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Radiometric calibration

Coco Rulinda (CGIS-NUR) for PGD 2009Based on slides by Boudewijn van Leeuwen, ITC-RSG-GTS

Advanced Remote Sensing – PGD 2009June 2 2009

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The Remote Sensing process

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Image Data

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Why Radiometric calibration?

• There are a number of important reasons to calibrate remote sensing data. The raw sensor DNs are simply numbers, without physical units. Each sensor has its own gain and offsets applied to the recorded signals to create the DNs. To do inter-sensor data comparison, they must be converted to at-sensor radiances. This step is called sensor calibration. If we desire to compare surface features over time, or to laboratory, or field reflectance data, corrections must be made for atmospheric, solar and topographic correction. We call this entire calibration and correction process radiometric calibration.

Subject 1Date 1

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Radiometric calibration process

• Convert sensor’s DNs to at-sensor radiances requires sensor calibration information

• Convert at-sensor’s radiances to radiance at earth surface difficult to achieve: view path atmospheric conditions at the time and locations of the image and sensor is required.

• Correct from atmospheric, solar and topographic effects from surface radiance to surface reflectance

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At-satellite Calibration

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Radiance (L)

• Energy measured by the sensor in Watt per square Meter per Steradian per Micron

(Wm-2sr-1μm-1) or Milliwat per square

Centimeter per Steradian per Micron (mWcm-2sr-1μm-1)

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DN to Radiance Conversion

Lmin = minimum radiance (in Wm-2sr-1μm-1 )Lmax=maximum radiance (in Wm-2sr-1μm-1 )QCALmax=maximum DN value possible (=255)QCALmin=minimum DN value possible (=0 or 1)

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Gain Settings

• Scene specific• Usually described in

the header file• To prevent saturation• Band 6 Low• Band 6 High

When converting DNs to radiances always check the gain settings!

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Radiance to Reflectance

Conversion of Radiance to Reflectance, why?• Reflectance (ρ) = wavelength dependent ratio

between reflected and incoming energy• Unitless (0 – 1 or 0 - 100)• Normalization of Sun Angle• Normalization of irradiance

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Reflectance (ρ)

• Reflectance = ratio between reflected and incoming energy

= Measured EnergyIncoming Energy

=LλЛd2

ESUNλcos θzLλ= Radiance at sensor (in Wm-2sr-

1μm-1 )d2= Earth-Sun Distance (AU)θz = Solar Zenith Angle (deg)ESUN = Band dependent

Exoatmospheric Irradiance (Wm-2μm-1 )

At-satellite reflectance ≠Surface reflectance

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Earth – Sun Distance (d) - LUT

• Distance Sun to Earth = ± 149 mln Km = 1 AU (Astronomical unit)J= Julian day Julian day: January 1st=001Sin is in radian January 2nd=002

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Exoterrestrial Solar Irradiance (ESUN)-LUT

• Mean Irradiance for a specific bandwidth• Watt per square meter per micron (Wm-2μm-1 )

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Reflectance (ρ) - All

• Lλ = Radiance at sensor (in Wm-2sr-1μm-1)• D2 = Earth-Sun Distance (AU)• ESUN = Band dependent Exoatmospheric Irradiance (Wm-

2μm-1)• Θz = Solar Zenith Angle (deg)

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Radiance to Temperature

• Brightness Temperature; temperature of a blackbody

(= perfect emitter)• At-satellite temperature ≠surface temperature

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Temperature (T)

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Sensor Calibration

• Pre-flight calibration• Sensors degrade, so there is a need for:

– In-flight calibration• Internal blackbody references; multiple calibrated

lamps or panels• Solar calibrators• Hot and cold black body reference• Cross Calibration with space, airborne and ground

measurements

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Calibration Updates

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More information

• Landsat 7 Science Data Users handbook:http://ltpwww.gsfc.nasa/IAS/handbook/handbook_toc.htmlNOAA Solar Position Calculatorhttp://www.srrb.noaa.gov/highlights/sunrise/azel.html