Although a number of on-board calibration sources are included in GOME-2, a thorough on-ground calibration is required prior to launch.

The most important measurements are:

• the characterisation of the bi-directional scattering function (BSDF) of the diffuser in the calibration unit
• the characterisation of the polarisation response of the instrument as a function of the different optical paths for solar calibration and Earth nadir view, as a function of the scan mirror position and wavelength

These calibrations are performed in a thermal vacuum chamber at the instrument calibration contractor's site (TNO/TPD).

Other measurements include comprehensive instrument characterisation and consistency cross checks. In particular, full radiance and irradiance calibration using Earth and sun observation paths are performed with NIST calibrated light sources for the Polarisation Unit (PU) and the main channels.

Additional calibration includes:

• full characterisation of the straylight behaviour (adjusting the instrument to account for the effect of stray light)
• the wavelength calibration
• the field of view
• the instrument response function

The instrument calibration is complemented by cross checking the results using the NASA sphere as calibration source and measurements of scattered sky light offered to the instrument by light fibre.  
 

absolute spectral radiance: 2.4 % (250 nm) 1.6 % (655 nm) absolute spectral irradiance: 2.3 % (250 nm) 1.5 % (655 nm) BSDF measurements: better than 1.2 % all polarisation properties: better than 1.2 % instrument slit function

 
Calibration requirements:

Under ambient conditions:

• scan mirror dependencies of polarisation and radiometric resolution: 1.5%
• straylight characterisation field of view