# WorldView-2

WorldView-2 acquisitions are calibrated using the Orfeo Toolbox.

The process applied is the same for PAN and MS acquisitions.

The WorldView-2 product handbook provides the formula below to convert the DN into radiance:

$L_{(b)} = gain_{(b)} \times DN_{(b)} \times { abscalfactor \over effectivebandwith } + offset_{(b)}$

Where:

$$gain_{(b)}$$ and $$offset_{(b)}$$ are values taken from the table below:

 Band Gain Offset pan 0.942 -2.704 coastal 1.151 -7.478 blue 0.988 -5.736 green 0.936 -3.546 yellow 0.949 -3.564 red 0.952 -2.512 rededge 0.974 -4.120 nir1 0.961 -3.300 nir2 1.002 -2.891

And $$abscalfactor$$ and $$effectivebandwith$$ are extracted from the acquisition metadata.

The conversion from $$DN$$ (for Digital Numbers) to spectral radiance (or ‘TOA radiance’) $$L$$ is done with the formula below:

$L_\lambda = gain \times DN + offset$

As such, the calibration uses:

$gain = gain_{(b)} \times { abscalfactor \over effectivebandwith }$

and

$offset = offset_{(b)}$

$$L$$ 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 table below:

 Band Thuillier 2003 pan 1571.36 coastal 1773.81 blue 2007.27 green 1829.62 yellow 1701.85 red 1538.85 rededge 1346.09 nir1 1053.21 nir2 856.599

## Input

The input is an ingested WorldView-2, see details here.

## Output

The output is a STAC item geojson ... including the assets ...