Nigel Fox, NPL Fellow at the National Physical Laboratory, discusses the climate-focused TRUTHS mission, including its achievements so far and the benefits it will deliver for the future.
Led by the UK Space Agency (UKSA) in partnership with several European states, the Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS) satellite mission is designed to improve climate change modelling.
Delivered by the European Space Agency, TRUTHS will be a ‘standards laboratory in space’, setting the ‘gold standard’ reference for climate measurements. Carrying a cryogenic solar absolute radiometer and a hyperspectral imaging spectrometer, as well as a novel onboard calibration system, TRUTHS will make continuous measurements of incoming solar radiation and reflected radiation to evaluate Earth’s energy-in to energy-out ratio.
To learn more about the mission and the progress made so far, The Innovation Platform spoke to Nigel Fox, NPL Fellow in Optical Radiometry and Earth Observation at the National Physical Laboratory (NPL), who conceived the mission and is its lead investigator.
Can you briefly outline the TRUTHS mission, its main collaborators, and its key objectives?
TRUTHS is a UK proposed and led ESA Earth Watch mission, delivering hyperspectral analysis-ready data (ARD) of incoming and Earth/Moon reflected solar radiation in the short-wave (UV-SWIR) spectral range of unprecedented fiducial quality – a world first!
The resultant data will serve as a reference benchmark of the short-wave state of the planet against which near- and long-term change can be reliably detected in as short a time as possible and signatures of ‘information’ (type/health of vegetation, aerosols/air quality, land/water pollution, mineralogy, greenhouse gas (GHG) emissions, etc.) can be robustly detected from complex backgrounds of solar reflected signal and noise.
TRUTHS’ ‘gold standard’ reference data not only serves the Earth observation (EO) community through its own data but, uniquely, it upgrades the Earth observing system as a whole, through the provision of reference calibration. The mission directly responds to the requests of the Committee on Earth Observation Satellites (CEOS) and the Global Climate Observing System (GCOS).
The mission:
- Enables the results of climate action/mitigation to be assessed in a robust manner in as short a timescale as possible, ensuring timely evidence-based adaptation investments.
- Facilitates the improved parametrisation of climate models – carbon cycle, radiation imbalance, reducing uncertainty in climate and impact forecasts.
- Enables a harmonised satellite observing system – with reduced uncertainty and increased confidence – maximising value for money of all the world’s space assets.
- Provides the means to trust and integrate high spatial/temporal resolution commercial satellite data into the mainstream science observing system – leading to new localised information services.
- Enables the creation of new innovative services and associated economic growth from commercial and agency satellites – in principle through UK data architecture such as EODH.
Through providing high accuracy calibration of existing infrastructure, e.g. Moon and Earth deserts, TRUTHS extends the value of the mission backwards and forwards in time beyond that of its nominal 5–8-year life. Allowing recalibration and harmonisation of existing data from the 1980s and that of future missions will enable artificial intelligence (AI) tools and digital twins to fully exploit the archives of world data and lead to TRUTHS becoming a mission with an effective life and impact spanning a century.
The mission is implemented by ESA and funded by a consortium of European Nations led by the UK – Spain, Switzerland, Greece, Romania, and Czech Republic.
The industrial consortium developing the mission is led by Airbus UK and consists of various companies (approximately 25, of which nine are SMEs), including: Telespazio UK, CGI, SSTL, AVS-UK, RAL space, NPL, Deimos-UK & Romania, Teledyne-UK, Thales Alenia space- Switzerland, PMOD- Switzerland, Sener Aerospatial Spain, Toptec Czech Republic, Sonvision Romania, ISD Greece, and Helia Photonics Ltd.
What are the issues associated with the capabilities of current EO systems and how will TRUTHS work to improve these?
Most existing satellites deliver data within their individual specifications. However, whilst these are often adequate for the specific missions’ individual applications when conserved independently, there are often biases between similar missions from different space agencies and/or from the same space agency between different satellite sensors. Often, these differences do not necessarily pose significant issues until data from different sensors needs to be combined together to create longer-time base studies. For example, as needed for climate, or for services where results from two or more satellites are required to improve sampling. For these, any biases need to be assessed and removed, potentially through harmonising to one sensor. But which one?
Similarly, when absolute values of change are required, it is important to have a common reference from which all the world’s satellites can evidence against and derive consensus information that may be required to support climate action, underpin financial risk, and/or address potential litigation issues. The more confident (accurate) the data, the more reliable and trustworthy the information and action is.
For many climate applications, the signal/trend to be detected is so small it can take many decades for it to be at a level that is large enough to confidently be detected above background noise of the Earth system as a whole and the instruments undertaking the measurements. Studies have shown that the uncertainty requirement for many long-time-base climate observations is typically around 10X lower than current sensors achieve.
However, the main challenge in the current observing system is the residual bias and how to establish a common reference that can be relied upon now and into the future and accepted by all. The International System of Units (SI) is, in principle, designed to do this and most satellites seek to calibrate against this system before launch. However, whilst every effort is made to ensure data from satellites is accurate, the shock of launch and harshness of space generally leads to small but significant changes in performance which need to be assessed and corrected before the data can be exploited.
Many public-funded EO satellite missions tend to be of a sufficient size to have their own on-board calibration systems to help address changes in performance, such as bias. However, the on-board systems are also subject to change. Smaller satellites, typical of those from commercial organisations, generally do not have the capacity to host such systems and rely solely on calibration/validation against vicarious externally viewed references, such as deserts. Public sector missions, because of on-board system drift, similarly rely on deserts and similar targets for their initial in-flight calibration checks and contribute to ongoing monitoring.
These challenges limit the uncertainty attainable in flight to be, at best, around 2-3% and often significantly more than this. As it flies its own on-board calibration system that replicates what is typically done pre-flight on the ground, including a primary reference standard of the SI, TRUTHS can achieve uncertainties closer to 0.3%.
This improved accuracy can then not only deliver climate quality hyperspectral data for many applications, but can also be used to upgrade and harmonise the rest of the world’s space assets. This is achieved by having a unique orbit that allows TRUTHS to observe the same target as other satellites at the same time as they do over many locations on the planet. In this way, by viewing the same sight at the same time, TRUTHS observations can be compared with those of the other satellite and any difference identified and corrected.
TRUTHS will ensure that biases are well understood, enabling sensor data to be harmonised and interoperable and, in many cases, with a much higher accuracy than it previously had, facilitating improvements to its data products and often increased utility and opportunities for new applications. The result will be a future climate-ready observing system and one where commercial satellites can be harmonised and integrated into the mainstream, leading to not only improved science but also commercial opportunities for new services.
Can you go into more detail about NPL’s involvement in the mission?
The mission was conceived by myself some 25 years ago and I am the mission’s lead investigator. I also lead a consortium of international scientists, funded by ESA, to undertake science and sensitivity studies related to the design of the mission.
NPL is also part of the industrial consortium undertaking calibrations and engineering design support to Airbus and others on calibration and performance related activities and will ultimately undertake the final calibration of the mission before launch in conjunction with RAL Space. In addition, NPL is supporting the development of the ground segment and how the data from the mission will be quality controlled and certified.
The NPL team is also developing algorithms to support the simulation of the mission performance as it is built, and these will also form part of the data processing system of the mission once it is launched. This latter step, whilst not unique, will be the first time that such transparency and rigour has been applied to the uncertainty evaluation of a satellite mission. As the calibration of other sensors is a key objective of the mission, NPL is also developing the algorithms and tools to allow this process to be undertaken.
Can you explain more about the achievements so far in the development process of the mission, and outline the next steps and estimated timeline for further development?
The mission, following a few small updates in the next month, will have passed through its so-called preliminary design review (PDR) and completed phase B2 in readiness to progress into phase C/D where the designs of the payload are finalised, fully prototyped, and start to be built into space-ready hardware.
To date, all aspects of the mission have been subject to detailed design and review: from the satellite platform, the compatibility with the likely satellite launcher, how the data will reach the ground, be processed and delivered to users, and the payload. For TRUTHS, the payload and the means to achieve the unprecedented uncertainty has been the dominant task of the last few years. Although there have been several iterations, current designs and, in some cases, prototype hardware have shown that they are capable of meeting the stringent objectives of the mission.
Going forward, the detailed design of the ground segment will start to materialise and further refinements of the payload will be undertaken.
The mission, as with all ESA projects, is subject to a funding review at the ESA ministerial in November, where funds from ESA Member States will be requested to secure the future of this highly innovative, world first mission, led by the UK and due for launch in 2031/32.
The mission can be considered the world’s first metrology laboratory in space, providing a gold-standard reference for the world’s optical EO sensors, embedding trust into a future interoperable global Earth observing system.
Please note, this article will also appear in the 23rd edition of our quarterly publication.
