Virtual periscope
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Virtual periscope is a system that allows submerged submarines to observe the surface above them without having to come to a shallower depth, as is required by traditional periscopes.
The system, described in a patent as "Virtual Periscope",[1] was tested in 2005 aboard USS Chicago (SSN-721). It employed a small camera mounted on the sail of the submarine that uses the surface of the ocean as a lens, collecting light from above the surface and refracting it below. High-speed signal processing software assembles an image of what is on the surface. At the time, the system's resolution did not allow ship identification, only indicating that something was on the surface. Objects 30 meters (98 feet) tall could be seen at about a distance of 1,600 meters (one mile). Sufficient light was available when a camera was positioned shallower than 30 to 60 meters (98 to 197 ft).