The ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) Earth observation mission is an atmospheric limb sounder with three independent hyperspectral imagers. A spectrally tunable Fabry-P´erot Interferometer cascade in the ultraviolet (250 nm to 355 nm), and two Acousto-optic tunable filters covering the visible (440 nm to 675 nm) and near-infrared (600 nm to 1020 nm) respectively perform measurements to obtain atmospheric ozone data. This paper addresses the ALTIUS Instrument numerical model developed in MATLAB by the European Space Agency to confirm the compliance status regarding the Instrument Level zero requirements, perform sensitivity analyses, assist in design trade-off exercises, and monitor the industrial instrument development progress. The model employs a modular approach, whereby the optical assembly is separated into individual functions representing the optical units within the instrument channels, including the modeling of less conventional spectral tunable filters adopted within the ALTIUS Mission. The model is designed to be versatile to follow the hardware driven input along with the project life cycle. Finally, this paper provides examples of how this model has contributed to instrument design decision confirmation, presents a selection of current Level zero performance results of the ALTIUS instrument channels (signal-to-noise ratio, spectral response function, full width half maximum, etc.).
The ALTIUS Mission (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) aims at the development of a limb sounder to monitor the distribution and evolution of stratospheric ozone at high vertical resolution in support of operational services and long-term trend monitoring. The ALTIUS Instrument novel concept consists of three hyperspectral channels using active tunable spectral filters to perform observations with a spectral resolution ranging between 1nm and 10nm. The spectral filters are using Acousto-Optic Tunable Filters (AOTFs) in the Visible (440-675nm) and NIR (600-1020nm) range, and a cascade of Fabry-Pérot Interferometers (FPI) in the UV (250-355nm) range. The ALTIUS Mission is currently in Phase C, with a Critical Design Review (CDR) planned in mid-2023 with Redwire Space N.V. as Mission Prime and OIP Sensor Systems N.V. as Instrument Prime. The paper presents the key technical challenges faced in the development of the ALTIUS Instrument up to its current CDR maturity level. Beyond a full system overview, detailed insight is provided of its optical concept, the choice and development challenges of its optical tunable spectral filters, the associated control electronics, the ALTIUS Instrument assembling (and alignment), its integration and testing strategy, the on-ground calibration plan and, finally, a summary of the achievable L0/L1 performances. A brief description of the ALTIUS Instrument in-flight calibration strategies will be presented too, along with a flavor of the stringent cleanliness and contamination control measures envisaged to ensure stable optical performances. Finally, the paper presents lessons learned from the subsystem qualifications activities and Instrument STM environmental test campaign, as well as an overview of the foreseen project milestones towards launch.
The ALTIUS Mission (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) aims at the development of a limb sounder based on a small satellite concept to monitor the distribution and evolution of stratospheric ozone at high vertical resolution in support of operational services and long-term trend monitoring. The ALTIUS instrument consists of three spectral imagers flying at an altitude of approximately 700 km Sun-Synchronous Orbit. The three hyperspectral channels are based on Acousto-Optic Tuneable Filters (AOTFs) in the Visible (440-675nm) and NIR (600-1020nm) range, and a cascade of Fabry-Pérot Interferometers (FPI) in the UV (250-355nm). The use of tuneable active spectral filters will allow the ALTIUS Instrument to perform observations with a spectral resolution ranging between 1nm and 10nm in an extremely versatile operational concept. This paper presents the key technical challenges to be controlled on the design and technologies of the ALTIUS 2D imager for limb sounding. A particular insight will be given on its optical concept including the choice of tuneable spectral filters in each channel and the key development of optical elements such as mirrors, filters and coatings, ensuring the straylight performances of the instrument. The paper also provides a deep dive into the structural and thermal design of the instrument ensuring the pointing accuracy, and the way to achieve L1 radiometric performances from L0 instrument performances through calibration needs and data processing strategies. Along the lines, the stringent cleanliness and contamination control and related envisaged strategy to ensure sustained optical performances will be also described
The ALTIUS Mission (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) aims at the development of a limb sounder based on a small satellite concept (i.e. PROBA-Next small platform).
ALTIUS will monitor the distribution and evolution of stratospheric ozone at high vertical resolution in support of operational services and long term trend monitoring. It will provide detailed stratospheric profile information at high vertical resolution, which is a valuable addition to ozone total column for data assimilation systems based on nadir sounders used by operational centers. In addition, some secondary scientific mission objectives are targeted, including measurements of vertical concentration profiles of other atmospheric species.
The ALTIUS Mission was first proposed by the Belgian Institute for Space Aeronomy and, after several studies in different programmes within ESA, is now being developed as an element of ESA’s Earth Watch Programme currently with the participation of Belgium, Canada, Luxembourg and Romania.
The ALTIUS Mission concept consists of three spectral imagers flying at an altitude of approximately 700 km Sun- Synchronous Orbit on-board the next generation of PROBA platform. The ALTIUS Instrument shall allow observation of the Earth’s atmospheric bright limb in an extended spectral region from the Ultraviolet to the Short Wave Infrared (SWIR). In addition, the ALTIUS instrument shall perform solar and stellar occultation observations in the dark limb.
The ALTIUS Instrument three hyperspectral channels are based respectively on Acousto-Optic Tuneable Filters (AOTFs) in the Visible (440-800nm) and SWIR (900-1800nm) range, and a cascade of Fabry Perot Interferometers (FPI) in the UV (250-370nm). The use of tuneable active spectral filter shall allow the ALTIUS Instrument to perform observations with a spectral resolution ranging between 1nm and 10nm in an extremely versatile operational concept.
The use of the AOTFs and Fabry Perot technologies constitutes a novelty in the limb sounding missions and its development critical aspects as well as its performances have been proven in the frame of an extensive Instrument predevelopment phase.
In this phase the critical technologies necessary to ensure the performance and functionalities of the ALTIUS Instrument have been designed, developed and qualified. In particular the following pre-developments were undertaken to pave the way towards Instrument Preliminary Design Review:
Design, manufacturing and qualification of the VIS AOTF Assembly and RF control electronics;
Design, manufacturing and qualification of the UV FPI assembly stack and control electronics;
Development and qualification of UV filters and coatings.
Design and manufacturing of sensor electronics based on selected CMOS sensor for UV and VIS channels;
Design, manufacturing and partial qualification of the Instrument mechanism;
Bread-boarding of the full VIS channel for functionality and end-to-end performances verification.
This paper presents the status and main results of the above-mentioned ALTIUS Instrument pre-development activities. In particular: major outcomes during the development process, achieved performances and lessons learned for the continuation of the ALTIUS Instrument flight model design are being highlighted.
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