# GeoEye-1

GeoEye-1 acquisitions are calibrated using the Orfeo Toolbox.

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

The GeoEye-1 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 0.970 -1.926 blue 1.053 -4.537 green 0.994 -4.175 red 0.998 -3.754 nir 0.994 -3.870

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 1610.73 blue 1993.18 green 1828.83 red 1491.49 nir 1022.58

## Input

The input is an ingested GeoEye-1, see details here.

## Output

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