The Earth’s climate is changing. We face grave risks and there are already signs of change in the increased frequency of extreme weather conditions globally.

Remote sensing of the Earth from space is the main way we have to obtain the trustable global data needed to understand climate change and to provide information that enables policy makers to adopt appropriate mitigation and adaptation strategies.

Detecting a climate trend for key observable parameters of the Earth system requires observations over decades. The natural variability of the Earth’s climate system means that, even with a perfect instrument (no uncertainty), measurements must be taken over multiple decades to ensure that any detected change is large enough to be separated from short-term climate anomalies, long-term cycles, or volcanos etc. To obtain multi-decadal records, observations need to be stable and accurate, often within a few tenths of a percent per decade. In many cases, this requires measurement uncertainties that are normally only realisable in the laboratories of National Metrology Institutes (NMIs), such as the UK National Physical Laboratory (NPL).

Figure 1: A conceptual value chain for impacts of climate change and mitigation: emissions and sinks of greenhouse gases

Domestic policy has committed the UK to achieving net-zero emissions by 2050, and internationally, the Paris Agreement requires significant decarbonisation to keep average global temperature increases well below 2°C (ideally below 1.5°C). Meeting these extremely challenging goals requires immediate, sustained and significant annual drops in greenhouse gas emissions and a simultaneous increase in carbon sinks (in soils, forests and ocean phytoplankton or through artificial sinks/extraction). The UN Sustainable Development Goal framework balances climate needs – and related urgent environmental challenges caused by, for example, pollution, habitat and biodiversity loss and freshwater reduction – against the human need to “leave no one behind” and to allow fair, and sustainable, economic and social development across the whole world.

To mitigate and adapt to climate change and to provide a timely “feedback loop” on the climate response to human actions, decision makers in governments, industry and non-governmental organisations need access to trustworthy differentiating information about the historical, current and future state of the climate and status of anthropogenic greenhouse gas emissions, water and land use changes. Such information is provided as a result of direct observations and collated in ‘inventories’, near real-time analyses and climate services, in longer duration climate data records of essential climate variables (ECVs) and in the outputs of climate models (as predictions or reanalyses of historical climates).

Figure 1 shows a conceptual value chain for how environmental information is used in decision making. Climate policy in governments and commerce leads to changes in greenhouse gas sources (emissions) and sinks (land use). Direct observations of sources and sinks provide a short feedback loop (golden arrows in Figure 1) for policy decisions.

Those changes influence the climate of the Earth and, over longer timescales, observations of ECVs are used in climate models to give information on the past, current and future climate state. Integrated assessment models combine the physical/biochemical climate models with economic and societal behaviour models to inform both climate and economic policy on longer timescales.

“the value chain is the basis of the growing market of commercially delivered ‘climate services’, which was estimated in 2016 to be 24B€”

The shift in policy focus to net zero and the urgent timescales, means that now, more than ever, the two feedback loops need to be robust, internationally consistent and linked to references that are stable over decades. Furthermore, the value chain shown in Figure 1 is the basis of the growing market of commercially delivered ‘climate services’, which was estimated in 2016 to be 24B€.

The 50+ ECVs defined by the Global Climate Observing System (GCOS) are quantities that must be monitored as part of an integrated observing system. These quantities include the measurement of the specific anthropogenic forcing agents (e.g., greenhouse gases), the quantities that define the Earth cycles (carbon, water, energy balance) and quantities that define the state of the atmosphere, land and oceans. More than half of these ECVs require remote observation from satellites, a further quarter need some satellite measurements.

Monitoring the ECVs, with sufficient accuracy to detect a climate trend
in the shortest possible time, is the primary focus of the World’s climate observing community and places a major challenge to the world’s space agencies.

Metrology, the science of measurement, must be employed to establish a robust, global and stable climate observing system. Metrology, through the global NMIs, has ensured that the SI units have been stable for nearly 150 years and derived measurements are consistent and coherent worldwide. This has been accomplished through application of the key principles of metrological traceability. These principles rely on comparisons between independent measurements, clear traceability pathways that are well-documented and peer reviewed, and robust uncertainty analysis following the principles of the Guide to the Expression of Uncertainty in Measurement (the GUM). These metrological principles can, should and are being applied to the observations of our environment and our climate, albeit with necessary interpretation and adjustment, to provide quality assurance to the observations that society relies on.

NPL has collaborated with the climate observing community to introduce these principles not only to the calibration of instruments that make climate observations, but also to establish metrological traceability to ECVs derived from a combination of such observations through complex processing algorithms, often involving Earth system models. NPL helped lead the establishment of the Quality Assurance Framework for Earth Observation (QA4EO), which was endorsed by the Committee on Earth Observation Satellites (CEOS) in 2010 and its core principles have been widely adopted by the broader observation communities. NPL chairs a sub-group of CEOS and the European Metrology Network for Climate and Ocean Observation, which coordinates the European NMI community’s efforts in responding to climate observation.

Radiometry for calibration

Figure 2: NPL primary standard cryogenic radiometer

One of the core functions of NMIs, is to establish, maintain and disseminate SI base units and derived quantities. For optical satellite sensors, NPL provides calibration and characterisation of instruments and artefacts, therefore ensuring confidence in the accuracy and reliability of satellite sensors in orbit.

At the heart of NPL’s facilities is a cryogenic radiometer, the base reference for most radiometric scales in the World’s NMIs. The cryogenic radiometer, see figure 2, is a 40-year-old NPL invention based on the 100-year-old concept of Electrical Substitution Radiometry (ESR), which compares the heating effect of light to that of an electrical heater. A well-designed ESR operating at ambient temperatures will typically have uncertainties around 0.1%. This technology is widely used, including in many space-borne instruments measuring the amount of incoming radiation from the Sun (solar irradiance).

A cryogenic radiometer is an ESR cooled to temperatures generally <20 K (–250 °C). At these temperatures, many residual sources of uncertainty are reduced, allowing uncertainties of better than 0.01% (10× improvement). The concept of flying a cryogenic radiometer in space and replicating the terrestrial calibration chain on board a satellite lies at the heart of the TRUTHS satellite mission described below, where it both serves as a primary reference standard and also provides a direct measure of solar irradiance. A prototype of this Cryogenic Solar Absolute Radiometer (CSAR) is on a platform in Davos, Switzerland at the World Radiation Centre of the WMO as a reference for terrestrial solar irradiance measurements.

“NPL has developed several specialist capabilities and facilities tailored to the pre-flight calibration and characterisation of satellite instruments”

Figure 3: schematic representation of STAR-cc-OGSE

NPL has developed several specialist capabilities and facilities tailored to the pre-flight calibration and characterisation of satellite instruments. The most recent of these is the Spectroscopically Tuneable Absolute Radiometric (STAR) calibration and characterisation (cc) Optical Ground Support Equipment (OGSE) designed and built-in collaboration with the company M-Squared Lasers. The STAR-cc-OGSE, figure 3, is designed as a transportable facility that can calibrate and characterise satellite imagers in vacuum facilities with both white light and a monochromatic source built from a unique laser which can dial up any wavelength from the ultraviolet to the shortwave infrared.

The radiometric output of the STAR-cc-OGSE is fully traceable to the NPL cryogenic radiometer and can provide unprecedented uncertainties well below 0.5 %. Through the low uncertainty and spectral flexibility, the system can characterise many aspects of satellite sensors simultaneously, for example, radiometric gain, spectral bandwidth (including for spectroscopy) and straylight. The first customer for the facility is Airbus France who will shortly be using it to characterise and calibrate the French led (UK partner) MicroCARB mission designed to measure greenhouse gases.

Fiducial Reference Measurements

However well satellite sensors are calibrated in laboratories prior to launch, it is important to monitor their performance once in orbit, as the stress of launch, and the harsh environment of space, cause most sensors to degrade. Such post-launch calibration and validation (CalVal) has been carried out for 40 years using vicarious calibration sites – sites that are either extremely stable (the Saharan desert, Antarctica) or that are monitored with instruments at the time of satellite overpasses. More recently, the term “Fiducial Reference Measurement” (FRM) has been coined to describe the special case of instrumented in-situ sites that are explicitly focussed on providing SI-traceable CalVal of satellite sensors. Such sites are characterised using instrumentation that have taken part in comparisons to evidence their uncertainty and traceability and FRMs are particularly important to underpin the more demanding climate related observations.

RadCalNet, the radiometric calibration network of CEOS, is one such FRM. RadCalNet currently comprises five sites (in the USA, France, Namibia and two in China), with permanent instrumentation measuring both the ground reflectance and the atmospheric conditions to provide reference values of top-of-atmosphere reflectance against which satellites can be compared. Site operators use their own data for satellite CalVal, but additionally provide a subset of that data to the RadCalNet processing facility for processing in a common way. RadCalNet makes both the raw ground reflectance and the processed top-of-atmosphere reflectance data available publicly and at no cost on the RadCalNet portal. NPL established and is operating the Namibian site on behalf of ESA, and, as a metrology institute, has led the process of peer review and comparison of the different RadCalNet sites. NPL worked with several of the sites to develop metrologicallyrobust uncertainty assessments.

NPL has also supported other FRM initiatives. The FRM4STS project established traceability and good practices for surface (land, ocean and ice) temperature measurements from satellites, including a comparison of the world’s best field instruments in NPL’s laboratories in Teddington and also at field sites in Africa and the Arctic. Similarly, NPL supported traceability of ocean reflectance, so called “ocean colour”, through the FRM4SOC project and through supporting the uncertainty evaluation of the European satellite calibration reference, the Boussole ocean buoy. Ocean colour gives information about the health of the ocean and the amount of phytoplankton, which absorb a significant fraction of atmospheric carbon dioxide.

NPL also leads an ESA project called FRM4VEG, which considers the other major carbon dioxide sink – vegetation. FRM4VEG is establishing international harmonisation of observations of some key land ECVs. NPL, along with the University of Oxford and others, has established a UK test site at Wytham Woods in Oxfordshire. The site has long been established as a research site for many ecological and biodiversity applications and has one of the best-catalogued vegetation inventories in the world for a natural woodland site of this size. NPL has installed an automated network of sensors to monitor solar radiation interactions with the vegetation and also undertakes regular campaigns to characterise the site above the canopy via a tree-top platform and use of drones. NPL worked with UCL to use a lidar to digitally scan the woodland so that a virtual representation could be created to more closely model the interaction of light and provide a reference for satellites.

TRUTHS – metrology in space

The ideal scenario for a climate observing system is to establish SI-traceability on-board the spacecraft so that satellite sensors can be regularly calibrated to a trustworthy reference standard. While it would be too costly to introduce such capability to each individual satellite, it is possible to launch a small number of very well-calibrated SI-traceable satellites, which act as “NMIs in orbit” and then use those to calibrate other satellites from orbit.

The concept of an NMI-in-orbit satellite, originally proposed by NPL nearly 20 years ago, is now starting to become a reality. In the USA,
a demonstrator mission called CLARREO-Pathfinder is being built by NASA for flight on the International Space Station. China is also now developing its version of an SI-traceable sensor and Europe has recently started the first phase of the TRUTHS mission, implemented as part of the ESA Earth Watch programme. TRUTHS, conceived and scientifically led by NPL, is predominantly funded by the UK in partnership with Switzerland, Czech Republic, Greece and Romania.

The main observational instrument in TRUTHS is a hyperspectral imaging spectrometer (HIS), The HIS observes climate-relevant processes related to the atmosphere, oceans, land and cryosphere and provides new insight for societal understanding of the grand challenges: food, water, natural resources, health and disaster management. TRUTHS provides the data to test and constrain climate models and retrieval algorithms upgrading the capability of both satellite and surface-based observing systems.

What differentiates TRUTHS from other complementary missions is that it achieves, in orbit, the uncertainties needed for climate-quality data. The TRUTHS payload includes an on-board calibration system, which mimics the Earth-based SI-traceability provided at NMIs, including a primary standard cryogenic radiometer and the STAR system. TRUTHS provides on-board radiometric accuracies at 10× better than those available in current imager sensors which will, in turn, halve the time required for climate scientists to make unequivocal determinations of changes to the Earth’s temperature. Most importantly, TRUTHS can continuously recalibrate the HIS in orbit, maintaining SI-traceability for the entire duration of the mission

The TRUTHS mission plans to establish a set of internationally accepted SI-traceable reference targets, including the Sun, Moon and the Earth’s deserts. These, and the matchups that occur when the TRUTHS orbit overlaps with another mission, will be used to calibrate other satellites, both those from the space agencies and the microsatellites of commercial operators.

This metrology-focussed mission demonstrates the increasing recognition of how important metrology is to EO and climate and how meeting the goals needed for climate requires ambitious, innovative solutions and a strong partnership between all expertise of the community: academia, engineering and metrology.

The Earth is warming, fuelled by anthropogenic actions; however, there remain questions about the scale and timing of impending temperature and its impacts. We do not know if there will be critical thresholds or tipping points with non-linear repercussions; for example, will melting of the permafrost in Siberia release large volumes of methane? We do not know if the actions we are currently taking will be adequate to even meet the 2°C minimal targets of Paris 2015 or if we can verify and/or observe the results of these actions.

Much of the uncertainty is a direct result of the complexity of the climate models and the inadequacy of the data that feeds and constrains them. This is a metrology challenge that needs urgent action but is not quite that traditionally practised by NMIs. NPL has, over the last few decades, sought to be more responsive to the needs of society and recognises that much of the metrology of the future concerns not only the direct measurements but also the information derived from them, along with the integrity and provenance of all the intermediary transformational steps.

Data science is an area of strength for the UK attracting significant investment, and NPL is exploiting its use in metrology and developing methods to bring metrological rigour in its use for societal applications. The huge volumes of EO and climate data and their critical impact for society make this a fruitful testbed for a new metrological data framework.

Within the UK, and internationally, NPL is engaged directly with academics, industry and public sector scientists/engineers, and in some of the key coordination committees of both metrology and satellite Earth observation communities, to help provide the necessary metrological input to support decision making. NPL is continuing more than two decades of effort to bring together experts from classical metrology domains centred around the SI units, with data scientists, and environmental domain experts to create a multi-disciplinary focus on metrology needs for climate and Earth Observation.