2. Radiometry
Radiometry = science of electromagnetic radiations
Interest for Remote Sensing
Physical interpretation of signals
Enhance detection
Estimate physical values related to ground and / or
atmospheric effects
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3. Radiometry : from DN to TOA reflectance
Goal : using physical measures
TOA reflectance (Top of Atmosphere)
TOC reflectance (Top of Canopy)
Calibration of images : DN (digital numbers) converted into
TOA reflectance
Application of calibration coefficient inserted into metadata
files → Luminance
Normalisation with solar effect →
Reflectance
Example for Spot :
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5. Luminance
Definition
Emitted power per unit area and per unit solid angle
Unit
W.m-2.sr-1
Numerical example:
Luminance of the sky (visible): ~ 6 W.m-2.sr-1
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7. Radiometry : from TOA reflectance to TOC reflectance
Atmospheric correction : inversion of surface TOC reflectance
Need to caracterise atmospheric conditions
➢TOMS / TOAST data : (grid : 1,25°lon x 1°lat ~ 135 x 110
km) : stratospheric Ozone amount
NCEP meteo data : (grid : 2,5°lon x 2,5°lat ~ 270 x 270
km) : Atmospheric Pressuree, water vapor, wetness
SeaWiFS or MODIS data : (grid : 5’ x 5’ ~ 9 x 9 km) ) :
Optical aerosols thickness
Cimel of aeronet network : information
about aerosols, and water amount
Absorption Ozone
Scattering Cloud
Emission Water vapor
Molecules
Aerosols
Reflection orfeo-toolbox.org
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8. Atmospheric effects
Atmosphere
Thickness ~100 km
Filter descending and ascending radiations
Composition of the atmosphere:
Molecules
nitrogen (78%), oxygen (21%), argon, carbon oxides,
ozone, water vapor (highly variable, even locally) ...
Aerosols
small solid or liquid particles suspended in the
atmosphere. Microparticles, water particles, ice crystals,
smoke ...
Dimensions: mainly from 0.1 to 10 µm
Abundance and type varies geographically and with time orfeo-toolbox.org
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11. Atmospheric effects
Three main phenomena, depending of wavelength
Absorption
Diffusion (in the visible and NIR, up to ~ 3 µm)
Emission (in the thermic IR, from ~ 3 µm)
Optional : refraction, usually neglected for high elevation
acquisition and not included spatial resolutions greater than
10 cm
The contribution of the signal from the ground in the measured
signal is disturbed by filtering effects and luminance of the
atmosphere
Need to correct measures
Need to understand the phenomena
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12. Radiometry : 6S model
Atmospheric corrections : invert the TOC surface reflectance.
Simulation with radiative transfer code
6S code
Diffusion : code of the successive diffusion orders
Absorptions : O2, CO2, H2O, O3, N2O, CH4
Spectro data at 10 cm-1« AFGL atmospheric absorption
line parameters compilation » (1991)
Look-Up Tables : refl. TOC -> TOA
Mesure
ρTOA
Simulations TOA
Géo d’observation
Conditions atmosphériques
ρTOC
0 0,01 0,02 0,03 0,8
Valeur sol interpolée orfeo-toolbox.org
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14. Radiometry : 6S parameters
Atmospheric parameters => parameters of radiative transfer
The zenithal and azimutal solar angles that describe the
solar incidence configuration (in degrees)
The zenithal and azimuthal viewing angles that describe the
viewing direction (in degrees)
The month and the day of the acquisition
The atmospheric pressure
The water vapor amount, that is, the total water vapor
content over vertical atmospheric column
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15. Radiometry : 6S parameters
Atmospheric parameters => parameters of radiative transfer
The ozone amount that is the Stratospheric ozone layer
content;
The aerosol model that is the kind of particles (no
aerosol, continental, maritime, urban, desertic)
The aerosol optical thickness at 550 nm that is the is the
Radiative impact of aerosol for the reference wavelength
550 nm
The filter function that is the values of the filter function
for one spectral band, from λinf to λsup by step of 2.5
nm. One filter function by channel is required. This last
parameter are read in text files, the other one are directly
given to the class. orfeo-toolbox.org
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16. Radiometry : 6S parameters
Spectral sensitivity file
Data provided in OTB-Data (not with Monteverdi)
➢OTB-Data/Input/Radiometry/
For each band with a step of 0.25 nm, the spectral band
sensitivity of the instrument
➢Input file : values in the Atmospheric parameters window
➢No input file : iso sensitivity (1 everywhere) or default file
automatically loaded (>OTB V3.10)
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17. Aeronet data
AERONET (AErosol RObotic NETwork)
This program is a federation of ground-based remote sensing
aerosol networks established by NASA and LOA-PHOTONS
(CNRS) and is greatly expanded by collaborators
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18. Aeronet data and OTB
Aerosol optical depth data are computed for three data quality
levels
Level 1.0 (unscreened)
Level 1.5 (cloud-screened)
Level 2.0 (cloud screened and quality-assured).
Extract Aerosol thickness (épaisseur) and Water amount
Download site
http://aeronet.gsfc.nasa.gov/cgi-bin/webtool_opera_v2_new
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19. Radiometry – Reference reflectances
Mesure of the reflectance of a source
Principle : use a source and measure reflected luminance
➢ Direct measure : known source
➢ Indirect measure : use of a known reference reflectance
surface (spectralon, or BaSO4)
Measures in laboratory
➢ Artificial source : directional, mobile (lamp, laser)
➢ Sensor (radiometer or spectro-radiometer) : mobile
Direct measure or indirect measure
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20. Radiometry
Spectral signature of the chlorophyllian vegetation
Proche
Visible Moyen Infra-Rouge
Infra-Rouge
REFLECTION due à : pigment Structure
Teneur en eau
de la feuille cellulaire
ABSORPTION due à : Chlorophylle Eau
Longueur d’onde (µm)
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21. Radiometry processing - Synthesis
3 processing steps
Digital Number to Luminance
Luminance to TOA reflectance
TOA reflectance to TOC surface reflectance
Many parameters available in the metadata of sensors
Importance of aerosols to quantify the effects of the
atmosphere
Difficulties
Aerosol model used?
Validation of results?
Ground truth?
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28. Use case 2 : Spectral viewer
Menu File > Open
./img_kalideos_reunion/2009-03-21/IMAGERY.TIF
Menu Viewer > Spectral viewer
Left click : select your area
Right click : select pixel
Spectral angle menu > Select one curve ID
➢ Click on Compute button
File > Save (the Spectral angle file). Use Float data type
Create several reference spectral angle images : roads, houses, sea,
clouds, vegetation, bare soil
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