ESA project: lunar spectral irradiance measurement and modelling for absolute calibration of Earth Orbiting optical sensors

The ROLO model has been found to be the most reliable extraterrestrial lunar irradiance model so far. However, some comparison exercises have proven that the ROLO model has an uncertainty in the absolute scale of 5-10 %, which might be related to the absolute calibration of the model derived from observations of the star Vega. There is another possible source of errors related to the atmospheric extinction correction. The different zenith angle for the Moon and Vega during this star-based calibration method leads to lunar reflectance spectra with unexpected band-to-band deviations, introducing the spectral (absolute) band scalings.

This uncertainty in the model appears as a systematic error (bias) but it is hypothesized that there exists also a phase angle dependence of the ROLO calibration (up to 6% according to Vitticchie et al. (2013) when comparing ROLO and SEVERI lunar irradiances). Other authors found a similar dependence with phase angle (Vitticchie, 2013, Lacherade, 2014, Barreto et al. 2016). It is inconclusive as to whether the systematic error was the result of instrument calibration of the instruments involved in such comparisons or errors with the ROLO model itself.

The conclusion of all these previous works is that the uncertainty of the ROLO model for absolute spectral irradiance is poorer (5-10%) than for relative spectral irradiance (≈1%). The long-term aim should be to provide SI-traceable absolute irradiance for the Moon.

Due to the excellent photometric stability of the lunar surface, any model which provides an absolute irradiance of the Moon, accounting for phase and libration, could indeed be used to re-calibrate and re-analyze historical data for any satellite that routinely obtained images of the Moon. This would result in a much higher degree of accuracy of our historical climate record and reduce the uncertainty in our climate forecast. This is the main reason for the ESA project launched in 2017.

The objective of this project is to use ground-based measurements taken with a Cimel CE318-TP sun-moon-sky photometer to improve upon the modelling of the lunar disk irradiance variations through its cycles (to < 2 % uncertainty), including polarization measurements. This project involves nocturnal measurements at the high-mountain stations IZO and TPO (2373 and 3555 m a.s.l., respectively), precise calibration at a National Metrology Institute (NPL, London), and high-quality correction for atmospheric effects to obtain extra-terrestrial lunar irradiance. This project is currently ongoing, and it is planned to continue for at least 4 more years (until 2023), to account for the phase/libration lunar cycle.

This project was assigned to a consortium of three members: University of Valladolid/Izaña Atmospheric Reseach Center, National Physical Laboratory (NPL, London) and Flemish Institute for Technological Research (Vlaamse Instelling voor Technologisch Onderzoek, Belgium).

Figure 1.- Participants in the ESA project in a meeting held in Tenerife, March 2018.

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