VRSS-2

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:

\[ L_{(b)} = { DN_{(b)} \over K_{(b)} } - { C_{(b)} \over K_{(b)} } \]

For the conversion from \(DN\) (for Digital Numbers) to spectral radiance (or ‘TOA radiance’) \(L \), the following formula is used :

\[ L_\lambda = gain \times DN + offset \]

As such, the calibration uses:

\[ gain_{(b)} = { 1 \over K_{(b)} } \]

and

\[ offset_{(b)} = - { C_{(b)} \over K_{(b)} } \]

\(L \) in \(W/m^2/steradians/micrometers\) with \(b\) being a band ID.

To convert TOA radiance to TOA reflectance, the following formula is used :

\[ R(b) = { {\pi \times L(b) \times d \times d} \over {ESUN(b) \times cos(θ)} } \]

where :

\(L(b)\) 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 ...