FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring)

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring)

In 2017, ESA has chosen two concepts, FORUM and SKIM, to be developed further and compete to be the ninth Earth Explorer mission. Thanks to new technical developments, the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) candidate would measure radiation emitted from Earth across the entire far-infrared part of the electromagnetic spectrum. Significantly, it measures in the 15–100 µm range, which has never been done from space before. ¹⁾

These observations are important because Earth emits infrared radiation to space, which is affected by water vapor and cirrus clouds, which, in turn, play key roles in Earth’s temperature.

FORUM’s benchmark measurements would improve our understanding of the greenhouse effect and, importantly, contribute to the accuracy of climate change assessments that form the basis for policy decisions.

The SKIM (Sea-surface Kinematics Multiscale) monitoring candidate would carry a novel wide-swath scanning multibeam radar altimeter to measure ocean-surface currents. Uniquely, it uses a Doppler technique, which offers more direct measurements than conventional satellite altimeters.

These new measurements would improve our understanding of vertical and horizontal ocean–surface dynamics over the global ocean every few days. This would lead to better knowledge of how the ocean and atmosphere interact – for example, how atmospheric carbon dioxide is drawn down into the ocean.

SKIM would have particular relevance for understanding the rapidly changing Arctic Ocean, and for observing equatorial regions where conventional satellite altimeters are unable to provide useful measurements of currents.

The Earth observation scientific community is invited to participate in a European Space Agency (ESA) User Consultation Meeting at the Robinson College, University of Cambridge in Cambridge, United Kingdom on 16–17 July 2019. This consultation forms a critical input to the decision-making process that will lead to the selection of ESA’s ninth Earth Explorer mission. ²⁾

Mission Study Phase

• September 24, 2019: Following a rigorous selection process, ESA has selected a new satellite mission to fill in a critical missing piece of the climate jigsaw. By measuring radiation emitted by Earth into space, FORUM will provide new insight into the planet’s radiation budget and how it is controlled. ³⁾

- The FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission was one of two concepts competing to be ESA’s ninth Earth Explorer mission.

- Earth Explorers use innovative measurement techniques to yield new insight into different aspects of the Earth system and the interactions that bind the system as a whole. Fundamentally, they are designed and built to fill knowledge gaps identified by the scientific community, so, importantly, the community retains a key role in the selection and development process.

- After a two-year feasibility study phase, both FORUM and its competitor, the Sea-surface Kinematics Multiscale monitoring (SKIM) concept, were presented and discussed in detail with the scientific community at a User Consultation Meeting in Cambridge, UK, in July.

- Wolfram Mauser, who chaired ESA’s Advisory Committee for Earth Observation on behalf of Martin Visbeck, said, “Both mission concepts are outstanding in the value they would bring to science, and are technologically ready to be built, so it was difficult to recommend which one should be implemented. - Nevertheless, FORUM promises to improve climate models and, therefore, climate prediction. So with the issue of climate change a major global concern, we finally decided to recommend this concept – and we are very happy that ESA has taken our recommendation.”

- Earth’s surface temperature is driven by the radiation balance at the top of the atmosphere, but this balance has been disturbed by the emission of greenhouse gases that are trapping heat in the atmosphere that would otherwise escape into space. — More than half of this outgoing longwave energy is in the far-infrared part of the electromagnetic spectrum – and this has not, so far, been measured.

- Filling this gap, FORUM will measure across Earth’s entire far-infrared part of the electromagnetic spectrum.

- These measurements are important because Earth’s outgoing radiation at these wavelengths is strongly affected by water vapor and ice clouds, which in turn, play a key role in regulating surface temperatures.
Measurements from this exciting new mission will improve confidence in the accuracy of climate change assessments that form the basis for future policy decisions.

- Josef Aschbacher, ESA’s Director of Earth Observation Programs, said, “FORUM will measure, for the first time, the far-infrared part of the electromagnetic spectrum from space, thus allowing us to better understand the energy balance of our planet. FORUM will bring great benefits to climate science. - Better understanding the complexity of our climate system and filling gaps in our knowledge is of critical importance as the consequences of climate change are far-reaching, affecting all facets of society and the natural world.”

- The design of the mission will now be fine-tuned, and then built.

• On 16 –17 July 2019, the Earth observation science community gathered at Robinson College, University of Cambridge, UK, for ESA’s Earth Explorer 9 User Consultation Meeting to discuss the merits of the FORUM and SKIM mission concepts before one is finally selected to be built as ESA’s ninth Earth Explorer mission. ^{4) 5)}

- The philosophy of developing Earth Explorer missions centers on the involvement of the scientific community from the very beginning of a mission’s life. Being proposed by the scientific community and realized through the user-driven selection process ensures that this series of state-of-the-art missions addresses the most urgent Earth science questions of our time.

- This meeting provides the opportunity for the scientific community to debate the new mission concepts – each of which promises to deliver novel information on how our planet works and bring benefits to society.

- The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission would measure radiation emitted from Earth across the entire far-infrared part of the electromagnetic spectrum. These benchmark measurements would improve our understanding of the greenhouse effect and, importantly, contribute to the accuracy of climate change assessments that form the basis for policy decisions.

- The Sea-surface Kinematics Multiscale monitoring (SKIM) candidate would carry a novel wide-swath scanning multibeam radar altimeter to measure ocean-surface currents. These new measurements would improve our understanding of vertical and horizontal ocean–surface dynamics over the global ocean every few days. This would lead to better knowledge of how the ocean and atmosphere interact – for example, how atmospheric carbon dioxide is drawn down into the ocean.

Development Status

• June 28, 2022: ESA has awarded a contract worth €160 million to Airbus in the UK to build the Earth Explorer FORUM satellite. This exciting new mission will yield unique insight into the planet’s radiation budget and how it is controlled – thereby filling in a critical missing piece of the climate jigsaw. ⁶⁾

- FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) is ESA’s ninth Earth Explorer mission.

- Realised within ESA’s FutureEO programme, Earth Explorers are pioneering research satellites. Each of these extraordinary missions carries innovative space technology and each, without exception, has exceeded their original science objectives.

- Time and time again, they show how novel spaceborne instruments and measuring techniques can return an astonishing wealth of scientific findings about our planet – which, in turn, benefits society at large.

- FORUM promises to be all of this and more – importantly, bringing new understanding to atmospheric processes linked to climate change.

- Earth’s surface temperature is driven by the radiation balance at the top of the atmosphere, but this balance has been disturbed by the emission of greenhouse gases, trapping heat in the atmosphere that would otherwise escape into space.

- More than half of Earth’s outgoing longwave energy is in the far-infrared part of the electromagnetic spectrum but this particular part of the spectrum has never been measured before.

- Filling this gap, FORUM will measure across the far-infrared part of the electromagnetic spectrum.

- These measurements are important because Earth’s outgoing radiation at these wavelengths is strongly affected by water vapour and ice clouds, which in turn, play a key role in regulating surface temperatures.

- Measurements from this ambitious new mission will improve confidence in the accuracy of climate change assessments that form the basis for future policy decisions.

- Following the first development phases and its final selection as the ninth Earth Explorer, ESA has awarded the contract to build the FORUM mission to Airbus in the UK.

- At a ceremony marking the contract signature, held yesterday at the Houses of Parliament in London, Simonetta Cheli, ESA’s Director of Earth Observation Programmes, said, “We are thrilled to award the industrial contract to Airbus in the UK as Prime Contractor for FORUM, with OHB in Germany responsible for the instrument. FORUM adds to our highly successful family of Earth Explorer missions and, by acquiring novel information, will bring great benefits to climate science.”

- Jean-Marc Nasr, Head of Space Systems at Airbus added, “This critical Earth observation mission to measure infrared radiation from the Earth for the first time, will give scientists and climatologists the data they need to improve their global warming forecasts.

- “It builds on Airbus’ heritage in designing and manufacturing cost efficient small Earth observation missions including Copernicus Sentinel-5P and is the sixth Airbus primed Earth Explorer mission for the European Space Agency.”

- George Freeman, Minister for Science, Research and Innovation, said, “This important new mission to further improve the accuracy of climate forecasts and view our planet through new eyes is another illustration of UK space tech expertise. Scientists at Imperial College London provided key support to ESA in defining FORUM’s science objectives and the satellite is set to be built by Airbus in Stevenage.

- “This is a significant industrial contract which demonstrates the UK’s strengths in Earth observation technology and satellite manufacturing, as well as our global leadership in tackling climate change.”

- Dominique Gillieron, ESA’s Earth Explorer Programme Manager, noted, “FORUM is a single-satellite mission that will carry a Fourier Transform Spectrometer that can measure across Earth’s entire far-infrared part of the electromagnetic spectrum.

- “These measurements are important because Earth’s outgoing radiation, at these wavelengths, is strongly affected by water vapour and ice clouds, which in turn, play a key role in regulating surface temperatures.

- “It will be the very first time that this part of the spectrum will be measured from space with this accuracy and this will greatly contribute to climate research.

- “With FORUM’s important role to play, this contract is a significant milestone and we look forward to the build ahead.”

• September 2020: FORUM aims at measuring the Earth’s Top-Of-Atmosphere (TOA) emission spectrum in the mid- to far infrared spectral range (100 to 1600 cm^-1, i.e. 6.25 to 100 µm), filling the current observational gap in the 100 to 667 cm^-1 (i.e. from 15 to 100 µm) spectral region of the far-infrared. These measurements will improve the understanding of the climate system by observing, for the first time, most of the spectral features of the far-infrared contribution. These observations are highly relevant for the Earth radiation budget, in particular for the water vapor contribution to the continuum absorption in the rotational band, the cirrus cloud properties, and the ice/snow surface emissivity. ⁷⁾

- The main research goal of the FORUM mission is the evaluation of the role of the far-infrared in shaping the current climate and thus the reduction of uncertainty in predictions of future climate change. This will be addressed by: a) building a highly accurate global dataset of far-infrared radiances to validate present-day state as captured by Earth system models; b) using these measurements to understand and constrain the processes that control far-infrared radiative transfer and hence Earth’s greenhouse effect; c) updating the parameterizations of these processes for implementation in radiative transfer codes, and ultimately in Earth system models; and d) characterizing critical feedback mechanisms.

Forum Mission Architecture

- Figure 5 depicts the main mission architectural elements of FORUM. The space segment consists of a single satellite carrying two optical instruments: the FORUM Sounding Instrument (FSI) and the FORUM Embedded Imager (FEI). The satellite will fly in a loose formation with MetOp-SG (A1) in a 29-day repeat-cycle Sun-synchronous orbit, with a Mean Local Solar Time at the descending node of 09:30 and an average orbit altitude of 830 km. Vega-C is the baseline launcher in a dual-launch configuration.

- FORUM will perform continuous step-and-stare nadir-looking observations (see Figure 6) of spectrally-resolved TOA radiances with a 100% duty cycle along the orbit. The primary instrument is the FSI, a single-pixel Fourier Transform Spectrometer (FTS) sampling the electromagnetic spectrum from 1600 cm^-1 to 100 cm^-1 (i.e. 6.25 µm to 100 µm) with a spectral resolution better than 0.5 cm^-1. The FSI single 15 km diameter circular ground sample is co-located with the ground sample of the FEI, a single-band-infrared-imager with central wavelength at 10.5 µm and a bandwidth of 1.5 µm, which is used for scene heterogeneity determination. Both instruments share several common units: the entrance aperture, the pointing mechanism, the thermal and mechanical subsystems and the radiometric calibration devices.

- The space segment is designed for a nominal lifetime of four years with sufficient propellant for six years. The FORUM satellite is largely based on existing three-axis stabilised Earth Observation platforms, with specific mission adaptations. This ensures a streamlined satellite development meeting the stringent programmatic boundary conditions while minimizing the development risks.

- The Ground Segment uses the generic ESA’s Earth Explorer ground segment infrastructure, comprising:

a) The Flight Operation Segment (FOS), responsible for operations and safety of the spacecraft during all mission phase. This includes the Telemetry, Tracking and Command (TT&C) ground station and the Flight Operations Control Centre (FOCC)

b) The Payload Data Ground Segment (PDGS), which includes the Science Data Acquisition Station, the Processing and Archiving Element and the Mission Planning and Monitoring Element.

- The FORUM FOS will receive orbit and mission planning data from the MetOp-SG’s FOS to ensure consistent formation-flying operations. A single high-latitude ground station (e.g. Svalbard or Kiruna) will receive the FORUM scientific data via X-band. The PDGS will distribute to the users the data, processed up to Level-2, within 24 hours nominally or within three hours under request.

Space Segment

The FORUM space segment consists of a satellite carrying the FSI (FORUM Sounding Instrument) and the FEI (FORUM Embedded Imager) instruments, flying in loose formation with MetOp-SG (1A). For the satellite platform, two concepts lead by Airbus Defence and Space UK and Thales Alenia Space UK have been analysed during the Phase A preparatory activities. Both concepts, as developed in Phase A, largely rely on recurrent platform with specific adaptations to reduce implementation costs and development time.

The accommodation of the payload drives the physical satellite configuration, which is based on a classical platform and payload modules design. As shown in Figure 7 and Figure 8 both concepts mount the payload on top of the platform in launch configuration. In orbit, the satellite flies, in nominal mode, with the launcher interface ring panel facing the Sun. Such configuration guarantees an unobstructed view of the payload towards Earth, minimizes Sun illumination on the payload and maximizes the power generated along the orbit.

Key Mission Requirements

Table 1 summarizes the main spectral, geometric and radiometric observation mission requirements at Level 1c.

The absolute radiometric accuracy required in the FORUM mission is very demanding, particularly in the spectral region between 300 cm^-1 and 1100 cm^-1 and at the lower end of the dynamic range (i.e. 190 K), where a goal accuracy of <0.1K is required.

The FSI precision is expressed by the Noise Equivalent Spectral Radiance (NESR) with a goal requirement below 0.4 mW/(m².sr. cm^-1) in the spectral range between 200 cm^-1 and 800 cm^-1.

The distance between two consecutive FSI spatial samples (see Figure 6) is 100 km, and less than 20% of the overall samples along the orbit are used for calibration purposes to meet the stringent radiometric accuracy requirement.

Payload

Two payload concepts depicted in Figure 9 and Figure 10 have been analysed during the Phase A preparatory activities by Airbus Defence and Space GmbH and OHB, respectively.

The FSI and the FEI share several common units, such as the entrance aperture, the pointing mechanism, the thermal and mechanical subsystem and the radiometric on-board calibration devices.

The mechanical accommodation is driven, in both concepts, by the geometrical and thermal stability required to meet the pointing knowledge and the radiometric requirements. For both concepts, the payload includes:

- A very stable optical bench to minimise thermoelastic distortions. It supports all the optical elements, the pointing mechanism, the interferometer, the calibration units and the detection chains. The optical bench is isostatically mounted on the platform. The optical bench together with the optical units is kept at around 293 K for both Concepts A and B.

- Nadir and deep-space baffles to minimise undesired radiation inside the instruments from the Earth and Sun, respectively.

- A calibration blackbody accommodated at the entrance of the instruments. The blackbody view is selected by rotation of the pointing unit.

The design of the FSI consists of a compact optical instrument. The optical design allows covering the complete FORUM spectral range with the required spectral resolution while keeping the beam divergence small for performance optimisation. The FSI instrument is composed of the following subsystems:

- A pointing unit allowing to select the different payload views: nadir, blackbody or deep space

- a three-mirror reflective telescope

- a four-port double pendulum interferometer

- Two focusing optics to direct the light onto the two FSI detectors.

- 2 pyroelectric detectors and associated electronics

- The front telescope optics has three main functions:

- To reduce the beam diameter to keep the beamsplitter and the cube corners in the interferometer at a manufacturable size.

- To relay the aperture stop onto the entrance pupil of the interferometer at the cube corners.

- To provide a well-corrected intermediate image plane at the field stop, which optimises spatial performance (Integrated Energy).

The interferometer is a four-port double pendulum type interferometer for both concepts. This choice reliably provides sufficient stroke and beam diameter, with a simple configuration and is robust against environmental loads. In the pendulum mode of operation, the corner cubes located at the tip of both arms move in opposite directions resulting in an Optical Path Difference that produces the interferogram.

The FSI detectors are single pixel DLaTGS (Deuterated Lanthanum α Alanine doped TriGlycine Sulphate) pyroelectric detectors operated at ambient temperature as the rest of the payload. The detector noise is the main contributor to the required NESR radiometric performance.

The FEI, a single-band-infrared-imager centred at 10.5 µm with a bandwidth of 1.5 µm is used for scene heterogeneity determination. It is a refractive uncooled imager using a 2D µ-bolometer array detector. The FEI acquires at least five images during each FSI dwell time; these are symmetrically distributed with respect to the satellite nadir direction at the middle of the FSI dwell time (coinciding with the FSI LoS). Each FEI image has a footprint of at least 36 x 36 km² and is sampled with a GSD (Ground Sampling Distance) at the nadir image of a 0.6 km x 0.6 km, along- and across-track respectively. The FSI single pixel is co-located within the footprint of the FEI.

In order to mature the critical technologies some key technology development activities have been carried out during the Phase A, with emphasis on technologies and materials for the beamsplitter unit, the detection chain, the on-board blackbody and interferometer mechanism of the FSI.

Platform

Two platform concepts were developed during the Phase A preparatory activities of FORUM. Concept A (see Figure 7) is based on a full reuse of the Copernicus Sentinel-5P platform and the reuse of the Astrobus-M equipment. This choice provides large margins in terms of payload accommodation. Concept B (see Figure 8) is based on the adaptation of the Proteus 150/300 standard platform and the Copernicus Sentinel-3 and Earth Explorer FLEX equipment. Concept B provides an optimized solution with sufficient margins in terms of accommodation, power and fuel availability.

This paper presents an overview of the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, selected in September 2019 as the ninth Earth Explorer mission of the European Space Agency. The main mission architecture elements have been introduced, specifically the platform, the payload and main mission performance.

References

1) ”Two new Earth Explorer concepts to understand our rapidly changing world,” ESA, 15 November 2017, URL: https://www.esa.int/Our_Activities/Observing_the_Earth/Two_new_Earth_Explorer_concepts_to_understand_our_rapidly_changing_world

2) ”Earth Explorer-9 User Consultation Meeting, Robinson College, University of Cambridge, Cambridge, UK, 16-17 July 2019, ESA, 30 January 2019, URL: https://atpi.eventsair.com/QuickEventWebsitePortal/19m11---earth-explorer-9-user-consultation-meeting/website

3) ”A new satellite to understand how Earth is losing its cool,” ESA, 24 September 2019, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/A_new_satellite_to_understand_how_Earth_is_losing_its_cool

4) ”Earth Explorer-9 -Discussions on FORUM and SKIM, and close of meeting,” ESA, 17 July 2019, URL: http://m.esa.int/spaceinvideos/Videos/2019/07/Discussions_on_FORUM_and_SKIM_and_close_of_meeting

5) ”Earth Explorer 9 Candidate Mission FORUM —Report for Mission Selection,” ESA-EOPSM-FORM-RP-3549, 21 June 2019, URL: https://esamultimedia.esa.int/docs/EarthObservation/EE9-FORUM-RfMS-ESA-v1.0-FINAL.pdf

6) ”Contract secures design for ESA’s FORUM satellite,” ESA Applications, 28 June 2022, URL: https://www.esa.int/Applications/Observing_the_Earth/FutureEO/Contract_secures_design_for_ESA_s_FORUM_satellite

7) Bernardo Carnicero Domínguez, Charlotte Pachot, Hulk Oetjen, Flavio Mariani , Stefanie Riel, Andrea Tromba, Dulce Lajas, Dirk Schuettemeyer, Bernd Sierk , Nicolas Leveque, Manfred-Georg Kolm, Hans Korswagen, Winfried Posselt, ”The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission. ESA's 9th Earth Explorer,” Conference Paper, Researchgate, IGARSS 2020, 26 September-2 October 2020, Waikoloa, HI, USA, URL: https://tinyurl.com/2c7farm6
 

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002.