Improved Confidence on the PLEIADES In-flight Absolute Calibration Through the Merging of Different Vicarious Calibration Methods

Transcription

1 Improved Confidence on the PLEIADES In-flight Absolute Calibration Through the Merging of Different Vicarious Calibration Methods 22 nd CALCON TechnicalConference August 2013, Logan, UT Sophie Lachérade Bertrand Fougnie Charles Galindo Philippe Gamet Gwendoline Blanchet Laurent Lebègue

2 from SPOT to PLEIADES SPOT4 PLEIADES SPOT2 SPOT1 swath 60 km, 2 instruments East/West tilt +/- 27 SPOT 1-4 : 10m PA, 20m XS SPOT 5 : 2.5m PA, 10m XS swath 20 km, 1 instrument agile Satellite 70 cm PA, 2.80 XS SPOT1 SPOT2 SPOT3 SPOT4 SPOT PLEIADES

3 ISTANBUL (Extrait) E

4 The PLEIADES system System with a very high level of agility! Manhattan Dubaï: the highest tower of the world

5 The PLEIADES system System with a very high level of agility! Manhattan Dubaï: the highesttowerof the world Monument Valley

6 when Pleiades points the sky Jupiter and its moon

7 two other planets Saturne Mars

8 Moon (spatial resolution: 380m)E

9 The PLEIADES system (spectral bands) Normalized Spectral Response B0 B1 B2 B3 PAN Wavelength (µm)

10 PLEIADES Absolution calibration Goal: absolute radiometric calibration < 5% Methods : African deserts La Crau Moon Oceans Antarctica

11 The PLEIADES absolute calibration To reach this goal: 5 types of natural targets with 5 spectral behaviors Examples of average spectrum:

12 The PLEIADES absolute calibration Desert and Dome calibration methods are cross-calibrationmethods, using reference sensors Rayleigh method is an absolute calibration method based on the rayleighscattering above oceans (used only for the spectral bands from the blue to the red) La Crauis a site where an automatic station is settled, provided simultaneous measurements with satellite data (absolute calibration) The Moon is used mainly for multi-temporal calibration, but can be also used for cross-calibration, inter-band calibration To cross-check the results and the sites is very important to validate and improve the calibration of the sensors

13 PLEIADES Absolution calibration: our dreams What we would like to obtain Very good consistency of the results for each spectral band of the sensor PLEIADES-1A Green band Ak/Ak_ground /09/ /04/ /10/ /05/2013 Acquisition Date

14 PLEIADES Absolution calibration: the real life! What we obtain at the beginning of the calibration phase Dispersion of the results depending of the methods, sites and spectral bands PLEIADES-1A Green band 1.10 Ak/Ak_ground Whereisthe truth??? /09/ /04/ /10/ /05/2013 Acquisition Date

15 PLEIADES Absolution calibration How to improve confidence on the results First step: determine where the truth is Second step: understand why some methods/sites show biais The sites don t have the same spectral properties: Take advantage of them!!! Third step: confirme the previous conclusions with another methods

16 PLEIADES Absolution calibration How to improve confidence on the results First step: determine where the truth is Second step: understand why some methods/sites show biais Third step: confirme the previous conclusions with another methods How: spectral comparison of the Top Of Atmosphere reflectances betweenpleiades and ourreferencesensor(envisat/meris) Top Of Atmosphere Reflectance MERIS_desert MERIS_dome Dome: ozone absorption band Desert: inflexion point of the sand 0.1 PLEIADES_spectral response Wavelength (µm)

17 PLEIADES Absolution calibration How to improve confidence on the results First hypothesis: Oceans and Dome give the correct calibration results Comparison of TOA reflectance over dome sites between PLEIADES (with application of the 1 st hypothesis) and MERIS. Réflectance TOA MERIS PLEIADES-1A (based on Dome+Oceans) Ozone Wavelength (nm) Hypothesisnot correct. The TOA reflectancein green couldn t be higher than the blue and redone.

18 PLEIADES Absolution calibration How to improve confidence on the results Second step: understand why some methods/sites show biais Analysis of errors in the cross-calibration based on dome sites Solar irradiance (W/m²/µm) Thuillier 2002 PLEIADES_solar_irradiance spectral response Green Band Wavelength (µm) 0.0 Conclusion: the solar irradiance used for the dome processings was not enough precise! This errorwassufficientto createthisbiais of 7% for Domesites.

19 PLEIADES Absolution calibration How to improve confidence on the results After correction of the solar irradiance: Good consistency of the results for desert and dome cross-calibration methods Ak/Ak_ground /09/ /04/ /10/ /05/2013 Acquisition Date

20 PLEIADES Absolution calibration How to improve confidence on the results Second step: understand why some methods/sites show biais Analysis of errors in the cross-calibration based on ocean sites Calibration on ocean sites = modelisation of the rayleigh scaterring This methodmodelisesthe radiance receivedby the instrument in the equivalentwavelengthof the consideredspectral band Different errors/uncertainties are possible: -thismethodisnot enoughprecisedue to spectral rejection of the PLEIADES spectral responses -the PLEIADES spectral responsesare measuredonlyin 6 points of the fieldof viewon ground. Wedon tknow preciselythe spectral variation between these points. 1nm 1% in the calibration results Thesetwopoints couldexplainthe biais observedfor the oceanssites.

21 The PLEIADES system (spectral bands) B0 B1 Normalized Spectral Response B2 B3 PAN Rejection Wavelength (µm)

22 PLEIADES Absolution calibration How to improve confidence on the results Third step: confirm the previous conclusions with another methods How: Inter-band calibration based on Moon acquisition Differences between calibration results (%) Desert/MERIS Dômes/MERIS Rayleigh B0 B1 B2 B3 PAN PLEIADES Spectral bands Differences between Moon results and other methods Good consistency between Moon, Desert and Domes. Confirmation that there is a biais within the calibration over oceans.

23 PLEIADES Absolution calibration How to improve confidence on the results CONCLUSIONS: Quite difficult to assess calibration results better than 5% To improve confidence on the results, we need - Different sites (snow, desert, Moon, ocean ) - Different methods : absolute calibration cross-calibration inter-band calibration - Different reference sensors for cross-calibration Why: To be able to answer to the question: Where does the observed phenomena come from? (sites, methods, data ) To confirm analysis: Once the phenomena is unsterstood, it is necessary to confirm it by another method/site/reference sensor.

24 Where to get some PLEIADES images?