The ALTIUS mission: status and performance

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is an atmospheric limb mission being implemented in ESA's Earth Watch programme and planned for launch in early 2026. The primary objective of the mission is to measure high-resolution stratospheric ozone concentration profiles. Secondary objectives are the retrievals of stratospheric aerosols particle density, NO₂, water vapor and other minor species concentrations.

This innovative instrument consists of three spectral high-resolution imagers: UV (250-355 nm), VIS (440-675 nm) and NIR (600-1040 nm) channels. The UV channel uses a stack of four Fabry-Pérot interferometers, while the VIS and NIR channels each rely on an AOTF (Acousto-Optical Tunable Filter). Each channel can image scenes independently of the others at given wavelengths and with a moderate spectral resolution, and high spatial sampling. The agility of ALTIUS allows for series of observations at desired wavelengths carefully chosen to retrieve the vertical profiles of species of interest.

The instrument will perform measurements in different geometries to maximize global coverage: observing limb-scattered solar light in the dayside, solar occultations at the terminator, and stellar, lunar, and planetary occultations in the nightside.

The status of the ALTIUS mission will be presented as well as the foreseen quality of the Level-1 observations.  The quality of the retrieved profile densities will be discussed with a particular focus on the high vertical resolution that can be achieved using this instrument. The added-value of the native imaging capabilities of ALTIUS in terms of observations, and in-flight calibrations, will be highlighted.