1. Toward a synergy between on-orbit lunar observations
Sophie Lachérade
CNES
GSICS Annual Meeting – March EUMETSAT

2. 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
[-115°;115°]
166
PHR-1B
2013
970
AQUA/MODIS 7 m
[51°;55°]
108
MSG1
Vis-Nir-Swir 3
2500m
[-150°;152°]
393
MSG2
3000m
[-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 – March EUMETSAT
*Results on-going !

3. Lunar observations – Spectral response
PHR1A
PHR1B
MSG1
MSG2
MODIS
LANDSAT8
GSICS Annual Meeting – March EUMETSAT

4. 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 – March EUMETSAT

5. 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 – March EUMETSAT

6. 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 – March EUMETSAT

7. 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 – March EUMETSAT

8. 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: June
Apollo correction: Fig 8
GSICS Annual Meeting – March EUMETSAT

9. 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 – March EUMETSAT

10. 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 – March EUMETSAT

11. 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 – March EUMETSAT

12. 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 – March EUMETSAT

13. 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 – March EUMETSAT

14. 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 – March EUMETSAT

15. 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 – March EUMETSAT

16. 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 – March EUMETSAT

17. 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 – March EUMETSAT

18. 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