Toward a synergy between on-orbit lunar observations Sophie Lachérade CNES GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Number of measurements Lunar observations The SADE Lunar database at CNES: SENSOR Spectral range Nb of spectral bands Spatial resolution Acquisition Dates Phase angle range Number of measurements PHR-1A Vis-Nir 4 2.80m 2012-2013 [-115°;115°] 166 PHR-1B 2013 970 AQUA/MODIS 7 250-500m 2002-2013 [51°;55°] 108 MSG1 Vis-Nir-Swir 3 2500m 2003-2012 [-150°;152°] 393 MSG2 3000m 2006-2013 [-145;145] 366 LANDSAT8* 8 30m -7° and +8° 148 -> A lot of sensors with different characteristics. One common thing: there are able to look at the Moon ! PHR: Pleiades High Resolution GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT *Results on-going !

Lunar observations – Spectral response PHR1A PHR1B MSG1 MSG2 MODIS LANDSAT8 GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Lunar calibration – But what kind of calibration ? The current lunar reference, internationally used, is ROLO. Different calibration methods could be perfomed, based on lunar acquisitions, all needed the ROLO model : - Stability monitoring - Inter-band calibration - Absolute calibration - Cross-calibration GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Multi-temporal calibration Goal: guaranty the stability of the sensor better than 1% PHR1B_B0 All phases MSG1_VIS06 All phases -> Limitation of the ROLO model to take into account the phase angle. -> Can be bypassed by using a restricted phase angle (PHR:40°, MODIS:55°, LANDSAT8:7°) GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Multi-temporal calibration After the phase angle selection: MSG1_VIS06 Phase:-40°±5° PHR1B_B0 phase:±40° MODIS_555 phase:55° Good accuracy of the method for one chosen phase ! GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Multi-temporal calibration After the phase angle selection and adjustment of the temporal range: PHR-1A_B2 phase:40° MODIS_555 phase:55° 2 years of PLEIADES versus 11 years of MODIS -> Seasonal cycles observed both on MODIS and PLEIADES with similar shape and level. Is it a residual effect of the lunar librations modelised by ROLO? GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Interband calibration Goal: estimation of a simulated irradiance in one band knowing the irradiance in another band and the albedo of the Moon in these two bands. Accuracy to be achieved: better than the absolute one -> The accuracy of the method depends on the relative spectral accuracy of the albedo in the two bands Which ROLO to use ? Ref: The spectral irradiance of the Moon H.H. Kieffer and T.C.Stone The astronomical Journal, 129:2881-2901 2005 June Apollo correction: Fig 8 GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Interband calibration – Phase dependence Calibration of the Blue band of PHR1B (B0) from the other spectral bands of PHR1B PHR1B All phase angles PHR1B [-70°;70°] -> The phase dependence of ROLO has less influence on the interband calibration results than on the multi-temporal calibration results. 2% versus 5% for phase [-100°;100°] This allows us to perform an accurate interband calibration whatever the phase ! -> Useful for sensors which cannot choose their acquisition phase angle GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Multi-temporal calibration Goal: guaranty the stability of the sensor better than 1% PHR1B_B0 All phases MSG1_VIS06 All phases -> Limitation of the ROLO model to take into account the phase angle. -> Can be bypassed by using a restricted phase angle (PLEIADES:40°, MODIS:55°, LANDSAT8:7°) GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Interband calibration – Phase dependence Calibration of the red band of MSG1 (VIS06) from MSG1 VIS08 MSG1 All phase angles MSG1 [-70°;70°] -> The conclusion is less obvious on the MSG dataset but the residual dispersion may be due to the integration step at high phase angles -> When limiting phase angles to [-70°;70°], the conclusions are the same than for PHR : method with very few dispersion ! GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Interband calibration – Spectral dependence Interband calibration results for PHR1B (phase angles range: [-70°;70°]) and MODIS: PHR1B AQUA/MODIS The results highlights a dispersion depending of the reference band which is used. This dispersion is up to 3% but the absolute accuracy of the results strongly depend of the spectral knowledge of the Moon albedo. -> Necessary to compare these results with other calibration methods to estimate the accuracy of the method: -> very good consistence of the results other methods GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Lunar calibration – But what kind of calibration ? The current lunar reference, internationally used, is ROLO. Different calibration methods could be perfomed, based on lunar acquisitions, all needed the ROLO model : - Temporal stability observation - Inter-band calibration - Absolute calibration - Cross-calibration GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Absolute calibration ROLO versus on-orbit measurements -> AQUA/MODIS, PHR1A and PHR1B show a very good agreement PHR-1A and PHR-1B absolute calibration based on ENVISAT/MERIS -> Uncertainty of the absolute calibration of the ROLO model up to 10% at 55° GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Cross-calibration results PHR-1B versus AQUA/MODIS: Final accuracy: ±3% Retrieve calibration differences between MERIS and MODIS observed on desert sites (Lachérade et al., IEEE, 2013) Uncertainties for Moon cross-calibration due to the lack of correct spectral interpolation between PHR and MODIS (cross-calibration limited to a band-to-band cross-calibration) Estimation of the interband accuracy based on Moon acquisitions ≈ 3% GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Cross-calibration results Where is the truth ? Need of a lunar reference in the SWIR (LANDSAT8 – VIIRS ?) MSG2 versus AQUA/MODIS Very good agreement between cross-calibration results using the Moon (PHR1B and MODIS) and results obtained over desert sites. Dispersion of the interband calibration results using the SWIR band of MSG. GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

Lunar observations – Perfect sensor ? A perfect lunar reference sensor should be characterised by the following parameters: - its spatial resolution (PHR-LANDSAT8) A image of the Moon with a lot of pixels will decrease the uncertainties when integrating its irradiances - the range of phases it has/will observe(d) (PHR-MSG) To be sure to take into account the phase effect, it is better to cross-calibrate sensors using the same phase of the Moon - its spectral resolution in term of band width and spectral range (SWIR range needed to constraint the spectrum) (LANDSAT8-VIIRS?) The ideal sensor is characterised by a large number of spectral bands covering the full spectrum from 400nm to 2500nm - the accuracy of its absolute calibration (MODIS-LANDSAT8-VIIRS?) No current sensor corresponds to this ideal one !!! It is necessary to take advantage of several of them to derive a perfect lunar reference GSICS Annual Meeting – 24-28 March 2014 - EUMETSAT

CONCLUSIONS To do list: - Improvement of the lunar cross-calibration method by implementing an interpolation function in the MUSCLE/SADE tools - Analyses of the LANDSAT8 lunar dataset (VIS-NIR-SWIR bands) - Cross-calibration and interband calibration of all the sensors to better discriminate uncertainties linked to the lunar albedo and uncertainties linked to the methods - Improvement of the reference lunar model (based on available datasets): PLEIADES MSG MODIS / VIIRS ? LANDSAT8 Phase angle Effect Spectral range Inter-band calibration Absolute calibration Reference lunar Model ROLO