Proposed by BIRA-IASB as an Earth Watch element of ESA’s Living Planet Programme, Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere (ALTIUS) will continue the long-term and global-scale measurement of ozone, aerosols, and several reactive gases and greenhouse gases carried out so far by satellite missions reaching now their end of life (Fussen et al., AMTD 2016). On board of the agile PROBA micro-satellite to be launched in 2021 on a mid-morning polar orbit, this UV/visible/near-infrared hyperspectral imager will alternate measurements of limb-scattered sunlight with solar, stellar and planetary occultations, from which the vertical distribution of target trace gases and aerosols will be retrieved with the global sampling and at the vertical resolution required to improve numerical predictions of weather and climate, to assess environmental policy impacts, and to better understand connections between changes in atmospheric composition, transport and climate.
In order to assess the performance of the mission, to ensure proper use of the ALTIUS data, and to enable candidate users to readily evaluate the fitness-for-purpose of the data, a dedicated validation programme has to be organized, which is the subject of this introductory talk. While part of the quality assessment of satellite data consists classically in monitoring the outputs of the data production system, another part, referred to as independent validation, consists in checking the satellite data and their associated uncertainties against reference measurements provided by well-documented ground-based and balloon-borne systems, e.g., those in use in ground-based monitoring networks contributing to WMO’s Global Atmosphere Watch (GAW) like NDACC, SHADOZ, and TCCON. Direct intercomparison with other satellite data can be valuable in documenting mutual consistency with other sounders, and it extends usefully ground-based validation results to the more global domain. The lower altitude range of ALTIUS data on ozone, NOx, CH4, water vapour and aerosols could also be validated against aircraft measurements by the IAGOS/MOZAIC in-service system. The fact that ALTIUS is part of ESA’s Earth Watch element, which aims at facilitating the delivery of Earth observation data to operational services, adds operational and interoperability constraints beyond the scientific and research objectives of the mission, which will be discussed too.