ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere)

Quick facts

Overview

Summary

Mission Capabilities

The ALTIUS mission will carry one instrument onboard named the ALTIUS instrument which is a three-channel hyperspectral imaging spectrometer that will conduct a spectroscopic survey of the Earth’s atmosphere. The ALTIUS instrument observes Earth’s limb, allowing ozone to be viewed at different altitudes, thereby providing vertical profiles of different ozone concentrations and providing a 2D image of ozone in the atmosphere. 

Performance Specifications

The ALTIUS instrument will observe in the ultraviolet (UV) at wavelengths between 0.25 µm and 0.355 µm, visible at wavelengths between 0.44 µm and 0.675 µm (VIS) and near Infrared (NIR) at wavelength between 0.6 µm and 1.02 µm. By utilising Acousto-Optic Tunable Filters and Fabry-Perot interferometers, the instrument will have a spectral resolution better than 10 nm over the complete operational spectral range.

ALTIUS will be launched into a sun-synchronous orbit at an altitude of 688 km and have a repeat cycle of 3 days. 

Space and Hardware Components

ALTIUS will be built by QinetiQ Space and will use the Project for On-Board Autonomy (PROBA)-P-200. The satellite will be three-axis stabilised and will weigh just 266 kg. It will have dimensions 800 x 800 x 1200 nm and carry four 1N thrusters and two deployable solar panels with 120 W of power on board. 

The satellite will be operated from ESA’s European Space Security and Education Centre (ESEC) in Redu, Belgium, with the payload ground segment located at the Belgian Support and Operation Center (B.USOC) in Brussels, Belgium

Overview

The European Space Agency’s (ESA) upcoming Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere (ALTIUS) mission will aim to monitor the distribution and evolution of  stratospheric ozone in the Earth’s atmosphere. ALTIUS will provide support to services such as weather forecasting, and monitor long-term trends in atmospheric ozone.

Satellites in orbit are the only way of measuring recovery and change of atmospheric ozone in a consistent and systematic manner. Since the end of ESA’s Envisat mission in 2012, there are only a few instruments in orbit that provide profiles of ozone, and some of these missions will end in the next few years. The ALTIUS mission will fill a very important gap in the continuation of limb measurements for atmospheric science. 

Background: Atmospheric Ozone

In the 1970s, scientists discovered that the ozone layer was being deleted, particularly above the South Pole resulting in what is known as the ozone hole. It transpired that hydrofluorocarbons, used in refrigerators and air conditioners, were largely to blame. To address the destruction of the ozone layer, in 1987, the international community established the Montreal Protocol on ozone-depleting substances.

Thanks to the Montreal Protocol, the global consumption of ozone-depleting substances has since reduced by about 98%, and the ozone layer is showing signs of recovering. It is imperative that concentrations of stratospheric ozone, and how they vary according to the season, are monitored continually, to not only assess the recovery process, but also for atmospheric modelling and for practical applications including weather forecasting. 

While ozone depletion is not a major cause of climate change, the two are nevertheless linked. Ozone affects the temperature balance of Earth in two different ways. Firstly, it absorbs solar ultraviolet radiation which heats the stratosphere. Secondly, it absorbs infrared radiation emitted by Earth’s surface, effectively trapping heat in the troposphere. 

Spacecraft

ALTIUS will be built by QinetiQ Space and will use the Project for On-Board Autonomy (PROBA)-P-200 satellite bus currently under development at QinetiQ space. The bus is a three-axis stabilised multi-purpose platform, and the spacecraft will be in the small satellite category, weighing just 266 kg. The agility of this class of micro-satellite platform allows for atmospheric limb observations in different remote-sensing geometries from a low-Earth orbit.

The Proba-P-200 bus has dimensions of 800 x 800 x 1200 nm and is made of an Aluminium Honeycomb structure mounted on an AI milled bottom board. The bus carries four 1N thrusters and two deployable solar panels with 120 W of power on board.  The PROBA-P-200 has a built-in turn-key satellite control centre which is fully validated in the Assembly Integration and Verification (AIV). It offers a fully transparent automation system, which can easily be tailored by the end user. The SCC has shown flawless and fully automated operations in testing, demonstrating its relevance in terms of simplicity, autonomy and response to the mission needs starting from the AIV to routing operations.

Mission Status

The ALTIUS mission concept has been studied since 2006 by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), together with OIP Sensor Systems and Qinetiq Space Belgium. 

• June 2015: The payload successfully underwent the intermediate design review 
• 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. 
• December 2016: ALTIUS was submitted by Belgium to the European Space Agency’s (ESA) ministerial council as an element of the Earth Watch Programme. 
• 2022: ALTIUS is still in testing within ESA’s Earth Watch programme and financed mainly by Belgium with contributions from Canada, Luxembourg and Romania.

Sensor Complement

The ALTIUS instrument is a three-channel hyperspectral imaging spectrometer that conducts a spectroscopic survey of the Earth’s atmosphere in the ultraviolet (UV), visible (VIS) and near infrared (NIR) spectral ranges.  The ALTIUS instrument observes the ozone from side-on, at Earth’s limb, with a 1 km vertical resolution. This limb-sounding technique allows ozone to be viewed at different altitudes, thereby providing vertical profiles of different ozone concentrations, providing a 2D image of ozone in the atmosphere. In addition to nominal limb observations, the instrument will also perform solar and stellar occultation observations in the dark limb.  The channels that the instruments images in are provided in Table 1. 

The ALTIUS instrument will be based on the use of Acousto-Optic Tunable Filters (AOTFs) for the VIS and NIR measurements while spectral filtering will be done by a stack of Fabry-Perot interferometers for the UV measurements. The use of these filters will lead to performance with a spectral resolution better than 10 nm over the complete operational spectral-range.

The ALTIUS instrument will be built by Optique et Instruments de Précision (OIP) Sensor Systems and will observe the Earth’s atmospheric bright limb in different small wavelength bands by means of three separate instrument channels. The instrument is composed of two separated sub units. 

1. The optical subunit contains three 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. For thermal reasons, these electronics are not integrated in the same mechanical structure of the optics. 

The 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. The optical subunit will contain the following sections:

• A long front baffle to prevent out of field stray olight to enter inside the instrument, integrating periscope mirrors to orient the light towards the instrument entrance
• A mechanism which allows switching between the different observation-modes of the instrument by either integrating the optical path, a ND filter or a folding mirror.
• A front end optics (FEO) group, to guid the incoming light towards the spectral element.
• Spectral tunable filter to provide the spectral observation of the incoming light in small selectable spectral bands; being an acousto-optic tunable filter (AOTF) in the VIS and NIR channel or an Fabry-Perot interferometer (FPI) assembly in the UV channel. 
• A back end optics (BEO) group to focus the spectral image on the detector
• A detector which is mostly identical in each channel.

Ground Segment

The ground segment for data processing is developed by a consortium under the prime contractor Spacebel in belgium with scientific support from the Belgian institute for Space Aeronomy (BISA) and the University of Saskatchewan (USASK) in Canada.

The satellite will be operated from ESA’s European Space Security and Education Centre (ESEC) in Redu, Belgium, with the payload ground segment located at the Belgian user Support and Operation Centre (B.USOC) in Brussels, Belgium.

References 

1) “ALTIUS.” OIP Space Instruments, https://oipspace.be/solutions/spectrometers/atmospheric-analysis/altius/. 

2) “ESA - Altius.” European Space Agency, https://www.esa.int/Applications/Observing_the_Earth/Altius.

3) “ESA - The instrument.” European Space Agency, https://www.esa.int/Applications/Observing_the_Earth/Altius/The_instrument. 

4) “ESA - Operations and data flow.” European Space Agency, https://www.esa.int/Applications/Observing_the_Earth/Altius/Operations_and_data_flow.

5) “ESA - Introducing Altius.” European Space Agency, https://www.esa.int/Applications/Observing_the_Earth/Altius/Introducing_Altius. 

6) “ALTIUS - Gunter's Space Page.” Gunter's Space Page, 11 January 2022, https://space.skyrocket.de/doc_sdat/altius.htm.