Objects in mirror are really far away
Earth-bound mirrors calibrate spacefaring satellite sensors
A satellite is only as good as its vision.
Satellites must be able to see the earth well enough to produce crystal-clear images and collect the detailed data scientists use to forecast the weather and perform advanced analytics.
Raytheon has developed a unique means for calibrating the sensors aboard orbiting satellites: Packing a parking lot back on Earth with mirrors. Called the Specular Array Calibration method, or SPARC, it turns the parking lot into a cluster of virtual stars, each reflecting the sun back at the sky. As the satellite constellation flies overhead, assuming normal operating conditions, onboard sensors scan the mirrors for spatial accuracy, using light spectrums ranging from infrared to ultraviolet.
“Satellites are like giant eyes in the sky, staring down at every corner of the globe,” said John Coogan, a Raytheon engineering fellow. “Similar to how a pair of glasses helps our eyes bring blurry pages on a book into focus, calibration brings clarity to the satellite’s sensors, ensuring accuracy.”
The extreme shaking during launch and the harsh conditions of space create stress on sensors that makes calibration even more important. Many conventional calibration methods date back to the first Earth-observing satellites of the late 1950s. To fix and fine tune sensors, operators would typically take a satellite out of normal orbit, rotate it upside down and point it at a known star, like the sun. Performing such maneuvers has to be done in a tight window of time, at the risk of damage or failure.
There's been a greater need for much more accurate satellite imagery of the Earth in recent years. The level of detail found in some observations is refined enough to change everyday life. Satellites help monitor agricultural and industrial production around the world, support economic forecasts, help make weather predictions more accurate and observe disasters to help emergency responders prepare for storms. Even national security depends on precise satellite observations.
Calibration with SPARC determines how well a satellite can see large objects, from ships entering and leaving busy ports to the containers sitting on the decks of those ships. If satellites are not calibrated, then you can’t determine when and what things are changing.
“The challenge is no longer simply observing areas like shipping ports and counting how many ships are entering and leaving,” said Stephen Schiller, a Raytheon engineer and SPARC inventor. “What’s important is knowing that on Monday, a satellite imaged a ship with 50 blue containers leaving the port, but on Wednesday it never stopped and now has 49 blue containers and one red container. That change could be insignificant, or it could be the difference in identifying a potential smuggling operation."
SPARC calibration produces data that is more accurate because it's been collected from different sensors on different days, according to Schiller.
And SPARC is a hardy system. The mirrors are built to be more impervious to weather and environmental conditions than other calibration methods. Take, for example, the Salar de Uyuni, one of the flattest places on Earth. The vast plain of white, cemented, reflective salt in the mountains of Bolivia has provided international space agencies with a working calibration target for years. However, the appearance of the lake bed is continually changed by rain and wind.
“As long as there’s no major cloud coverage or the surface isn’t turned into a mush causing low reflectivity, calibration using natural features like Salar de Uyuni works great,” said Coogan. “But the surface of the sun is always hot and it doesn’t have clouds. Calibrating using the SPARC method is not only easier on the satellites, it provides better insights faster and more often, at a fraction of the cost to satellite operators. That means more data more often, with greater precision.”
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