Imaging engineers at South Dakota State University are testing a prototype device that may soon be used to calibrate sensors on Earth-imaging satellites, according to Larry Leigh, director of the SDSU Image Processing Laboratory. The partnership with Labsphere, a New Hampshire-based electro-optics company, will also help Leigh and his team conduct research to validate new remote sensing products.
“It’s a two-way street,” Leigh said. “Labsphere wants our opinion about its hardware, which also allows us to pursue some new research areas.” Labsphere is a global provider of systems, materials, and services for photometry and radiometry applications including remote sensing, light metrology, image sensor characterization and spectroscopy.
The Field Line-of-Sight Automated Radiance Exposure, or FLARE, system uses convex mirrors to redirect the sun’s rays toward satellite sensors and take radiometric measurements. The goal is to offer clients a cost-effective means of calibrating optical sensors on unmanned aerial vehicles, airborne manned aircraft and orbiting satellites.
“The company has an approach that is unproven,” Leigh explained. “The science says it works; the algorithm says it works, but they have to put an instrument in the field to make sure they can achieve results the industry can get behind.”
The SDSU Image Processing Laboratory is one of only three university laboratories in the nation doing radiometric satellite calibration.
Signals from satellite sensors produce digital images composed of pixels. Each pixel measures the amount of energy reflected or emitted from Earth, Leigh explained. Before a satellite is launched, the sensors are calibrated; however, while in orbit, the sensors can drift or change.
Imaging engineers take measurements when the satellite passes over a specific location and run those measurements through an atmospheric model to predict what the satellite sensors see. Based on those readings, engineers adjust, or calibrate, the sensors.
Validating FLARE calibration
The FLARE system arrived in the latter part of June via a semitrailer and was installed within a few days at the site 3 miles north of Arlington. The FLARE system is 16 feet in diameter and sits upon a 20-foot-diameter concrete slab. A tower with additional instrumentation stands near the device.
The deployment site is strategically located along the path of Landsat 8 and the Sentinel 2 satellites, so the researchers can collect data every eight days—provided there are no clouds obscuring the sun.
“This is an alpha product,” Leigh said, noting the company is working to reduce the device’s size to increase its mobility. To do the testing, Leigh is working with Chris Durell, Labsphere’s business development director for remote sensing.
“FLARE is a next-generation tool for satellite and airborne imager calibration” Durell said. “It will automate and improve the process of Earth remote sensing and lead to digital imager calibration becoming more accurate, easier and less expensive by several factors.”
Furthermore, Durell continued, “Better calibration means getting more insightful data from every sensor image. Calibrated images means better climate science, weather prediction, agricultural mapping and other vital, beneficial information.”
When a system in the FLARE network is tasked by the customer through the cloud portal, the rectangular panels flip to reveal the mirrors and the platform rotates to focus the sun’s light toward the satellite. The process takes only a few mouse clicks and minutes versus the hours and days needed today through classic means, Durell added.
To validate the instrument, Labsphere runs FLARE reflectance measurements “through its algorithm to provide a solid calibration factor,” Leigh explained. Meanwhile, the SDSU imaging engineers take measurements using “the traditional method we’ve been using for 20 years to see if we come up with the same answer.”
Leigh anticipates the testing will take at least 18 months, but the partnership may continue longer than that.
Calibrating new surface products
Access to the FLARE hardware will help Leigh and his group determine how to validate ground-level reflectance data.
Traditionally, satellite sensors are calibrated based on top-of-the-atmosphere reflectance; however, demand is increasing for products that use ground-level reflectance. For instance, these products can help farmers evaluate the health of a corn crop or identify areas affected by disease, Leigh explained.
“It’s a new area when it comes to satellite sensors,” he said. The Europeans and Australians are trying to figure it out and Rochester Institute of Technology, the University of Arizona and the SDSU calibration groups are coming up with the American approach to ground reflectance validation.
“We want to know how well the sensors and algorithms are doing that,” said Leigh, noting lab clients, such the U.S. Geological Survey, are interested in utilizing this capability. “That will open new potential avenues for calibration and validation of another level of products that the industry wants.”
Labsphere, Inc. is headquartered in New Hampshire, US, with a satellite facility in Shanghai, China, and a global network of distribution partners. Founded in 1979, it is part of the Halma group of companies. Labsphere provides innovative solutions for a wide range of applications including LED/SSL lighting, laser power measurement, remote sensing, imager/consumer camera, automotive, defense and security, and health and biomedical optics to both production and research environments. For more information, visit www.labsphere.com.