30 April 2018
One of the main functions of Earth-observing satellites is to monitor change over time, by imaging the same scenes repeatedly. But how do scientists know if differences between images reflect changes in the light received by orbiting instruments, or changes to the instruments themselves? Calibration is an ongoing challenge, and perhaps the best calibration target is the Moon – because while the surface of the Earth is always changing, the light coming from the Moon has been the same for millions of years, barring asteroid impacts.
But while the Moon is a more stable calibration target than relatively featureless parts of Earth such as stretches of desert or salt flats, its light still varies – the Moon’s orbit isn’t circular and the face it shows to Earth changes slightly over the 28-day lunar cycle, so there is actually an 18-year lunar light cycle.
The USA, China and Europe are working to build better models of lunar light and so improve the accuracy of optical instrument calibration. The European Space Agency has installed an instrument – a solar photometer, designed to measure airborne particles but adapted to work at night – high on 3,718m Mount Teide in Tenerife, to measure the light from the Moon every night for the next two years.
Overseeing the project for ESA is Marc Bouvet. He says that the performance of orbiting instrments “can gradually drift, due to radiation or lens contamination for instance, or mechanical changes”.
“Our objective is to observe the full disc of the Moon across a two-year period, to be able to create a model of lunar radiation that is much more accurate. Today’s best modelling effort has a 10% uncertainty associated with it; we want to be able to get that down to around 2%.”
It is hoped that the project, being undertaken by a consortium incorporating the UK’s National Physical Laboratory, Spain’s Valladolid University and Belgium’s Flemish Institute for Technological Research (VITO), may deliver an improved model by the end of the decade.
The second lunar calibration conference, held in China last year, generated a lot of excitement about US, Chinese and European efforts to accurately model lunar light, adds Bouvet: “Success would lead us to being able to link past, present and future optical Earth-observing missions to a common calibration reference, allowing easier cross-comparisons of data, enriching our overall picture of the terrestrial environment.”