ALTIUS atmospheric limb sounding mission: Spectral filters calibration and flight instrument integration

The Altius Mission (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) is dedicated to the development of a limb-sounding satellite instrument designed to monitor the vertical distribution and long-Term evolution of stratospheric ozone with high vertical resolution. This mission supports both operational atmospheric services and long-Term trend monitoring. The Altius instrument features an innovative design composed of three hyperspectral channels, each utilizing active, tunable spectral filters to achieve spectral resolutions between 1 nm and 10 nm. The spectral filtering is accomplished through Acousto-Optic Tunable Filters (AOTFs) for the visible (440-675 nm) and near-infrared (600-1020 nm) ranges, and a cascade of Fabry-Pérot Interferometers (FPIs) for the ultraviolet (250-355 nm) range. Currently in Phase D, the mission is nearing the completion of flight hardware manufacturing, with the integration of the flight instrument actively underway. This paper will first present calibration results of the spectral filtering subsystems, including the AOTFs and the FPI assembly. Significant adaptations to the test plan and setup were required to measure the spectral response function down to the 10-5 level. Lessons learned from these calibration campaigns and their implications for Level 0/Level 1 instrument performance will be discussed. Next, the status of the flight instrument integration and ongoing preparations for the ground calibration campaign will be detailed. Finally, an overview of upcoming project milestones leading to the targeted 2027 launch will be presented.