The Remarkable Use of Moonlight to Calibrate Earth-Observing Satellite Sensors
Did you know that the USGS RObotic Lunar Observatory (ROLO) project helps Earth-observing satellite operators monitor the performance of satellite instruments throughout their lifetimes in orbit? The ROLO system uses moonlight to calibrate satellite imaging sensors (cameras), including those on Landsat 8 and 9, NASA research satellites, and meteorological satellites that provide weather pictures.
The lunar calibration technique operates using light-reflecting properties, called albedo, of the lunar surface. Calibrating satellite sensors requires techniques that can be verified in orbit, and the ROLO system predicts the brightness of the sunlit Moon with the high precision needed for this task.
The concept of calibrating sensors against the Moon was formulated at Astrogeology during the Apollo era. The observatory facility was built in the early 1990s and operated every month during the bright phases of the Moon until September 2003. More than 110,000 Moon images and more than a million images of stars were collected. These observations were used to build the lunar calibration system in use today.
“Until just a few years ago, USGS was the only institution that had a lunar calibration capability,” said Thomas Stone, Lunar Calibration Project Scientist and ROLO’s principal investigator. “I work with instrument teams from all over the world to promote using the Moon as a calibration reference.”
The capability to use moonlight in this way is recognized as a valuable asset by the international remote sensing community. It can place all satellite sensors on the same scale; in other words, if the sensors could all look at the same Earth scene at the same time, they all would register the same measurement. But since space-based sensors can't view the same thing at the same time, it is useful to have a common target to view that is a known quantity. That target can be the Moon, and the known quantity is the Moon's brightness, provided by the ROLO project.
Having a common calibration reference for all Earth-observing sensors has important implications for studying the Earth's climate and climate change. NASA has defined a set of Essential Climate Variables that need to be monitored all over the world at all times. This can only be accomplished by observing the Earth's surface from space. However, the signatures of climate change are very small, and it takes many years of precise observations to detect them. But satellites operate in the harsh environment of outer space, and their sensors are known to degrade with time. Extraordinary efforts have been developed and applied to monitor the sensors' performance in orbit and to maintain their calibration over mission lifetimes.
Because the Moon has been orbiting the Earth for billions of years, its surface has been extremely weathered by micrometeorite bombardment and unfiltered sunlight. This means that its light-reflecting properties have reached a very stable state, with no detectable degradation presently occurring. That stability is the key to lunar calibration, as it enables predicting the Moon's brightness with high precision for any observations taken by instruments. Those predictions constitute the lunar reference that is provided by USGS, and a lunar calibration is a comparison of measurements of the Moon taken by a sensor against this reference.
The ROLO project has successfully developed and demonstrated a system to use the Moon as a calibration target, despite the continually changing lunar brightness. This system is actively employed by the operators of Earth- observing satellite instruments, through interactions with USGS. Lunar calibration meets the needs for inter-consistency and long-term stability of measurements by Earth-observing sensors that are required to accomplish the climate measurement task. To learn more about ROLO, visit this website.
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