What is more, the light that is reflected by the objects must also
cross the atmosphere before being analysed by the satellites
sensors, and this journey also perturbs the signal. These perturbations
are due to the presence of gases and dust that can absorb and/or
reflect specific wavelengths, thereby changing the radiations
spectral properties. What is more, the electronic processing of
the rays that reach the sensors is also accompanied by some perturbations.
Consequently, it is actually rather difficult to get accurate radiometric
values from the data recorded by Earth-observing satellites.
Now, it is sometimes very useful to be able to calibrate these
data precisely, for example to compare the data recorded by different
satellites or recorded by the same satellite at different times,
and several solutions do exist to try to overcome these flaws. Some
of them are based on complex mathematical models that describe the
main interactions involved. These models are effective.