Birds and Bees Use It — Why Not Autonomous Vehicles?

By Terry Bollinger, MITRE, and Keith Hammack, Office of Naval Research

What do birds and bees do that you may want your autonomous vehicle to do? See polarized light, of course. Birds, bees, and many other forms of life on land, in the air, and even in shallow seas can see and use polarized light to help them navigate in their environments. Since polarized light is free, easy to detect, and abundant, why not use it to help autonomous vehicles glean additional localization and navigation information from their land, sea, air, and shallow-water environments?

This is exactly the question that a company called Polaris Sensor Technologies in Huntsville, Ala., is trying to answer. Using a Small Business Innovation Research (SBIR) contract from the Office of Naval Research’s Expeditionary Maneuver Warfare and Combating Terrorism Department, Polaris is looking at how polarization-assisted navigation could help ground vehicles find their bearings in the absence of GPS or other location methods. Success in ground vehicles also would benefit autonomous vehicles traveling through empty stretches of sea or air when GPS cannot be used or trusted.

Seeing the Light

So what is polarized light, and why is it relevant to autonomous navigation?

Human eyes and most cameras treat light as if it consists of a lot of little particles of various colors. While that works well for imaging and many other uses, light is actually a wave whose dynamics closely resemble those of a jump rope. If you grasp one end of a jump rope and move it quickly back and forth, you will see simple sine waves travel down its length. The orientation of these waves—vertical, horizontal, or anything in between—depends on the orientation of the invisible line along which you moved your hand. The equivalent light waves are said to be “linearly polarized” and, like the waves on the jump rope, can be vertical, horizontal, or have any angle between. Alternatively, you could have moved your hand in clockwise or counterclockwise circular patterns, which would have sent corkscrew-shaped waves down the rope. The light wave equivalents of these are said to be “circularly polarized,” and they, too, come in clockwise and counterclockwise versions. Linearly polarized versions of light can be distinguished by using ordinary polarizing sunglasses, and the two forms of circularly polarized light can be seen by using the two lenses of the most common form of 3-D movie glasses.

Most natural light is a mixture of randomly oriented vibrations, which is why we can usually ignore polarization. However, there are two important natural sources of linearly polarized light. The first is light reflected from flat surfaces. Properly processed, this form of polarized light can make manmade objects stand out in a way that is not possible with any other video processing method. Since manmade objects are a relatively new thing in the world, there is no analogue in natural life to this use of polarized light.

The second natural source is the sky, which scatters the light of the sun in a mathematically precise polarization pattern that captures both the direction of the sun and latitude information. Linear polarization in this case behaves roughly like having small arrows painted onto every part of the sky. These polarization arrows make it possible to discern which way is north even when only small fragments of the sky are visible. Birds, bees, fish, and insects all make use of this handy resource to help them navigate. Replicating such capabilities with modern sensors and computer hardware is main focus of this SBIR effort by Polaris. While not a complete substitute for GPS, careful use of this free and inconspicuous source of natural geo-location information can provide an autonomous vehicle with valuable real-time updates and verifications of where it is and what direction it is pointing.

Clues from the Past

If polarized light is so useful for navigation, why hasn’t it been used in the past? The answer is that it has, but without critical advantages made possible by modern computing and data fusion methods. In fact, the very first use of polarization for navigation was by people who sailed the North Atlantic hundreds of years ago: the Vikings. By employing a naturally occurring form of crystalline limestone called Iceland Spar or “sun stones,” Viking ship navigators could see and use polarized sky light on overcast days to help find locations and bearings.

Modern military use of polarized light navigation began in 1948 with the Pfund Sky-Compass, a polarized light compass that was intended for use during summer flights in the Arctic. Crossing the Artic was a particularly good application for polarized light navigation, since magnetic compasses fail in that region, the polar sea is featureless, and in the summertime the stars are not visible. Scandinavian Airlines has made the most use of this technology, since many of its flights require it to cross the kind of barren landscapes that baffle conventional navigation systems.

Moving Forward

The focus of the new work by Polaris is taking examples and hints from past work and moving them into a modern era of radical miniaturization, inexpensive and large computational capacities, and improved algorithms for comprehending and merging the information from polarized light with other resources. One example is that because entire global maps now can be stored in small computational devices, the location and orientation information from polarized light can be used in combination with new resources, such as miniaturized inertial guidance, to verify and validate locations in ways that were not possible in the past. Autonomy itself, with its ability to assess and evaluate data sources in more human-like ways, also adds an important ingredient by allowing an autonomous vehicle to assess and weigh the value of multiple information sources in new ways. Polarized light holds real promise in military and commercial applications to provide critical location data even when other forms are unavailable.

About the authors:

Terry Bollinger is a MITRE employee working full time on autonomy research and technologies for the Expeditionary Maneuver Warfare and Combating Terrorism Department at the Office of Naval Research. He was also chief scientist for a small team at the Offi ce of the Secretary of Defense that looked for emerging commercial technologies. Keith Hammack manages research programs in ground vehicle autonomy, fuel efficiency, mobile power, mobility, and survivability for the Marine Corps, Naval Expeditionary Combat Command, and Naval Special Warfare Command. He was previously deployed as a combat company commander with the U.S. Army.