VRSS-2 acquisitions are calibrated using the Orfeo Toolbox.
DN to Radiance
The VRSS-2 metadata files provide the formula below to convert the DN into radiance:
For the conversion from \(DN\) (for Digital Numbers) to spectral radiance (or ‘TOA radiance’) \(L \), the following formula is used :
As such, the calibration uses:
\(L \) in \(W/m^2/steradians/micrometers\) with \(b\) being a band ID.
Radiance to Reflectance
To convert TOA radiance to TOA reflectance, the following formula is used :
\(L(b)\) is the spectral radiance for band b
\(pi\) is the famous mathematical constant
\(d\) is the earth-sun distance (in astronomical units) and depends on the acquisition’s day and month
\(ESUN(b)\) is the mean TOA solar irradiance (or solar illumination) in \(W/m^2/micrometers\)
\(θ\) is the solar zenith angle in degrees.
The values for the \(ESUN(b)\) are provided in the acquisition metadata.
The input is an ingested VRSS-2, see details here.
The output is a STAC item geojson ... including the assets ...
RW01 - OTB Optical calibration