Sensor Technology Opens New Horizons

Signal, Jun 2008 by Kenyon, Henry S

Innovative optics, radar systems cut through the fog of battle.

In the future, there will be no place to hide from the U.S. military. Two prototype sensor technologies may soon allow warfighters to observe enemy units at great distances and to track their movement inside buildings and urban areas. These systems benefit from recent developments in optics, radar, algorithms and processing to pull images out of desert heat distortion or to create maps of entire neighborhoods rapidly, greatly increasing soldiers' situational awareness.

These systems are part of an effort by the Defense Advanced Research Projects Agency (DARPA) to increase warfighters' operational capabilities while decreasing an adversary's ability to surprise or outmaneuver U.S. forces. The Super Resolution Vision System (SRVS) program will develop a sniper/reconnaissance telescope capable of observing targets at longer distances than currently possible with unaided optics, while the Visibuilding program will design a radarbased surveillance system to map buildings and track movement in those structures.

Atmospheric turbulence in the form of heat shimmer can greatly limit the distance ground-based electro-optical and infrared systems can observe and track terrestrial objects. The SRVS relies on an atmospheric quirk and raw signal processing power to stitch a complete image out of distorted fragments. The technology exploits a phenomenon known as turbulence-generated micro-lensing, which creates a brief, high-resolution "lucky" image out of a distorted scene.

Scientists have been aware of this effect since the 1970s, explains SRVS program manager Dr. Jennifer C. RickUn. If an object is scanned long enough with a high-speed camera, lucky images will appear. However, she notes, it soon became evident that it was impractical to scan for such images because the statistical probability of reliably capturing a lucky image was very low. In the late 1990s it was demonstrated that although a complete image could not be regularly captured with this technique, it was possible to retrieve a piece of it. A highspeed camera could theoretically pick out the good parts of a distorted image and assemble them into a coherent, high-resolution picture. She adds that SRVS is now possible because of increased processing power and advances in signal processing algorithms. "It's really an enormous signal processing issue," she says.

If a warfighter were using an SRVS-based system to observe a cluster of buildings and vehicles, the optics would pull an image together from scattered light that may not even appear in the scope's center of focus. Ricklin explains that because of the refractive properties in the atmosphere, light does not travel in a straight path but zigzags every time it encounters small areas of differing atmospheric pressure and refractivity. Atmospheric turbulence, most commonly visible as heat shimmer, is most active in the first few meters above the ground and decreases exponentially with altitude. "You're taking advantage of the fact that some rays that would not be captured by the [telescope's] optical diameter are accidentally refracted into the focal plane from the outside. It happens very randomly and only for parts of the image, not the entire image. But if you have the algorithms and the hardware, you can take advantage of this," she says.

The program recently passed its important first phase, which demonstrated the basic practicality of the technology. With the go-no-go tests complete, the next phase will focus on refining the technology. The final goal of the program is to develop a reconnaissance-spotter scope in the same form factor as the existing scope used by U.S. forces. Ricklin notes that this is a twofold problem. The first challenge is to demonstrate that this physical phenomenon can be exploited, which had never been done before outside of a laboratory. The second goal is to develop a system and fit it into a form factor with the necessary processing and service requirements for use by warfighters. "Nobody wants to carry an optical bench on their backs. It needs to be quite ruggedized," she says.

During phase one of the program, DARPA scientists conducted tests at Sandia National Laboratories in New Mexico to determine how far U.S. Army and Marine Corps snipers could see with conventional optics. Ricklin notes that no one had ever actually asked snipers how far they could see a target through atmospheric turbulence. Although the marksmen said they could pick out objects at hundreds of meters, she shares that their claims had never been scientifically measured. The Sandia tests measured the differences between current capabilities and SRVSaided viewing. Although she cannot provide any specific details, she said that the capability greatly increased the distance over which snipers could accurately detect targets. She adds that the SRVS's optical technology can be applied to any type of system that is affected by atmospheric turbulence, such as long-range surveillance systems and weapons sights.