For the human eye, the underwater environment may look like a dull-blue, featureless space. However, an enormous landscape of polarization patterns comes into view when observed through a camera that is engineered to see the world through the eyes of numerous animals that live in the water.
Researchers at the University of Illinois have formulated an underwater GPS technique by using polarization information gathered with a bio-inspired camera imitating the eyes of the mantis shrimp. The findings, published in Scientific Advances, are the first to reveal passive underwater GPS using the polarization properties of underwater light. This technology could pave way for new possibilities for marine navigation and understanding of the migratory behavior of aquatic animals.
The camera, a variation of a polarization imager known as Mantis Cam after the shrimp that inspired it, exploits how light refracts, or bends, when it travels through the surface of water and bounces from water molecules and particles.
We collected underwater polarization data from all over the world in our work with marine biologists and noticed that the polarization patterns of the water were constantly changing. This was in stark contrast to what biologists thought about underwater polarization information. They thought the patterns were a result of a camera malfunction, but we were pretty sure of our technology, so I knew this phenomenon warranted further investigation.
Viktor Gruev - Study Leader, Illinois professor of Electrical and Computer Engineering, Professor of the Carle Illinois College of Medicine
After getting back to the lab, Gruev and graduate student and co-author Samuel Powell established that the underwater polarization patterns are a result of the sun’s position corresponding to the location where the recordings were gathered. They discovered they can use the underwater polarization patterns to approximate the sun’s heading and elevation angle, allowing them to work out their GPS coordinates by knowing the time and date of the filming.
“We tested our underwater GPS method by pairing our bio-inspired camera with an electronic compass and tilt sensor to measure the underwater polarization data at a variety of sites around the globe, depths, wind conditions and times of day,” said Gruev, who also is affiliated with the Beckman Institute for Advanced Science and Technology at the University of Illinois. “We found that we can locate our position on the planet within an accuracy of 61 km.”
This technology may herald new ways for people and robots to better navigate underwater using visual indications from polarized light. “We could use our underwater GPS method to help locate missing aircraft, or even create a detailed map of the seafloor,” Powell said. “Robots swarms equipped with our sensors could provide a low-cost means of underwater remote sensing – it would certainly be more cost-effective than current methods.”
This research could also pave the way to new understanding about the migratory behavior of a number of marine species.
“Animals like turtles and eels, for example, probably use a slew of sensors to navigate their annual migration routes that take them thousands of miles across oceans,” Gruev said. “Those sensors may include a combination of magnetic, olfactory and possibly – as our research suggests – visual cues based on polarization information.”
Another feature of this technology is its potential to assist researchers in understanding how pollution may change the migratory paths of animals sensitive to polarized light.
“It is very likely that increased pollutants in the air and water alter underwater polarization patterns, causing the undersea environment to appear different from what many animals have learned,” Gruev said. “Our underwater GPS method may provide insights into how some long-distance migratory animals, such as whales, might get confused and end up in the wrong places.”
For instance, more whales are getting marooned close to the California shore, where they have never been seen before, Gruev said. “Perhaps pollutions is the indirect culprit for this reason, as it affects the underwater polarization patterns necessary for migratory behavior.”
The study was supported by National Science Foundation and the Air Force Office of Scientific Research. Gruev also directs the Biosensors Lab at Illinois.
(Credit: University of Illinois)