ESA publishes an Announcement of Opportunity (AO) for the calibration and validation of ALTIUS. This AO calls for proposals addressing quality assessments of the Level-1 radiometric data, the geophysical validation of mesospheric O3 and stratospheric NO2, BrO, OClO, H2O, NO3, aerosol extinction and temperature, as well as complementary validation of stratospheric ozone data. The deadline for proposal submission is November 1, 2026.