While GPS can be used to determine one's geographical coordinates on land, such is not the case underwater. Scientists have developed a possible alternative, however, in the form of a system that analyzes the manner in which sunlight is polarized by water.
GPS functions by calculating the user's position relative to four or more satellites, all of which are orbiting the Earth at known locations. The system only works if it can receive radio signals from those satellites, though, and those signals don't travel well through water. As a result, the technology can't be used underwater by divers, submarines, ROVs or anything else.
Sunlight travels through water much better than radio waves, although the liquid does polarize the light.
This means that instead of traveling in multiple directions as they would in the air, the light waves all travel in one direction, along a single plane. That direction is determined chiefly by the angle at which the sunlight hits the surface of the water, which varies according to the date, time of day (elevation of the sun) and geographical location.
With this fact in mind, a University of Illinois Urbana-Champaign team led by professors Viktor Gruev and David Forsyth started out by taking approximately 10 million photographs with an underwater camera that was equipped with a special lens. That lens revealed the underwater polarization patterns in four locations, namely a freshwater lake in Illinois, coastal waters in the Florida Keys, a bay in the city of Tampa, Florida and a lake in the country of North Macedonia.
The photos were taken in water conditions ranging from clear to murky, at various depths on various dates at various times – each photograph was tagged with this data. They formed a database which was used to train a neural network, that learned how polarization patterns vary in a predictable fashion according to the above factors.
That network was subsequently able to determine the geographical location at which an underwater photo was taken by the camera, based on the date, time and polarization pattern. The system is currently accurate to within a range of 40 to 50 km (25 to 31 miles), although that figure should improve significantly as the technology is developed further. It is limited to use at a maximum depth of about 300 m (984 ft), as sunlight doesn't penetrate much farther.
"We are showing for the first time, you can geolocate yourself, or a camera, in a number of different conditions, whether in open ocean waters, clear waters or low visibility waters, at day, at night, or at depth," said Gruev. "Once you have a sense of where you are, then you can start exploring and use that information to have a better understanding of the underwater world or even how animals navigate."
A paper on the research was recently published in the journal eLight.
Source: University of Illinois Urbana-Champaign via EurekAlert
