Stella Maris is a new technology to replace submarine periscopes with a camera system that sees what is going on above the water’s surface, clearly and without distortion. The system, an acronym of Stellar Marine Refractive Imaging Sensor, is “a novel approach to a virtual periscope as it passively measures water and waves by imaging the refracted sun,” according to Associate Professor Yoav Y. Schechner, part of the Technion’s department of electrical engineering, who is leading the research team developing the system.
Submarines, intended to be stealthy, hidden vessels that defend countries’ shores, have one big Achilles heel — the periscope, which needs to rise from underwater in order to see what is happening above the ocean surface. That is the only way to get a clear picture of ocean-surface activity. Images viewed from underwater taken by cameras will be distorted by water-surface waves.
Water refracts light, and light refraction is the essence of images taken by video or still cameras. Waves are not constant — they are random and can vary in intensity at any time, with the distortion they cause varying randomly as well. That distortion makes cameras useless for monitoring the sea surface and makes periscopes the only alternative.
Shechner’s team found a solution. “Distortions that are random in space, time and viewpoint are created when viewing objects through a wavy water-air interface. Such distortions are also created in turbulence. In both cases, the distortion is caused by dynamic refraction. We study how this situation can be handled. This is important to submariners, to avoid the salient use of a physical periscope. Stella Maris counter the distortions caused by water waves.”
The technology actually mimics solutions that have been employed to view outer space from earth, notably the Shack-Hartmann astronomical sensor, correcting for atmospheric layers that distort the view of space from earth. Telescopic images are degraded by the atmosphere, while underwater images are distorted by the wave surface. To correct for the atmospheric distortions, telescopes use sensors that seek out the position of a “guide star” filtered through pinholes.
The sensor observes the deviations of portions of the star imaged through the pinholes in relation to other parts, based on the star’s size and position, and corrects for the distortion. That correction is applied to other images seen by the telescope, so users are able to get an accurate view of space.
Schechner and his team have developed something similar for underwater image sensing. The Stella Maris system uses an imaging sensor attached to a camera, with the sun as its guide star. Sun rays and objects on the ocean surface are projected through the pinholes to the diffuser, which is imaged by the camera. By lining up the sun rays, the system can see what the distortion level is and correct for it, applying the correction to the distorted image. Te result is an accurate view of what the camera is actually looking at.
“Raw images taken by a submerged camera are degraded by water-surface waves similarly to degradation of astronomical images by our atmosphere. We borrowed the concept from astronomers who use the Shack-Hartmann astronomical sensor on telescopes to counter blurring and distortion caused by layers of atmosphere,” said Schechner. “Stella Maris is a novel approach to a virtual periscope as it passively measures water and waves by imaging the refracted sun.”
Schechner said the system could have uses beyond submarines. “Submerged on the sea floor, Stella Maris could be useful for marine biology research where and when viewing and imaging both beneath and above the waves simultaneously is important. Stella Maris could, for example, monitor the habits of seabirds as they fly, then plunge into water and capture prey,” he said. “There are many ways to advance the virtual periscope,” says Schechner, who added that while the system requires sunlight, the team is working on furthering the technology by developing ways to gather enough light from moonlight or starlight to be able to use the system at night.
Click below for a video explaining the Stella Maris system: