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24 2. Remote Sensing with MODIS any unit. Radiometric calibration converts instrument digital numbers (DN) to physical radiance values (W.sr −1 .m −2 ). Since CZCS (Coastal Zone Color Scanner), most space instruments relie on on-board, in-flight internal calibration systems that include a solar diffuser plate oriented towards the sun. The solar irradiance reaching the diffuser is then used as a radiance reference to adjust the sensor absolute radiometric calibration. Furthermore, pre-flight calibrations are not necessarely optimal in the actual space environment of the sensors and might require further adjustments. On many instruments, the continous exposure of the diffuser plate to solar radiations and bombardment by space particules, induces a progressive degradation that needs to be accounted for in the overall system (see section ). This can be achieved using an additional diffuser to monitor the primary diffuser degradation (MERIS) or by comparing lunar observations to the light scattered by the solar diffuser (MODIS and SeaWiFS). In addition to absolute and relative radiometric calibration, vicarious calibration techniques are used to minimize errors between satellite-derived data and stable ground targets with known radiance values. Typical calibration sites include desertic regions, ice sheets, clouds and ocean targets contaminated with sun glint. Assuming the availability of an extensive in situ data set and a reliable atmospheric correction, the sensor response can be adjusted so that the values of a geophysical variable estimated from satellite radiances, Figure 2.7 – The ocean color from satellite imagery is determined by the organic constituents of the ocean upper surface. These images acquired by MODIS illustrate phytoplancton blooms in the South Atlantic Ocean, off of the cost of Argentina. Phytoplancton blooms offer a wide variety of colors ranging from turquoise blue to dark green. The strong color variations are due to the pigment composition of each phytoplancton specie and their depth in the eutophic layer.
25 coincide with those derived from ground measurements. 2.4.2 Atmospheric correction Prior and after its reflection on the Earth surface, solar radiation is subject to molecular and aerosol scattering. The cumulation of these processes, combined with the high altitude of satellites, reduces the magnitude of the radiance reaching the sensor. The received signal, known as Top-Of-Atmosphere (TOA) can be expressed as a sum of radiance contributions related to several processes. Atmospheric Correction then estimates the radiative portion of these effect prior to the generation of surface reflectances. Atmospheric correction is particularly complex in the case of ocean color remote sensing where the TOA radiance can be written in a simplified form as : L T OA = L Rayleigh + L Aerosol + L Rayleigh−Aerosol + TL Glint + tL W ater (2.7) In the previous equation, - T and t are the direct and diffuse atmospheric transmittances between the ocean surface and the instrument. The portion of energy that reaches the sensor is related to the optical thickness of aerosols contained in the atmopshere - L Glint is the specular reflection of sun light on the oceanic surface towards the sensor viewing direction - L Rayleigh and L Aerosol both translate scattering effects of light and depend on atmospheric pressure, temperature and polarization. - L Rayleigh−Aerosol accounts for coupling processes between Rayleigh and Mie scaterring. - L W ater is the signal to be retrieved after atmopheric correction and corresponds to the water-leaving radiance The L W ater term represents a weak portion (less than 10%) of the TOA signal and its extraction from the combined surface/atmosphere system highly depends on the accuracy of the atmopheric correction. In fact, a 1% error on the measured L T OA leads to a 10% error on the estimated L W ater . In addition to scattering effects, many gazes present in the atmosphere such as ozone, oxygen and water vapor can absord solar radiation in specific regions of the electromagnetic spectrum. The effective removal of atmospheric contribution from the TOA signal is a major requirement for the generation of operational land and ocean colour products. 2.4.3 Sun glint The greatest obstacle for the generation of ocean color products is the presence of sun glint. Sun glint is the specular reflection of sunlight off the ocean surface and into the satellite sensor. This optical phenomena occurs when the ocean surface directs the solar radiation in the exact viewing direction of the sensor and as such, depends on the sea surface state, the sun position and the satellite viewing geometry. The high intensity of
Page 1: École Doctorale d’Informatique,
Page 5: 5 Résumé Nou abordons dans cette
Page 8 and 9: 8 TABLE DES MATIÈRES 3.5 Frequency
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112 5. Application : Restoration of
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- Alaska Labrador Siberia Restorati
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128 Conclusion
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130 BIBLIOGRAPHIE A. Chambolle. An
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132 BIBLIOGRAPHIE E. Hochberg, S. A
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134 BIBLIOGRAPHIE P. Perona and J.
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