Aiming for the Moon - to accurately monitor the Earth

So, ensuring the accuracy of measurements of these signals is crucial.

On 4-8 December, scientists from around the globe will take part in a meeting hosted by EUMETSAT to discuss progress on this challenge.

The meeting aims to share knowledge and expertise in the field of lunar calibration, that is, using satellite instruments’ observations of the Moon to determine the accuracy of observations of the Earth’s systems.

Using the Moon to calibrate satellite instruments

Unlike the Earth, the Moon has several advantages that make it a very powerful calibration target for satellite instruments.  Firstly, the Moon has properties that make it an exceptionally stable target over time; secondly, it has no atmosphere and there is no impact from human activity to influence sensors’ observations, EUMETSAT Radiometric Calibration expert Dr Sébastien Wagner said.

The lunar cycle is also well known. Combining this knowledge and the extreme stability of the Moon surface allows scientists to establish numerical models to simulate very accurately the lunar brightness at any point in time. In return, those simulations can serve as references for comparisons with lunar measurements from an instrument in space.

“When an instrument has the possibility to observe the moon on a regular basis, it offers a unique opportunity very accurately assess the instrument’s calibration and temporal stability, which is essential for Earth observation missions,” Wagner said.

“We want to work also on defining intercalibration algorithms that can allow us to tie all of the instruments that we have to a common reference, using the Moon.”

To achieve those goals, the scientists are working to define more and more accurate models that would simulate what a satellite’s instrument should see when it targets the moon to determine the accuracy of the instrument.

To do so, they need to “characterise” the Moon’s brightness over time, by getting as many independent and accurate measurements of it as possible.

“The only limitation comes from the models and the measurements that are used to infer those models,” Wagner said.

“We are trying to get as many measurements as possible to calibrate the models.

“Measurements that are traceable. Once you have the models, you can apply them to your instruments.”

Up to 70 scientists are expected to take part in the meeting, either in person at EUMETSAT’s headquarters in Darmstadt, Germany, or online.

It is hoped the meeting will lead to a series of recommendations, actions and decisions to help improve the accuracy of satellite systems and reduce the uncertainties of instruments’ measurements and reference data.

Reducing uncertainty

The “why” of this international effort goes back to the original point about the need for confidence in the accuracy of satellite data used for climate and weather monitoring.

Satellites’ instruments can degrade over time and scientists who use their data need to know whether they are seeing results of a change in what an instrument is measuring, or a change in an instrument’s accuracy.

Improving the lunar models to serve as references against which satellite instruments can be calibrated will help determine the accuracy of those instruments’ observations of the Earth system.

Main image: A complete view of the Moon, taken by the Flexible Combined Imager on Meteosat Third Generation – Imager 1, on 1 September 2023 at 10:50 UTC