Regularization scheme in ALTIUS retrieval algorithms

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 2026.

The instrument consists of three imagers: UV (250-355 nm), VIS (440-675 nm) and NIR (600-1040 nm) channels. Each channel is able to take a snapshot of the scene independently of the other two channels, at a desired wavelength and with the requested acquisition time. The agility of ALTIUS allows for series of high vertical resolution observations at wavelengths carefully chosen to retrieve the vertical profiles of species of interest.

ALTIUS 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 primary objective of the mission is to measure high-resolution stratospheric ozone concentration profiles. Secondary objectives are the retrieval of mesospheric ozone, stratospheric aerosols particle density and extinction coefficient, NO₂, NO₃, BrO, OClO, water vapor and temperature profiles.

The Level-2 retrievals use a Levenberg-Marquardt algorithm coupled to a Tikhonov regularization scheme to limit noise amplification during the inversion process. The type and strength of regularization has a direct effect on the retrieved profile vertical resolution. Therefore, a trade-off has to be found between noise removal amplitude and vertical resolution. A study on the different regularization constraint types is performed and the regularization strength is determined using the retrieval noise error in stellar occultation mode for O₃ and NO₂ density and aerosol extinction retrievals. The ability of Tikhonov regularization to remove the effects of scintillation on the retrieved profiles is discussed. Moreover, a re-processing of GOMOS data is performed to test the validity of our approach.