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From IASI to IASI-NG: improved radiometric precision and spectral resolution
To improve radiometric performance, the detection chain noises must be limited, and a bigger field is needed to limit the size of the pupil and thus keep the instrument compact.
Spectral resolution improvement
Taking into account the requirement of a bigger field to improve the radiometry, compensation of the field effects is mandatory to improve the spectral resolution. This improvement requires opto-mechanical compensation based on the Mertz principle which enables a very large instantaneous field-of-view to be covered and thus a great quantity of sounding points to be acquired at the same time.
Instrument principle: Mertz compensation
According to the Mertz principle, field effects compensation within a large angular field is realized by interposing a slide with parallel sides in the beam in the path difference: this principle amounts to removing the defocus of the mobile mirror at the exit of the interferometer.
The slide thickness must be adapted continuously to the path difference, thus varying with it, for example by introducing prisms in one of the arms of the interferometer.