Stars on the ground

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 Intelligence & Space, one of four businesses that form Raytheon Technologies, employs 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 Intelligence & Space 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.

“We’re creating an array of stars on the ground that you can use as calibration references,” said Stephen Schiller, senior systems engineer and calibration scientist at RI&S Space Systems. “That way, you won’t have to turn the sensor away from Earth, which can disrupt the science it’s trying to capture.”

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.

Using SPARC, scientists could fuse sensor data to paint an accurate view of changes here on Earth, from monitoring human activity to changes in the weather.

“Say you’re monitoring algae blooms in areas like Lake Erie,” continued Schiller. “When you get an accurate reading of the colors in the image, you can identify which species of algae are being produced in these blooms. When you have multiple colors, you can better understand the concentration level of the bloom. And as the blooms are observed over time, you can see the growth or depletion in response to environmental conditions.”

The patented calibration method has been tested with commercial, civil and DoD sensor systems. It was first licensed to Labsphere Inc., where it is planned to be used as an operational service for the remote sensing community in the fall of 2020.