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Recent Earth Observation instruments require a highly accurate knowledge of their Instrument Spectral Response Function (ISRF). This translates into lengthy and costly characterisation programs during the Assembly, Integration, and Test phase of the instrument. In addition, potential changes of ISRF after launch suggests to use an on-board equipment dedicated to this accurate characterisation in flight. This topic has been studied by Thales Alenia Space and TNO in the frame of a TRP study funded by ESA during years 2019 and 2020. This paper first recalls the potential application cases and the related target performances of the study. Then we identify conceptual solutions, such as operating tuneable laser diodes, Fabry-Perot interferometer, microresonators. Next, we describe the principle of the selected solution: a Fourier Transform Spectrometer (FTS), used as a calibration light source. Indeed, a single FTS can cover a very large spectral range with an extremely high spectral accuracy. But FTS are known as complex and sensitive devices: we have designed a simplified solution, well adapted to space applications. In a second step, we describe the flight hardware designed by Thales Alenia Space, targeting optimised manufacturing and implementation. The main performances, being spectral resolution and signal to noise ratio (SNR), are discussed. Breadboard activities also took place in TNO in order to validate the main hypotheses. Innovative data processing has been tested, that only makes use of interferogram data, while maintaining a high accuracy. Description of the breadboard as well as the first test results are presented.
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