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The ALTIUS Mission (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) aims at the development of a limb sounder to monitor the distribution and evolution of stratospheric ozone at high vertical resolution in support of operational services and long-term trend monitoring. The ALTIUS Instrument novel concept consists of three hyperspectral channels using active tunable spectral filters to perform observations with a spectral resolution ranging between 1nm and 10nm. The spectral filters are using Acousto-Optic Tunable Filters (AOTFs) in the Visible (440-675nm) and NIR (600-1020nm) range, and a cascade of Fabry-Pérot Interferometers (FPI) in the UV (250-355nm) range. The ALTIUS Mission is currently in Phase C, with a Critical Design Review (CDR) planned in mid-2023 with Redwire Space N.V. as Mission Prime and OIP Sensor Systems N.V. as Instrument Prime. The paper presents the key technical challenges faced in the development of the ALTIUS Instrument up to its current CDR maturity level. Beyond a full system overview, detailed insight is provided of its optical concept, the choice and development challenges of its optical tunable spectral filters, the associated control electronics, the ALTIUS Instrument assembling (and alignment), its integration and testing strategy, the on-ground calibration plan and, finally, a summary of the achievable L0/L1 performances. A brief description of the ALTIUS Instrument in-flight calibration strategies will be presented too, along with a flavor of the stringent cleanliness and contamination control measures envisaged to ensure stable optical performances. Finally, the paper presents lessons learned from the subsystem qualifications activities and Instrument STM environmental test campaign, as well as an overview of the foreseen project milestones towards launch.
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