, the "Virginias",
already use vidcams and other sensors in the periscope, so that what is up
there is displayed on large video screens. But the flatter, floating device,
would be less visible than a periscope, and would enable the sub to remain
farther from the surface, and detection.