For more than 15 years, the Moon has been evaluated as a valuable radiant calibration source for Earth observation sensors. The absence of an atmosphere, its temporal stability and high and predictable availability makes it an ideal target for the vicarious calibration of optical sensors. The photometric stability of the surface of the Moon is estimated to be of the order of 10 9 per year.
In 2017, ESA launched an initiative to derive the new “Lunar Irradiance Model of ESA” (LIME). The model is based on automated measurements of the total lunar irradiance, taken at high altitude using a multispectral CIMEL CE318-TP9. The instrument was calibrated and characterized at the National Physical Laboratory, against SI-traceable standards. The University of Valladolid mounted the instrument on the Pico Teide in Tenerife, where it continues to perform nightly lunar irradiance measurements for lunar phases angles between -99° to +99°, at a frequency of about 100 quality controlled measurements per year.
A follow-on project is currently on-going to take new measurements and develop further improvements to the model with better the spectral resolution, improved spectral interpolation between model wavelengths and a more detailed uncertainty analysis. It is also planned to develop, within the project, a reference implementation to be released in the form of an open source repository. The current status and achievements of this project are demonstrated in this presentation.
Up to now, the model has already been applied to calibrate different spaceborne sensors, mostly on an ad-hoc basis (Nemenam et al.). The PROBA-V was routinely calibrated during its mission life-time using vicarious calibration methods. As part of the operational radiometric calibration, the PROBA-V instrument has been taking lunar acquisitions (Sterckx et al.). For eight years, the moon was imaged twice a month at a constant phase angle, 7 degrees before and after full moon. This dataset has been compared with the LIME model during the instrument operational phase.
Since mid-2020, PROBA-V operational phase was transferred to the ‘experimental’ modus. With the instrument recording only about 25% of the nominal earth-view acquisitions, the freed capacity has been used to record extra lunar data. Up to now, 7 full lunar cycles of -90 degrees to 90 degrees phase angle with a granularity of 10 degrees have been acquired. This data is being be used to support the development and further understanding of the LIME model.
With the nominal calibration acquisitions of the moon at constant phase, only a small part of the model is exploited. This presentation will show how LIME is applied to calibrate the PROBA-V sensor and how the results of the experimental acquisitions contribute to the model development.
Neneman, M.; Wagner, S.; Bourg, L.; Blanot, L.; Bouvet, M.; Adriaensen, S.; Nieke, J. Use of Moon Observations for Characterization of Sentinel-3B Ocean and Land Color Instrument. Remote Sens. 2020, 12, 2543. https://doi.org/10.3390/rs12162543
Sterckx, S.; Adriaensen, S.; Dierckx, W.; Bouvet, M. In-Orbit Radiometric Calibration and Stability Monitoring of the PROBA-V Instrument. Remote Sens. 2016, 8, 546. https://doi.org/10.3390/rs8070546