GESat GEN1: In-Orbit Performance Review of a Compact SWIR Fizeau Interferometer for Facility-scale Methane Detection and Quantification

Abstract. The GESat GEN1 mission is a 16U microsatellite developed and operated by Absolut Sensing to demonstrate high-resolution methane monitoring using a compact SWIR Fizeau interferometer. GEN1 provides a 50m ground sampling distance to monitor facility-scale methane plumes. It is rated for an average detection rate of 500 kg.h⁻¹, with the capacity to resolve leaks as small as 100 kg.h⁻¹ in pristine environmental conditions. The system is designed to achieve a column retrieval precision of 120 ppbv, defined as the mean standard deviation of the retrieved column-averaged dry-air mole fraction of methane over operational conditions. This corresponds to a random error of approximately 6.5% relative to a background
concentration of 1850 ppbv.

The paper first presents the GEN1 mission and its interferometric payload, designed to enable selective methane sensing from a highly compact platform. It then describes the retrieval architecture, based on a physics-guided parametric algorithm that occupies an intermediate complexity  between matched-filter methods, as used for missions such as Tanager-1 and EMIT (Carbon Mapper, 2024), and full physics schemes using high accuracy radiative transfer code such as libRadtran or 4A/OP (Mayer and Kylling, 2005; Scott and Chédin, 1981; Chéruy et al., 1995). The proposed model generates top-of-atmosphere radiances in the SWIR [1550, 1700nm] region with no systematic bias relative to reference full-physics model and a random error below 0.2 ppbv in methane column retrieval. Moreover, on identical computing hardware, the physics-guided parametric approach achieves a processing speed approximately 20×103 times faster than the full-physics model. This strategy therefore provides a balanced trade-off between robustness, accuracy, and computational cost, enabling large-scale processing compatible with constellation-level operations while preserving radiometric fidelity required for quantitative methane retrievals. Supported by this instrument and retrieval design, representative in-orbit results are reported, including methane plume detections over industrial facilities, together with an assessment of current limitations and planned evolutions of the processing chain. Finally, the paper concludes by evaluating the mission’s performance against its design specifications.With more than 160 acquisitions processed, the system achieves an average precision of 111 ppbv, successfully meeting the mission’s primary target, with an average detection sensitivity of 450 kg.h⁻¹. Notably, several measurements achieved precision better than 80 ppbv, which enables an ultimate methane plume detection limit corresponding to emission rates below 100 kg.h−1 under ideal meteorological conditions. This on-orbit demonstration confirms the viability of the Fizeau interferometric approach for high-resolution monitoring and paves the way for future constellation deployment.