The underwater environment may appear to the human eye as a dull-blue, featureless space. However, a vast landscape of polarization patterns appear when viewed through a camera that is designed to see the world through the eyes of many of the animals that inhabit the water.
University of Illinois researchers have developed an underwater GPS method by using polarization information collected with a bio-inspired camera that mimics the eyes of the mantis shrimp. The camera, a variation of a polarization imager named Mantis Cam after the shrimp that inspired it, takes advantage of how light refracts when it passes through the surface of water and bounces from particles and water molecules.
The researchers collected underwater polarization data from all over the world and noticed that the polarization patterns were constantly changing. This was in contrast to what biologists had thought about underwater polarization — that the patterns were a result of a camera malfunction. Upon further investigation, they determined that the underwater polarization patterns are a result of the sun's position relative to the location where the recordings were collected. They found they can use the underwater polarization patterns to estimate the sun's heading and elevation angle, allowing them to figure out the GPS coordinates by knowing the date and time of the filming.
The underwater GPS method was tested by pairing the camera with an electronic compass and tilt sensor to measure the underwater polarization data at a variety of sites, depths, wind conditions, and times of day. They found that they could locate position on the planet within an accuracy of 61 km.
This technology may open up new ways for people and robots to better navigate underwater using visual cues from polarized light. The underwater GPS method could be used to help locate missing aircraft, or even create a detailed map of the seafloor. Robot swarms equipped with the sensors could provide a low-cost means of underwater remote sensing. The research could also lead to new insights into the migratory behavior of many marine species.
Another aspect of this technology is its potential to help researchers understand how pollution may alter the migratory paths of animals sensitive to polarized light. The underwater GPS may provide insights into how some long-distance migratory animals, such as whales, might get confused and end up in the wrong places. For example, more whales are becoming stranded close to the California shore, where they have never been observed before.