1. Bertrand Fougnie, Sophie Lachérade (CNES)
GSICS Annual Meeting
29th Feb.- 4th March 2016, Tsukuba, Japan
Discussion
Cross-calibration
(over the Moon)
Bertrand Fougnie, Sophie Lachérade (CNES)
Jack Xiong (NASA)

2. Context Digit = A. L Digit = A. Es. R
Radiance (or irradiance) is measured by sensors
physical quantity close to the measurement
depends on the observed target
but also depends on the illumination of the target (in general the sun)
Reflectance (or normalized radiance) is a property of the target
physical quantity close to the target
depend only the target properties
but doesn’t depend on the illumination
Some sensors are calibrated in reflectance R, other in radiance L
calibration = ratio measurement / reference
a calibration “in reflectance” adjust the product calibration x solar irradiance (A.Es)
Es is required to convert radiance into reflectance, or reversely
Many solar irradiances are used – in general, no common reference
What do you/we calibrate ? What does GSICS want to calibrate ?
Digit = A. L Digit = A. Es. R

3. Discussion Proposition (to be discussed):
A cross-calibration approach (result) must address the question
a cross-calibration result could simply show a difference between solar irradiances considered by both systems
cross-calibration between Pleiades and MODIS/VIIRS : results were impacted by the difference between solar irradiances considered by Pleiades and MODIS
Proposition (to be discussed):
If the reference sensor is calibrated “in reflectance”, cross-calibration should be done on reflectance
If the reference sensor is calibrated “in radiance”, cross-calibration should be done on radiance or irradiance
otherwise the result will be impacted by the difference between solar irradiances of both sensors.
Note : The GSICS reference MODIS is calibrated in reflectance
After the cross-calibration, every GPRC can use its own solar irradiance Es to convert radiance <-> reflectance

4. Example
Cross-calibration between Pleiades and MODIS : results were impacted by the difference between solar irradiances considered by Pleiades and MODIS
For the cross-calibration VIIRS-Pleiades the following cross-calibration was applied :
Reflectance to calibrate
corresp. ROLO reflectance
PHR1-B vs VIIRS
Reference reflectance
corresp. ROLO reflectance
Normalization factor for sun/moon/sensor distances