The ALTIUS mission: operational and scientific objectives
Didier Fussen, Noel Baker, Antonin Berthelot, Emmanuel Dekemper, Philippe Demoulin, Quentin Errera, Ghislain Franssens, Nina Mateshvili, Nuno Pereira, Didier Pieroux and Sotiris Sotiriadis
BISA
Adam Bourassa, Doug Degenstein, Nicholas Lloyd and Daniel Zawada
U.SASKATCHEWAN
Navarro Reyes, Daniel; François, Michael; Montrone, Luciana; Santandrea, Stefano; Frommknecht, Björn; Zehner, Claus; Wehr, Tobias
ESA-EO

ALTIUS is an original, groundbreaking mission which answers open questions and needs within the atmospheric remote sounding community. The project was proposed by the Belgian Institute for Space Aeronomy (BISA) and it is presently implemented as an element of the ESA Earth Watch program. The data processing algorithms will be developed in a collaboration between BISA, the University of Saskatchewan and the industry. Ozone and aerosol stratospheric profiles are the main operational objectives of the mission.
The truly innovative technology being developed through ALTIUS resides in the use of a high-performance microsatellite of the PROBA class that can operate in a multi-mode approach. Starting from a well-stabilized and controlled platform attitude, a single sensor can be optimized for a combination of limb scattering measurements and solar, stellar, and planetary occultations. The combination of an imager with an accurate attitude control allows for determining the tangent altitude by relating the instrument field of view to the satellite attitude. Moreover, occultation observations can also be carried out in inertial pointing without the need for an onboard tracking mechanism. Twilight observations will also be performed, to inter-compare occultation and limb retrievals for large solar zenith angles.
The ALTIUS instrument bridges urgent and significant gaps that exist in the availability of long-term satellite measurements which are essential to the climate research community.

ALTIUS is a limb sounder mission for the monitoring of the distribution and evolution of stratospheric ozone at high vertical resolution in support of operational services and long term trend monitoring. The ALTIUS mission will provide detailed stratospheric ozone profiles information at high vertical resolution, which adds valuable information to total column ozone used for data assimilation systems by operational centres based on nadir sounders.
The ALTIUS data will also be of high importance for the atmospheric modelling community, for use as input to climate models and their validation. It will reduce a gap in high-resolution limb observation data. It has the potential to contribute to the GCOS (Global Climate Observing System) ozone profile ECV (Essential Climate Variable). Off-line ALTIUS products are relevant for the validation of the Copernicus Atmosphere Monitoring Service (CAMS) model. Other secondary products, such as Aerosols extinction, will be provided as well for scientific interests.
The geophysical validation of the different ALTIUS products will provide also valuable scientific opportunities.
The ALTIUS mission is under implementation by ESA within the Earth Watch Program, with participation of Belgium, Canada, Luxembourg and Romania. Based on a PROBA-NEXT Platform, the satellite embarks an optical instrument made of three independent channels (VIS, UV and NIR) to observe the Earth limb through a field of view of 100 km x 100 km, measuring scattered sun light (bright limb) and atmospheric attenuation (solar and stellar occultation mode) in order to retrieve ozone and other species.
Project is going through phase C with an expected launch mid-2025.
The current Project status and plans covering flight model manufacturing, launch, commissioning, validation and routine operations will be presented.

The ALTIUS instrument is a three channel, imaging instrument that plans to provide global measurements of ozone limb profiles for the atmospheric community. The instrument utilizes acousto-optic tune-able filters (AOTF) in the visible and near infra-red channels and a 4-element stacked Fabry-Perot imaging complex in the UV channel. All three channels observe solar occultation and bright limb measurements while the UV channel additionally observes stellar occultation. The large dynamic range of signals coupled with novel imaging technologies presents interesting challenges for instrument calibration which are discussed.

Stratospheric aerosol plays an important role in the Earth’s radiative balance, stratospheric composition, and atmospheric dynamics. The background stratospheric aerosol layer is commonly perturbed by large volcanic eruptions, and more recently by smoke from large wildfire events in Australia and western North America. The limb scatter measurement technique used by ALTIUS is highly sensitive to stratospheric aerosol extinction from the cloud-top to altitudes approaching 40 km. Stratospheric aerosol is also an interfering species for the ozone retrieval; accurate knowledge of the stratospheric aerosol extinction is important to retrieve accurate ozone. Here we present the algorithms developed to retrieve stratospheric aerosol extinction from ALTIUS measurements developed at the University of Saskatchewan. The algorithm uses measurements in the visible and near-infrared spectra and builds upon heritage from the Optical Spectrograph and InfraRed Imager System (OSIRIS) and Ozone Mapping and Profiler Suite-Limb Profiler (OMPS-LP) missions. Recent scientific results using limb scatter stratospheric aerosol observations will also be presented, with an emphasis on how future measurements from the ALTIUS mission will add scientific value.

Ozone retrieval algorithm for ALTIUS observations
Didier Fussen, Noel Baker, Antonin Berthelot, Emmanuel Dekemper, Philippe Demoulin, Quentin Errera, Ghislain Franssens, Nina Mateshvili, Nuno Pereira, Didier Pieroux and Sotiris Sotiriadis

BISA

The retrieval of ozone number density profiles with a good vertical resolution is the major objective of the ALTIUS mission.
The preliminary retrieval algorithms will be presented for various observation geometries: solar limb scattering, solar occultation and star occultations.
For limb scattering, several innovative approaches will be introduced, including the use of principal components, the computation of the radiative transfer field by Montecarlo methods, the selected step-by-step spectral processing and an iterated chi-squared method.
For solar occultations, the processing of the full solar disk will be addressed in the case of a full spectral imager. For stellar occultations, the influence and the mitigation of star scintillation will be discussed.
The expected performances of ozone retrieval will be compared and discussed with respect to the mission requirements, in terms of SNR and averaging kernels.

Programmed as an element of ESA’s Earth Watch programme, Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere (ALTIUS) has been developed as a gap filler mission responding to the urgent need to continue the global, long-term of monitoring of stratospheric ozone, related species and aerosols at the vertical resolution of the order of a few km. Planned for operation in the 2025-2029 period, ALTIUS will continue the essential Climate Data Record (CDR) on stratospheric ozone started in the 1980s with the SAGE series and extended successively with other limb/occultation sounders on the UARS, Odin, Envisat, ACE, Aura, and OMPS-LP missions. Recognized by the Global Climate Observing System (GCOS) as an Essential Climate Variable (ECV), the vertically resolved ozone CDR is essential for large-scale research activities endorsed by the World Climate Research Programme (WCRP) under its core project Stratosphere-troposphere Processes And their Role in Climate (SPARC), and for stratospheric ozone assessments published every four years by the World Meteorological Organization (WMO) and United Nations Environment Program (UNEP) in the framework of the Vienna Convention for the Protection of the Ozone Layer. The Earth Watch programme aims at fostering the delivery of Earth observation data to operational services. As a contributor to this programme, ALTIUS will provide daily global measurements of the ozone profile in near real time to operational services like the Copernicus Atmosphere Monitoring Service (CAMS) and the Belgian Assimilation System of Chemical ObsErvations (BASCOE). In delayed mode ALTIUS will also provide consolidated ozone data records to the Climate Data Store (CDS) of the Copernicus Climate Change Service (C3S).

The present contribution introduces to the approach being developed for the necessary geophysical validation of ALTIUS ozone profile data within the operational constraints of the Earth Watch programme. After a review of mission and user requirements against which ALTIUS data will have to be validated, the different objectives of ALTIUS validation during the commissioning phase of the mission and beyond will be discussed. The validation approach includes classical comparisons to independent measurements collected from ground-based monitoring networks contributing to WMO’s Global Atmosphere Watch (GAW), namely, balloon-borne ozonesonde data for the lower part of the ALTIUS vertical range up to ~30 km, differential absorption lidar data (DIAL) for the stratospheric range up to ~45 km, and millimetre wave radiometer data for the stratospheric and mesospheric ranges. Comparisons with respect to observations from other satellites will extend the ground-based validation to the global domain and to quality features not detectable with point-like ground-based measurements. Quality assessments will be complemented by modelling support. A routine validation service for ALTIUS will include baseline monitoring of the geophysical data quality relying on an operational validation system, and in-depth validation by a dedicated team who will produce consolidated validation results and give analysis support to the evolution of the retrieval algorithms. An Announcement of Opportunity for the calibration and validation (Cal/Val) of ALTIUS will be published approximately two years before launch, with the aim to open the ALTIUS Cal/Val to the international community and to a wider range of external data and activities, to foster exchanges within the validation community and with the instrument and algorithm experts, and to promote the use of ALTIUS data.