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ALTIUS - Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere

The ALTIUS expedition consists of a 2D spectral imager, aboard a PROBA-like satellite, flying at an altitude of approximately 700 km Sun-Synchronous Orbit (SSO).

In the past decade, there has been a significant reduction in the number of atmospheric sounders with a high vertical resolution in space. This impacts the monitoring of long-term trends for essential atmospheric species, such as Ozone. In this frame, the ALTIUS mission was proposed by the Belgian Institute of Space Aeronomy to contribute to remote atmospheric trace gas monitoring with a high vertical resolution by the combination of limb-scatter measurements with solar, stellar, and planetary occultations.

The ALTIUS mission aims at demonstrating the potential of small scientific missions to measure the vertical concentration profiles of ozone and other trace gases in the Earth’s upper atmosphere.

The ALTIUS instrument will observe the Earth’s atmospheric bright limb in different small wavelength bands within the UltraViolet (UV), Visible (VIS) and Near InfraRed (NIR) spectral range by means of 3 separate instrument channels.

Wavelength: 250 – 1200nm.


The ALTIUS instrument, is a 3-channel hyperspectral imaging spectrometer to conduct a spectroscopic survey of Earth atmosphere in the UV, visible and NIR spectral ranges.

The ALTIUS payload is a limb sounder spectrometer, with 1 km vertical resolution over a wavelength range which ensures the detection of the different atmosphere constituents. Each channel contains a spectral filter, either a dedicated Acousto-Optic Tunable Filter (AOTF) or a stack of Fabry-Pérot interferometers, covering a separate section of the desired spectral range. In addition to nominal limb observations, ALTIUS will also perform solar and stellar occultation observations in the dark limb.

The ALTIUS payload will be based on the use of dedicated Acousto-Optic Tuneable Filters (AOTFs) in the Visible and NIR range, while in the UV spectral filtering will be done by a stack of Fabry Perot interferometers. By this way, the ALTIUS payload will perform observations with a spectral resolution better than 10 nm over the complete operational spectral-range.

The ALTIUS instrument contains three independent channels each imaging in a specific spectral range:
• UV channel images between 250nm and 355nm
• VIS channel images between 440nm and 675nm
• NIR channel images between 600nm and 1020nm



Type: Hyperspectral

Solution: Spectrometer

Field: Earth observation

Concept Details

The ALTIUS instrument is composed of two separated subunits:

  1. The optical subunit containing 3 optical channels operating respectively in the UV, VIS and NIR spectral ranges.
  2. The Channel Control Unit (CCU) subunit containing the driving electronics for the three channels, housed in a separate box, external to the optical subunit.

The three ALTIUS channels will be made of reflective optics, and will be built on the same type of architecture but corresponding to an independent optical design:
• A long front baffle to prevent out of field stray light to enter inside the instrument, integrating periscope mirrors to orient the light towards the instrument entrance.
• A mechanism which allows to switch between the different observation-mode of the instrument by either integrating in the optical path a ND filter or a folding mirror
• A front end optics group (FEO), to guide the incoming light towards the spectral element
• Spectral tuneable filter to provide the spectral separation of the incoming light in small selectable spectral bands; being an AOTF in the VIS and NIR channel or an FPI assembly in the UV channel
• A back end optics group (BEO) to focus the spectral image on the detector
• A detector, mostly identical in each channel

For thermal reasons most of the electronics for the ALTIUS instrument are not integrated in the same mechanical structure than the optics. As such the electronics design of the instrument is spread over

  • The optical unit which contains the active electro-optical elements of each channel, such as Focal Plane Assembly, Spectral filters, mechanism, housekeeping, etc
  • The channel control units (CCU), which contains all PCBs commanding the instrument

All the above elements are supported by mechanical structures. The optical unit contains the optical benches carrying the different channels, the external star trackers and a radiator (to passively regulate the thermal household of the instrument). The CCU electronics are maintained in a dedicated CCU box.


ALTIUS is a satellite mission aiming at the remote sensing of key atmospheric constituents at high vertical resolution, flying on-board a PROBA satellite at an altitude of approximately 700 km Sun-Synchronous Orbit. The mission will benefit from the high versatility of the PROBA-like platform to perform observations over different attitudes in a continuous way during an orbit. As such, a precise identification of the atmosphere species and tomography of the air masses can be performed. Its primary objective is the retrieval of ozone concentration profiles with a global coverage by the combination of limb-scatter measurements with solar, stellar, and planetary occultations. Secondary objectives focus on aerosols, NO2, H2O, CH4 and other trace gases.

ALTIUS is a component of the ESA’s Earth Watch Programme with launch expected in 2023 and a design lifetime of minimum 3 years.

The ALTIUS expedition is based on the scientific mission concept as initially proposed by the Belgian Institute of Space Aeronomy.

OIP’s Participation

OIP, as subcontractor to Belgian Institute of Space Aeronomy, participated in phases A, B0 and B1 of the ALTIUS project. These phases are completed.

When ALTIUS became element of ESA’s Earth Watch programme, the OIP work was continued into new contract, which made OIP, as the industrial prime contractor for the payload, responsible for the design, integration, alignment and testing of the ALTIUS instrument during the pre-development and CD phases of the project.


The ALTIUS mission concept has been studied since 2006 by the Royal Belgian Institute for Space Aeronomy, together with OIP Sensor Systems and Qinetiq Space Belgium up to phase B. The payload successfully underwent the intermediate design review (IDR) in June 2015. In December 2015, ALTIUS successfully passed two reviews which concluded that ALTIUS could meet the requirements of an operational ozone mission, and the mission is capable of furthering atmospheric and climate research with its additional objectives. In December 2016, ALTIUS was submitted by Belgium to ESA’s ministerial council as an element of the Earth Watch program. With the appointment of ALTIUS being element of ESA’s Earth Watch programme, a predevelopment activity could be kicked-off in the  the course of 2017. End 2019, beginning 2020 the Phase CD was started.

The payload is at PDR stage (Nov/2020).

The payload should be ready for launch by 2023.


ALTIUS is part of the European Earth Watch Programme (ESA), with QinetiQ Space [B] acting as mission prime and OIP acting as mission prime contractor for the payload.

Royal Belgian Institute for Space Aeronomy (IASB-BIRA) [B] is the principal investigator.

A Belgian Space experiment for atmospheric sounding

The ALTIUS instrument will be built by an industrial consortium on national and international partners (subcontractors) led by OIP, as industrial prime contractor.

The partners are AMOS [B*], Gooch&Housego [UK], VTT [FI*], ASRO [FI*], Teledyne-e2v [UK], QinetiQ Space [B*], HPS [RO*], Centre Spatial de Liège [B*], Antwerp Space [B*], Emtronix [LU*], Balzers [D], Glenair [D], Axon’/Capable [F/NL], ..

*means subcontractor


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