1. Coco Rulinda (CGIS-NUR) for PGD 2009
Radiometric calibration
Coco Rulinda (CGIS-NUR) for PGD 2009
Based on slides by Boudewijn van Leeuwen, ITC-RSG-GTS
June
Advanced Remote Sensing – PGD 2009

2. The Remote Sensing process
Orange ball: sensor

3. Image Data

4. 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.
Date 1
Subject 1

5. 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

6. At-satellite Calibration

7. 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)

8. 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)

9. 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!

10. Radiance to Reflectance
Conversion of Radiance to Reflectance, why?
Reflectance (ρ) = wavelength dependent ratio between reflected and incoming energy
Unitless (0 – 1 or )
Normalization of Sun Angle
Normalization of irradiance

11. Reflectance (ρ) = Measured Energy
Reflectance = ratio between reflected and incoming energy
= Measured Energy
Incoming Energy
=LλЛd2
ESUNλcos θz
Lλ= 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

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

13. Exoterrestrial Solar Irradiance (ESUN)-LUT
Mean Irradiance for a specific bandwidth
Watt per square meter per micron (Wm-2μm-1 )

14. 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)

15. Radiance to Temperature
Brightness Temperature; temperature of a blackbody
(= perfect emitter)
At-satellite temperature ≠surface temperature

16. Temperature (T)

17. 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

18. Calibration Updates

19. More information Landsat 7 Science Data Users handbook:
NOAA Solar Position Calculator