18.104.22.168 Radiometric Processing
The valid MERIS samples are digital counts resulting from the acquisition by MERIS of passive optical spectral radiance remote sensing. The objective of the radiometric processing is to estimate the spectral radiance which caused these counts. An inverse model of the MERIS acquisition is used for that purpose, using parameters stored in the Characterisation data base and the MERIS samples themselves. The MERIS acquisition model is described as:
Xb,k,m,f is the MERIS raw sample (not corrected onboard)
NonLinb,m is a non-linear function
Tf VEU is the amplification unit temperature
Tf CCD is the sensor temperature
g(T) and gc(T) are temperature-dependent gain terms (close to 1)
Ab,k,m is the "absolute radiometric gain"
Lb,k,m,f is the spectral radiance distribution in front of MERIS
Sb,k,m,f is the smear signal, due to continuous sensing of light by MERIS
Gb,k,m is a linear process representing the stray light contribution to the signal. For a given sample, some stray light is expected from:
· all the other simultaneous samples in the module, spread into the sample by specular (ghost image) or scattering processes
· the samples in the previous and following frames
C0 b,k,m is the dark signal (corrected onboard for temperature effects by the Offset Control Loop)
is a random process representative of the instrument errors and parasitic processes not accounted for in the other terms of the model.
All terms not indexed by f (frame) do evolve in time due to ageing, but with a much slower rate which allows to represent them, for a given Level 1b product, as fixed quantities retrieved from data bases.
The radiance sensed by MERIS Lb,k,m,f is, for a given set of target physical parameters and illumination and observation angles, proportional to the extraterrestrial Sun spectral flux. Because there is no absolute spectral measurement of the Sun irradiance simultaneous to MERIS acquisition, all results are produced with reference to a Sun spectral flux model which must be included in the product header.
The term Ab,k,m reflects all the amplification gains inside the instrument, which depend on:
· instrument programming (band settings, amplification programmable gains)
· components ageing
· components temperature
· power supply voltage
In order to provide for limitation or failure of the onboard temperature regulation, there shall be a residual correction for g(T), GC(T). In normal operation, T depends on the time elapsed since the Sun zenith angle has decreased below a threshold (80°) and can be predicted.