Impact of the design of a spectral imager based on partially sampled interferograms on the retrieve products

The emergence of spectral imaging concepts that are both sensitive and snapshot open up new perspectives in response to major challenges, such as air quality monitoring and greenhouse gas emissions. The imSPOC concept (Brevet n° : FR16/56162) is a static Fourier transform spectral imager. Its compactness, robustness, and lightness make it a promising concept for measuring greenhouse gases from space or assessing air quality. This concept is based on a paradigm shift, as it enables the acquisition of only portions of interferograms [1]. Consequently, it is necessary to transfer optimal estimation algorithms into the interferogram space: since spectral radiances cannot be retrieved from partial interferograms, it is necessary to directly fit measured interferograms to simulated interferograms in order to retrieve total columns from its measurements. Within the framework of the European Space Carbon Observatory project (H2020 SCARBO), it was demonstrated that the relevant geophysical information (CO₂ column for one camera and CH₄ column for the other) is indeed present in the identified interferogram portions [2]. In parallel, two prototypes were built, one dedicated to CO₂ and the other to CH₄. To improve the designs of these cameras, a co-design code allowing the propagation of instrumental errors to gas columns is essential.

In this work, we demonstrate how an approach based on the Cramer-Rao Lower Bound enabled a sensitivity increase of more than a factor of two for both the CO₂ and CH₄ cameras. The design of such instrumentation relies on both the selection of shape and spectral position of the filter, as well as the properties of the interferometric plate, which is an array of Fabry-Perot interferometers. We evaluated the impact of several design parameters: (i) the selection of the spectral shape and position of the filter, (ii) the selection of the maximum optical path difference, and (iii) the Fabry-Perot finesse of the interferometer, which enhances discrimination between different gas signatures. Further refinement of the design was achieved using the MEDOC retrieval model, incorporating additional parameters such as engraving depths, leading to an optimized repartition of the available optical path difference acquired by the spectral imaging sensor. Finally, the newl design for the CO₂ camera was benchmarked against the earlier CO₂ SCARBO design. The study found that the CO₂ scale factor random error has been reduced by more than a factor of two. Furthermore, performance has become more consistent across varying incidence angles. Additionally, correlation between albedo and retrieved CO₂ products has also decreased. This work received funding from the European Union’s H2020 research and innovation program under grant agreements No 769032 (SCARBO) and No 101135301 (SCARBOn).

[1] S. Gousset, L. Croizé, E. Le Coarer et al., “NanoCarb hyperspectral sensor: on performance optimization and analysis for greenhouse gas monitoring from a constellation of small satellites”, CEAS Space J., 11, pages 507–524 (2019)

[2] M. Dogniaux, C. Crevoisier, et al., “The Space CARBon Observatory (SCARBO) concept: Assessment of XCO2 and XCH 4 retrieval performance”. Atmospheric Measurement Techniques Discussions, 1-38.