you're reading...

ocean engineering

I saw the Sun!

Powell, S. B., Garnett, R., Marshall, J., Rizk, C., & Gruev, V. (2018). Bioinspired polarization vision enables underwater geolocalization. Science Advances, 4(4), eaao6841.

Figure 1 – Schematic of the polarimeter. The detector is set up underwater and allowed to record the light after it has passed from atmosphere to the water. (Adapted from Powell et al., 2018)

Being lost is no fun. Over the eons, humans have developed loads of ways to avoid straying from the beaten path. From traditional navigation tools like star charts to Google Maps, we generally have a good idea of where we are. Marine organisms have had to get by without smartphones. But they have managed to develop all kinds of interesting sensing mechanisms to navigate.

Dolphins and fish, for example, use acoustic cues to orient themselves locally. Other fish species and a host of other animals have developed special cell structures to geolocate based on Earth’s magnetic field. Visual light, however, is one information source that scientists have long assumed marine organisms use exclusively for activities like prey detection, predator avoidance, and communication.

Dr. Samuel Powell, a researcher at Washington University in St. Louis, recently proposed a more nuanced view of how ocean dwellers might use light. His recent study suggests that some animals could use subtle changes in the underwater light field for local positioning and long-range navigation. In particular, Powell and his colleagues hypothesize that such organisms could use the polarization angle of light to determine where they are.

Visible light from the sun is unpolarized as it enters the atmosphere. This means that the light wave is vibrating every which way. The light can be forced to oscillate only in one direction by a variety of physical processes. Polarized sunglasses and your very own acuvue oasys 1 day astigmatism do exactly that, protecting your eyes by making the light more coherent. Scientists have known for a long time that the ocean surface acts as a kind of polarizing lens ­– the state of light entering the water from the atmosphere is changed as it passes through the surface.

How specifically the light is polarized is a function of the angle at which the light hits the surface. Since the Earth is roughly a sphere, the angle of the sun relative to the ocean changes dramatically with latitude. Powell set out to demonstrate that the Sun’s heading and elevation angles could be determined from the polarization in the water alone. Using that information, Powell argues, an observer could reliably determine their location.

Figure 2 – An example of sensor measurements. The gray image on the left is the intensity recorded by the detector. The sun is blocked out by the disc in the middle. Each circle represent a different sensor output and is plotted in that color on the right. Notice how there is more energy in the light polarized at around 0 degrees. (Adapted from Powell et al., 2018)

Powell and his team collected measurements of light polarization using a fancy new instrument designed to mimic the eyes of mantis shrimp. Mantis shrimp, besides being crazy boxers, have specially evolved optical cells that can detect polarized light. Powell’s sensor measures polarization at a variety of angles over the entire digital sensor. In addition to taking a standard image, the polarimeter was able to measure polarization intensity and angle.

The system was sealed in an underwater housing and deployed off the coast of Australia, Hawaii, Florida, and Finland.  Divers set the polarimeter up between 2 and 20 meters at all times of day. Powell then devised a model that attempted to match the polarization data to infer Sun heading, elevation angle, and geographic location.

The group’s best model was able to estimate the heading and elevation with 41 and 23% error. If an observer where to use these estimates to navigate, they would stray off course by about 105 meters after 1 kilometers of travel. Likewise, the position estimate with these readings was off by an average of 1970 kilometers.

Figure 3 – Global position estimates generated by the model. The blue cross is the actual sensor position and the red x is the mean position estimate. Note how close the mean of the estimates is to the sensor location. (Adapted from Powell et al., 2018)

Powell’s team was able to substantially improve these estimates by only considering measurements made when the sun was above 40o off the horizon. Such measurements correspond roughly to the middle of the day. Powell argues that when the Sun is close to the horizon, the light it emits is subject to additional polarization effects from the atmosphere. When only considering midday estimates, the heading and elevation errors by more than half. The position estimates were with 442 kilometers of the actual sensor location.

Powell notes that these estimates were made on single observations. If such a method were to be used for actual navigation, the user would almost certainly be averaging the position estimates from numerous measurements. Under that scenario, the averaged model position estimates were within about 60 kilometers of the polarimeter location.

This method is clearly far from a perfect navigation system. Powell and his group did, however, demonstrate that it is possible to orient oneself underwater with light alone. Powell argues that animals with polarization sensitive eyes could already use that information to navigate. Furthermore, such sensing modalities could serve as the basis for future submarine navigation systems for autonomous vehicles. With that capability, the scientific instruments of the future could stay underwater for far longer, improving our ability to see beneath the waves.


No comments yet.

Post a Comment


  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 4 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com