Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) – A ‘gold standard’ reference to support the climate emergency

Nigel Fox1, Thorsten Fehr2, Paul Green1, Beth Greenaway3, Andrea Marini2, John Remedios4,
Jacqueline Russell5,

1National Physical Laboratory (NPL), Hampton Rd, Teddington, TW11 0LW, UK
2ESTEC, European Space Agency (ESA), Noordwijk, Netherlands
3UK Space Agency (UKSA), Polaris House, Swindon, SN2 1SZ UK
4National Centre for Earth Observation (NCEO), University of Leicester, LE1 7RH, UK
5National Centre for Earth Observation, Imperial College London, SW7 2BX, UK

Abstract
Introduction
Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) is a hyperspectral satellite mission explicitly designed to become a ‘gold standard’ reference for observing the state of the earth’s climate in the short-wave domain in support of the climate emergency. The UK-led mission, under-development within the ESA Earthwatch program (https://www.esa.int/Applications/Observing_the_Earth/TRUTHS), was conceived at the UK national metrology institute, NPL, more than 20 years ago in response to challenges highlighted by the worlds space agencies, through bodies such as CEOS, in relation to interoperability and accuracy. This led to the initial ‘calibration focus’ of the mission and the vision of creating an in-orbit SI-traceable reference or a ‘metrology/standards laboratory in space’. Such SI-Traceable satellites are now being called SITSats.

As the climate emergency started to emerge as a global priority, the value of TRUTHS’ unprecedented observational capabilities across the whole short-wave spectral domain, in addition to the enhancement of other missions by reference calibration, raised the needs of climate to become an explicit priority. The results of the 2007 US decadal survey helped to frame the most demanding observational objectives for TRUTHS towards addressing radiation balance and climate sensitivity resulting from feedbacks e.g. Cloud, albedo... It also initiated the long-standing partnership with the US sister mission CLARREO and the current Clarreo pathfinder mission.

What will TRUTHS do?
The high accuracy of TRUTHS together with its spectral and spatial resolution facilitate a new epoch in how the Earth is observed, delivering data not constrained to a single discipline but deliberately specified to allow it to be configured to support applications in and at the boundaries of Land, Ocean and Atmosphere to meet the exacting needs of climate. Encompassing it’s own ‘metrology laboratory in space’, TRUTHS sensors are regularly calibrated in-flight to a primary SI standard. This ensures that unlike other satellite sensors, TRUTHS’ ability to detect long-term changes and trends will not be constrained by sensor performance e.g. drifts and biases, but rather the size of the trend, above the background of natural variability. In this way, helping GCOS observational specifications to be achieved and the prospect of testing and constraining the forecasts of climate models in as short a time as possible.

TRUTHS will establish a fiducial data set of incoming and outgoing solar radiation which will:
• Provide, directly and through reference calibration, an SI-traceable operational observational benchmark of the state of the planet’s short-wave incoming and reflected energy and its contribution to the radiative balance including related forcings and feedbacks manifested within it. From this, human induced climate trends can be detected in as short a timescale as possible, limited only by natural variability.
• facilitate a transformation in radiometric performance and functionality of current, future (and some heritage) Earth observing systems to meet the specific needs of climate - through an SI-traceable, high accuracy, reference calibration in orbit, ensuring robust coherence and interoperability. This is the foundation needed to establish an ‘integrated Earth observing system’ and associated Climate Data Records (CDRs).
• deliver data of sufficient quality and flexibility to test and improve the retrieval of solar reflective Essential Climate Variables (ECVs), (particularly the carbon cycle on land and ocean) and other operational applications and services.
• provide a robust SI-traceable anchor to address the continued debate and uncertainty regarding the impact of solar radiation (spectral and total) on the atmosphere and consequently climate, in the near and medium term.
• serve as an enabler for the growth of next-generation ‘micro-satellites’ by providing a reference calibration for sensors too small for robust calibration systems of their own.

Payload/observations
The main instrument of TRUTHS is a hyperspectral imaging spectrometer (HIS), with continuous spectral extent across the UV-visible-short-wave IR (320 nm to 2400 nm) capable of up to 50 m ground instantaneous field of view. The HIS observes climate-relevant processes related to the Earth’s atmosphere, oceans, land and cryosphere and also solar and lunar irradiance. The novel on-board calibration system is seeking to enable all these observations to be made with a target uncertainty of ~0.3% (k=2) across the entire spectrum.

At the heart of this on-board calibration system is the Cryogenic Solar Absolute Radiometer, CSAR, operating at temperatures below -200 °C, this instrument, in common with similar instruments on the ground, provides the direct link to SI. It also provides daily measurements of the total integrated energy reaching the Earth from the Sun with an uncertainty goal of 0.02% (k=2).

The HIS of TRUTHS will primarily observe the full Earth at Nadir (pole to pole), the agile platform pointing to the sun or moon as the platform moves into the Earth’ shadow, to minimise observation gaps. On occasions, the platform will allow off-nadir pointing to match that of another sensor for simultaneous calibration and/or to characterise angular reflectance dependencies of the Earth’s surface.
The orbital track has been selected to be 90 degree precessing, non-sun-synchronous, with a repeat cycle of 61 days. Although adding complexity to the thermal control and power management, this orbit provides many opportunities to cross the paths of other satellites to enable improved cross-calibration due to simultaneity as well as diurnal sampling of the planet.

Status
Following a national competition, the mission was proposed by the UK and adopted into the ESA Earthwatch programme at CMin 19 with the additional partner nations of Greece, Switzerland, Czech Republic and Romania. Following an initial consultation with the prospective user community to prioritise observational requirements together with an intense design phase led by an Airbus Defence and Space consortium, the mission will complete its phase B1 in the summer of 2022. It is then expected to progress towards flight at the end of the decade with the next subscription at CMin 22.

Applications
In addition to short-wave climate radiation benchmark applications, TRUTHS will play a strong role in support of climate action and net zero ambitions. Its data will support the calibration and interoperability of GHG monitoring satellites, land use change classification and natural sinks such as oceans and land vegetation. Although the observation cycle of TRUTHS is not optimised for time critical applications such as for agricultural monitoring. TRUTHS supports these indirect applications by providing a high accuracy reference to assess and improve retrieval algorithms and improve and harmonise the performance of other sensors. Similarly, for the Oceans, TRUTHS will have the capability to make GCOS quality observations, in both type 1 and 2 waters without the need for post-launch system vicarious calibration, although not at the time frequency desired. It will however be able to complement the existing reference buoys, MOBY and Boussole, with calibrations to satellites made over different locations of the world’s oceans. In addition to primarily, Top of the Atmosphere, Level 1 products TRUTHS will also deliver a level 2 global surface reflectance product, with robust SI-traceable uncertainties.

Summary
In summary, this paper will provide an overview of the TRUTHS Mission; starting with the metrological principle and evolution of the concept, through the science and operational drivers, outline of the overall design, route to flight and longer-term vision as a founding element of an integrated international climate observing system. Subsequent papers in the session will provide more specific details of the current design, anticipated performance and operational characteristics.

TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) is an operational climate mission, aiming to enhance by up to one order-of-magnitude our ability to estimate the Earth Radiation Budget through direct measurements of incoming & outgoing energy with the ability to perform SI-traceable measurements of the solar spectrum. Another objective of TRUTHS is to establish a ‘metrology laboratory in space’ to create a fiducial reference data set to cross-calibrate other satellite sensors and improve the quality of their data.

The Definition Phase (A/B1) of the TRUTHS mission was presented as an Earth Watch Element at the time of the 2019 Ministerial Council (Space19+) and has been carried out in the 2020-2022 time frame. The study will culminate in July 2022 with the “Gate Review” involving all Participating States, with verification of technical and scientific maturity and the confirmation of its programmatic feasibility.

In this paper, the definition of Mission and System requirements and the conceptual design of a Satellite and Ground Segment will be detailed, as a result of the efforts at the Industrial consortium, The system study is accompanied by a wide program of technology pre-developments and supported by the establishment of science studies and creation of a Mission Advisory group to help elaborating the mission requirements and performance and eventually to advance in achieving the needed scientific readiness level (SRL-5). To take the advantages provided by a digital modelling environment, TRUTHS is adopting MBSE (Model-Based System Engineering) to support the requirements, design, analysis, verification, and validation activities

Upon a successful Gate Review , a program proposal for the Implementation Phases of TRUTHS mission (B2CDE1)will be prepared and submitted to the ESA Council at Ministerial level for subscription and implementation, with a target Launch date in Q1-2030.

“Traceable Radiometry Underpinning Terrestrial and Helio Studies”, TRUTHS, is an ESA funded Earth Watch Mission backed by the UK, Greece, Switzerland, Czech Republic and Romania. The Satellite and instrument study is focussed on providing a feasible and affordable design which addresses the TRUTHS objectives and delivers the key TRUTHS performance: delivering data acquired from space with state of the art radiometric accuracy of 0.3%.

The TRUTHS satellite is based on a rebuild of the Airbus CRISTAL mission, currently being implemented as part of the High Priority Copernicus Missions, which forms part of the next generation Airbus Platform product line. A cornerstone feature of the mission is the 90° inclined orbit which sets up an orbital plane that precesses with respect to the Sun by 360° per year, exposing the platform and instrument to a variety of Solar Angles. The platform is perfectly suited to such an orbit, being directly designed for the 92° inclined CRISTAL orbit and the similarly non-sun synchronous orbits of the CRISTAL platform predecessors: Cryosat and Jason-CS. The use of the advanced Airbus product line which extends the capabilities of previous missions to allow for high data rate throughput via X-band and controlled re-entry whilst still being underpinned by the strong heritage of the flight-proven Airbus Product Line.

The complexities of the TRUTHS operations and system design include Earth observation throughout the orbital dayside, Solar measurements every orbit, daily Lunar Measurements and biweekly calibration campaigns. Earth measurements are taken regularly with a nadir pointed view for up to 45% of the orbit, with the ability to image off axis to explore a variety of observation angles necessary for characterisation of pseudo-invariant sites (PICS) and other specific calibrated areas (such as RADCALNET), as well as upgrading the accuracy of existing satellites by direct, simultaneous observations of their swaths. These operations must be conducted under strict observational constraints to ensure the TRUTHS platform and instrument remain at performing temperatures and optimum performances.

The TRUTHS instrument is composed of a Hyperspectral Imager (HIS) and a dedicated On-board calibration system (OBCS) which includes the first Space-Based Cryogenic Radiometer (CSAR) that is the foundation of the TRUTHS instrument and provides SI-traceability on orbit. The HIS detector delivers a full 100km swath at 50m spatial resolution and a spectral resolution from 0.5nm to 6nmm across the spectral range of 350 nm to 2400nm, spanning from the UV to the infrared. The OBCS and CSAR allow for the absolute radiometric calibration of the HIS spectrum against an SI standard, a first in remote sensing for Space. The instrument will be constructed in the UK with components from across the key participating states and take advantage of the new UK-based facilities for pre-flight testing and characterisation.

The TRUTHS industrial concept is currently in the midst of the B1 Phase, developing both the design and specifications and developing emerging technologies to TRL 5 from the TRUTHS participating nations.

TRUTHS is at its core a climate mission, and it is currently in its early phase (Phase B1). TRUTHS is being led by the UK space industry, involving the UK Space Agency (UKSA), and delivered by the European Space Agency (ESA) to enable in-flight calibration of Earth Observation (EO) satellites. TRUTHS will help deliver improved confidence in Earth Observation data gathered from space and the forecasts driven by this data, through a novel hyperspectral imager and onboard calibration equipment, providing an SI-traceable reference point for TOA observations. During Phase A/B1, Airbus is leading the preliminary definition of the overall Ground Segment (GS), which has been carried out by CGI and Telespazio into the Payload Data Ground Segment (PDGS) and Flight Operations Segment (FOS) respectively. This definition phase has identified a number of places in which the TRUTHS mission ground segment will need to differ from the ESA/Copernicus norm in order to deliver the required capability in a cost-efficient and effective manner. This presentation will aim to describe some of these changes and elicit feedback for future phases.

On the PDGS side, the TRUTHS mission will require management of a data take of up to 10Tbits per day. The hyperspectral nature of the data means that, as higher level products are generated, the data storage needs will be many times higher. These numbers imply that the standard ESA/Copernicus approach of systematically generating and storing all products at higher levels will be prohibitively expensive. To resolve this, the PDGS definition proposes a more user-centric approach, making use of cloud/hybrid cloud technologies. The concept includes highly scalable on-demand processing and data caching in both highly controlled and experimental environments. The use of the data for long term calibration leads to requirements on traceability and verifiability that will require additions to the security model applied to data management. Such changes have ripple effects through various parts of the architecture (including data circulation, storage, archiving and monitoring), but also significant benefits for the development and integration of new algorithms and processing techniques.

On the FOS side, the TRUTHS mission will have stringent requirements on observation timing and pointing. EO instrument calibration relies on comparing, or extrapolating from, like for like observations. With this in mind, minimising the impact of differences in observation conditions (illumination angles, time of day, ground position etc.) and observation time could be managed at mission planning level. For this reason, the mission-planning element of the ground segment has to be iterated between the FOS and the PDGS in order to manage the mission requests within the mission constraints. The TRUTHS mission is being designed to maximise the calibration opportunities for a number of existing and future EO missions, which requires a powerful and extensive simulation exercise that takes into account predicted satellite positions in a future timeframe. The need for continued simulation runs as an input to mission planning in terms of observation prioritisation will be key. The FOS will be required to dictate the cadence of that planning process within the confines of the operational process. Automation of key processes will be important to minimise the impact on the mission planning lead times.

LEOP and commission of the TRUTHS mission will be conducted by ESA at ESOC before handing over to facilities hosted in the UK. The TRUTHS ground segment design will take advantage of advances in ground segment design applied by ESA. The design and performance of the UK facilities will have to reflect those at ESA to some extent. Overall, there is an opportunity to develop an efficient and innovative Ground Segment, designed to aid end-users effectively, and fulfil the aim of the TRUTHS mission being an operational climate mission.

Significant effort and budget is being expended on on-orbit hardware to monitor environmental and climate change. Sensor series from national and international space agencies are now being joined by a growing population of private sector sensors, providing timely, on-demand or targeted product markets driven by climate change and net-zero regulation. New applications and products are also being derived from existing sensors, addressing new issues albeit with sensors designed for another purpose.

We are entering a golden age, in terms of the volume & reach of EO-derived data, but how best to use and synthesise these data? Where datasets disagree, which should be more trusted, how does a user decide which data product best suits their needs?

The international community believe the answer to the quality assurance of EO data products lies in the rigorous application of metrology, traceable to an internationally agreed and ideally invariant reference. The International System of Units (SI) was developed to address such requirements, providing a reference framework tied to invariant constants of nature. Sensor pre-flight calibration and characterisation can be anchored to the SI through artefacts and traceability derived from National Measurement Institutes (NMIs). However, ensuring and maintaining SI traceability of sufficient accuracy in instruments orbiting the Earth presents a significant new challenge to the Earth Observation and metrology communities.

SITSats (SI-traceable satellites), a concept developed within the CEOS Working Group on Calibration and Validation (WGCV) and Global Space Intercalibration System (GSICS) of the operational agencies address this challenge. Multiple space agencies see the need for reliable reference measurements at all the frequencies that are used by instruments to sense the Earth, including visible/near infrared (Vis/NIR), Infrared, and microwave. The principle application area for these measurements, at the highest accuracy levels, is climate; from both the direct sensor measurements and for the calibration of other space-based instruments. However, any application requiring data interoperability, combination of sensors, and other relative information from single scenes, inherently needs to address and understand differences and changes in radiometric accuracy and biases.

SITSats are characterised by their ability to robustly evidence/verify, in-orbit, their uncertainty back to an SI ‘primary standard’. It of course also implied that the uncertainty level achieved is of sufficient quality that the satellite sensor can be considered a ‘reference’. This means that all factors contributing to the measurement uncertainty have a robust detailed uncertainty budget that can be verified in space. Ideally, the verification is performed by a regular calibration in-flight to a reference standard or instrument that has an uncertainty to SI significantly better than required by the source of error in the satellite sensors measurement budget. However, for some sources of error, this may only readily be achieved by pre-flight calibration/characterisation. This is acceptable providing there is clear documented evidence and knowledge on how this will evolve when operating in space. Any anticipated change will need to be well-understood and the resultant contributing uncertainty contained within the overall uncertainty budget.

In the context of the majority of sensors observing the Earth, the most critical source of uncertainty is the overall radiometric gain of the sensor i.e. the conversion from observed incident photon to geophysical units accounting for all the conversion losses in the instrument. Thus, most effort is focussed on determining this gain factor. The optimum way to achieve this is to replicate the calibration methods used pre-flight, in space, on-board the spacecraft, including the direct link to an SI primary standard. In effect creating a ‘metrology laboratory in space’ to provide regular verification and update of the calibration of the satellite sensor direct to SI.

In-orbit SI traceability is just one aspect, the other is the robust application of metrological principles to provide an evidenced, transparent and validated assessment of the final delivered product quality (manifest through its uncertainty). It is only through this comprehensive approach that data interoperability and the combination of products from different sensors can be achieved enabling a robust global climate observatory, that can be used by scientists and policymakers to understand our impact on the environment and the efficacy of enacted policies to limit its extent.

The ESA EarthWatch TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) mission is a solar-reflective band hyperspectral imager SITSat mission1 incorporating, for the first time, a primary SI standard. The TRUTHS mission development is in phase A/B1, with this presentation outlining the study’s efforts on the application of metrological principles to the sensor design, implementation of the route to SI traceability in the on-board systems design, pre-flight comparison activities and ultimately to the delivered data products together with how they will be evidenced and documented for users.

References

1. Fox, N.; Green, P. Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS): An Element of a Space-Based Climate and Calibration Observatory. Remote Sens. 2020, 12, 2400. https://doi.org/10.3390/rs12152400

The Earth Observing system of remote sensing satellites is now vital global infrastructure, gathering the underpinning environmental and climate information used to inform decision making across society. Data from the many satellites, both public and private, that together form the Earth Observing system must be combined to provide the comprehensive global monitoring required at all temporal and spatial scales. Within such a combined data record, however, observations from different satellites vary in quality and include measurement biases to an extent that limits their interoperability. To enable maximal, trustable use of EO datasets such measurement biases between sensors should be reconciled, with remaining measurement uncertainty determined and reported.

To attempt to achieve this, satellite sensors are routinely recalibrated against well-characterised references, such as in-situ measurements or other higher-quality satellites. The effectiveness of the current state-of-the-art on-orbit calibration methodologies is limited in several important ways, including the achievable uncertainty and degree of traceability to a common, internationally-consistent reference baseline, which would ideally be SI. This picture, however, will significantly change after the planned launch of TRUTHS, and similar SI-traceable satellite (SITSat) missions, which represent the next generation in terms of achievable measurement uncertainty on-orbit.

TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) is a planned hyperspectral climate mission, sensing in the solar-reflective spectral domain (UV-SWIR), which achieves high-accuracy SI-traceability on-orbit from a novel calibration system. The UK-led mission is currently under development by ESA as part of its Earth Watch program. It’s target measurement uncertainty of 0.3% (k = 2) will provide a step change in the quality of on-orbit observations of the Earth, Moon and Sun, which will be SI-traceable for the first time. TRUTHS also has the objective to act as a ‘gold standard’ reference against which to calibrate other satellite sensors.

In this presentation, the intercalibration objective of the TRUTHS mission will be introduced. This will be explored in terms mission requirements, including of intercalibration opportunities and orbit selection, as well as the operational processing planned to facilitate intercalibration for key space agency missions, such as Sentinel-2 and Sentinel-3.

The presentation will discuss the use, merit and limitations of different methods which will be enhanced by TRUTHS – for example, Simultaneous Nadir Observations (SNO), characterised Pseudo Invariant Calibration Sites (PICS), RadCalNet and oceans. For different methods and sensors, results of simulations using realistic data will illustrate the potential performance improvements achievable, accounting for effects such as differences in viewing and illumination geometry.