A positive Earth Energy Imbalance (EEI) is the energy, which is continuously stored by the Earth and will ultimately released to the atmosphere, causing global warming. The "imperative to monitor Earth’s energy imbalance” (von Schuckmann et al., 2016) has been continuously reported by the Earth’s climate community. The EEI has been identified to be around 0.5 to 1.0 Wm−2. To determine its exact value both the Total Solar Irradiance (TSI) and the Top of the Atmosphere (ToA) Outgoing Radiation (TOR) need to be measured with unprecedented accuracy and precision.However, so far, the EEI could not be determined as the measurements were not sufficiently accurate. This calls for improved instrument technologies as well as a traceable calibration chain of the space instrumentation. To pave the way in that direction, the ISSI International Team "Towards Determining the Earth Energy Imbalance from Space" has been established. We collect the current knowledge of ERB measurements and identify missing elements for measuring EEI from space. Specifically, we collect past and ongoing measurements of the ERB components obtained with instruments such as CLARA, RAVAN, SIMBA, GERB, and CERES. The goal is to evaluate the performance and uncertainty of each of the instruments to identify observational challenges that need to be overcome to be able to measure both TSI and the Earth’s outgoing radiation with the required accuracy to ultimately be able to determine the absolute level of EEI from space.

Across the broad potential user base for Earth Observation (EO) data, confidence in the quality of the available products is vital, particularly for users requiring quantitative measured outputs they can rely on. Particularly as the commercial EO sector rapidly expands, however, it is an increasing challenge for the user community to discern between the wide variety of product offerings in a reliable manner, especially in terms of product quality. In response to this ESA and NASA, through their Joint Program Planning Group (JPPG) Subgroup, have developed a common EO product Quality Assurance (QA) Framework to provide comprehensive assessments of product quality. The evaluation is primarily aimed at verifying that the data has achieved its claimed performance levels, and, reviews the extent to which the products have been prepared following community best practice in a manner that is “fit for purpose”. A Cal/Val maturity matrix provides a high-level colour-coded a simple summary of the quality assessment results for users. The matrix contains a column for each section of analysis (e.g., metrology), and cells for each subsection of analysis (e.g., sensor calibration). Subsection grades are indicated by the colour of the respective grid cell, which are defined in the key. Both ESA and NASA have on-going activities supporting the procurement of commercial EO data that make use of the joint QA Framework – to ensure decisions on data acquisition are made with confidence. On the ESA side, the Earthnet Data Assessment Project (EDAP) project performs data assessments on EO missions in optical, atmospheric and SAR domains. Similarly, the NASA Earth Science Division (ESD) Commercial Smallsat Data Acquisition (CSDA) Program, completed a pilot study in 2020, and has since entered sustainment use phase for some of the commercial data sets. In this presentation the joint ESA/NASA QA Framework is described, with some examples of its application to commercial EO products.

A new generation of satellites designed for low-uncertainty, SI-traceable measurements-termed "SITSats"-marks a major advancement in Earth observation (EO) capability. These missions aim to enhance the performance and interoperability of the EO "system of systems." Among them, the ESA Earth Watch Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS) mission is designed to serve as a "gold-standard" radiometric reference for cross-calibrating EO sensors in the solar reflective domain. In this work, uncertainties in cross-calibration comparisons arising from sensor characterization and design are investigated. A processing chain to prepare collocated data for uncertainty-quantified comparison is presented. This includes steps to perform spectral band adjustment and spatial resampling. Using the TRUTHS hyperspectral imaging spectrometer (HIS) as the reference and Sentinel-2 multispectral imager (MSI) as the target, a simulation study based on high-resolution imagery assesses achievable comparison performance. A subset of uncertainty effects driven by sensor characterization is propagated through the spectral and spatial processing using a Monte Carlo approach. Sentinel-2 data are assumed at 10-m resolution, which is most sensitive to the errors considered. The results highlight the importance of sensor characterization, particularly inherent in-flight wavelength knowledge for target sensors, in such comparisons. Results from the simulation analysis give uncertainty estimates (k =1) of 0.31% (blue), 0.50% (green), and 0.23% (red) for the combined error effects arising from sensor characterization and geolocation uncertainty for comparisons over the Libya-4 desert pseudo-invariant calibration sites (PICS) using an instantaneous 205-m square comparison region. Results for more heterogeneous scenes, such as rainforest, still achieve uncertainties of 0.6%-1.2% for the red-green-blue (RGB) bands over a 20x 200 m area. The uncertainty is driven largely by the spectral component-up to 1% due to the inherent Sentinel-2 wavelength knowledge of 1 nm across various representative scenes outside of the atmospheric absorption bands. While the impact of these uncertainties may decrease when considering a diverse range of scene types, they introduce systematic errors when scenes share similar spectral characteristics. The impact of some uncertainty contributions, for example, geolocation uncertainty, is shown to be substantially reduced by aggregating samples over larger regions or over longer time periods. This analysis supports the development of low-uncertainty, ideally SITSat-enabled intercalibration approaches needed to ensure radiometric consistency across missions for generating long-term climate data records.

This study aims to explore the forest aboveground biomass relationship to C-band backscatter. A one-hectare area in Wytham Woods, located west of Oxford, was selected for this research. The area has a total of 525 trees of seven different tree species. The Michigan Microwave Canopy Scattering Model (MIMICS), a two-layer radiative transfer model, was applied to simulate canopy backscatter responses in this deciduous UK forest. Model parameters related to forest structure were derived from previously published terrestrial laser (LiDAR) data. Simulated backscatter was performed for co-polarized and cross-polarized modes at a C-band frequency range and incidence angles (20 degrees to 45 degrees at 5 degrees increments). The research includes five objectives: i) backscatter sensitivity analysis from the variation of different model parameters; ii) backscatter seasonal effect under spring-summer (leaf-on) and autumn-winter (leaf-off) periods; iii) backscatter comparison between species as well as between simulated and satellite observations in a spatial pattern; iv) relationship between simulated backscatter and aboveground biomass; and v) relationship between MIMICS forest structure parameters and simulated backscatter. Sensitivity analysis results showed significant differences in backscatter from changes in leaf distribution, leaf thickness and water content (leaf, trunk and soil) for both polarization modes in leaf-on scenarios. Leaf-off scenarios presented significant differences from changes in branch distribution but only for the co-polarized mode. Seasonal variations presented significant backscatter differences between spring-summer and autumn-winter scenarios; additionally, backscatter differences among species for each seasonality in the co-polarized mode were observed. The computation of the grid resolution (20 m x 20 m) showed a range of backscatter values depending on the grid and incidence angle. Higher backscatter values were observed for the satellite data than for the simulated data. Finally, the aboveground biomass and the MIMICS input parameters presented high random variability and little systematic co-variation with the total backscatter on both simulated seasons, suggesting that more robust allometric equations for biomass estimation are required.

TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio-Studies) is an operational climate mission, aiming to enhance, up to an order-of-magnitude, our ability to estimate the Earth radiation budget, spectrally resolved to support attribution. Through direct measurements of incoming total and spectrally resolved solar irradiances and Earth reflected radiances, spatially resolved, it establishes ‘benchmarks’ against which change/trends can be detected in as short a time as possible. These fiducial reference data sets can be combined with data from other sensors and also serve as ‘gold standard’ references to anchor and upgrade the performance of other space sensors through in-orbit calibration. TRUTHS will become a founding member of a new class of satellites called SITSats, SI-Traceable Satellites, with payloads explicitly designed to achieve and evidence an uncertainty, in-orbit, at a level commensurate with the exacting goals of long-time-base climate studies. SITSats also facilitate interoperability and enhanced trust in the data from the Earth observation system as a whole, helping to provide observational evidence-based confidence in actions addressing the climate emergency.  The unprecedented uncertainty of TRUTHS’ globally sampled hyperspectral data underpins many additional applications: Establishing an interoperable, harmonised Earth Observing system incorporating agency and commercial satellites: large and small Top and Bottom of atmosphere reflectances impacting carbon cycle (e.g. land cover, ocean colour, vegetation, methane etc together with similar applications of other hyper/multi-spectral missions). Low uncertainty also facilitates improvements in retrieval algorithms. Transferring radiometric reference values to existing Cal/Val infrastructure (e.g. RadCalNet, Pseudo-Invariant Calibration sites, In-situ ocean colour reference observations; selected surface reflectance test-sites (fluxnet, …), both nadir and multi-angular) and Moon observations. The mission comprises an “agile” satellite capable to point and image the Earth, Moon and Sun from a 90°polar orbit by the Hyperspectral Imaging Spectrometer (HIS). The HIS provides spectrally continuous observations from 320 to 2400 nm, with a spectral sampling between 2 and 6 nm and a spatial sampling of 50 m. The payload utilises a novel SI-traceable on-board calibration system (OBCS), comprising of the Cryogenic Solar Absolute Radiometer (CSAR), able to realise SI-traceability in space and also measure incoming solar radiation. Together with other optical elements the OBCS links the HIS observations to the CSAR with a target expanded uncertainty 0.3% (k=2). TRUTHS is implemented by the European Space Agency (ESA) as a UK-led Earth Watch mission in collaboration with Switzerland, Czech Republic, Greece, Romania and Spain. The mission was conceived by the UK national metrology institute, NPL, in response to challenges highlighted by the worlds space agencies, through bodies such as CEOS addressing observational needs of GCOS. The mission is under development by an industrial consortium led by Airbus Defence and Space UK, with a target launch date of 2030 and minimal operations life-time of 5 years with a goal of 8 yrs. Together with FORUM (ESA) and IASI-NG (CNES/EUMETSAT) it will provide spectrally resolved Earth radiance information from the UV to the Far-Infrared in the coming decade, and in partnership with CLARREO-Pathfinder (NASA) and CSRB (CMA) inaugurate a future constellation of SITSats.

An international comparison of field deployed radiometers for sea surface skin temperature (SSTskin) retrieval was conducted in June 2022. The campaign comprised a laboratory comparison and a field comparison. In the laboratory part, the radiometers were compared with reference standard blackbodies, while the same was done with the blackbodies used for the calibration of the radiometers against a transfer standard radiometer. Reference values were provided by the National Physical Laboratory (NPL), traceable to the primary standard on the International Temperature Scale of 1990. This was followed by the field comparison at a seaside pier on the south coast of England, where the radiometers were compared against each other while viewing the closely adjacent surface of the sea. This paper reports the results of the laboratory comparison of radiometers and blackbodies. For the blackbody comparison, the brightness temperature of the blackbody reported by the participants agreed with the reference value measured by the NPL transfer standard radiometer within the uncertainties for all temperatures and for all blackbodies. For the radiometer comparison, the temperature range of most interest from the SSTskin retrieval point of view is 10 degrees-30 degrees C, and in this temperature range, and up to the maximum comparison temperature of 50 degrees C, all participants' reported results were in agreement with the reference. On the other hand, below 0 degrees C the reported values showed divergence from the reference and the differences exceeded the uncertainties. The divergence shows there is room for improvement in uncertainty estimation at lower temperatures, although it will have limited implication in the SSTskin retrieval.

An international comparison of field-deployed radiometers for sea surface skin temperature (SSTskin) retrieval was conducted during two weeks in June 2022. The comparison comprised a laboratory comparison and a field comparison. The field comparison of the radiometers took place on the second week at a seaside pier on the south coast of England. Six thermal infrared radiometers were compared with each other while continuously viewing the closely adjacent surface of the sea from the end of the pier. This paper reports the results of this field comparison. All participants' radiometers agreed with the reference value, evaluated as the simple mean of the participant-reported values, within the claimed uncertainties. The SSTskin variation during the 5-day period was within 3 degrees C around 18.3 degrees C, which is 2 times as large in range as in the previous comparison in 2016, while the mean of the difference from the reference value over the period evaluated for each participant was found to be within 0.07 degrees C, which is a 2-times improvement on the previous results. During the comparison an insignificant but noticeable abrupt shift in measured value occurred in one of the radiometers, which could not have been detected without comparison with other instruments. This demonstrated the effectiveness of having long-term stable internal reference sources in the instrument, a feature this particular radiometer did not have. The combined results from the laboratory comparison and the field comparison contribute to improve confidence in the retrieved SSTskin.

Upcoming SI-traceable satellite (SITSat) missions such as traceable radiometry underpinning terrestrial and helio studies (TRUTHS) aim to achieve unprecedented accuracy for SI-traceable measurements of the Earth-reflected radiation. These measurements will support the generation of low-uncertainty climate records and significantly improve the calibration of other sensors. In such a context, the calibration transfer rather than the reference sensor dominates the uncertainty budget. This study presents an end-to-end global intercalibration simulator capable of assessing the potential uncertainty for multiple scenarios that consider the interrelation of different error sources and match-ups. We first define the sensor-to-sensor match-ups through an orbital analysis that is followed by a top-of-atmosphere (TOA) radiance modeling of each match-up. Finally, we calculate the radiometric uncertainty based on different error sources combined globally. In this first implementation, we have calculated the match-ups of TRUTHS against observations by the Copernicus Sentinel-2A satellite over land areas throughout the year. We calculate the angular mismatch for both viewing differences and solar changes from different overpass times. We define multiple intercalibration scenarios based on temporal, angular, or cloud constraints. These first results show that considering overpasses up to 15-min difference, low cloud probability, and matching field-of-view (FoV), within 5 degrees, we sample most land areas with a mean error <0.1% and bias regression <0.5%. We have also restricted the sun zenith angle (SZA) to 60 degrees to minimize solar angle and view azimuthal dispersion over the poles. This also results in data gaps of several months that might be complemented with dedicated maneuvers or dedicated processing of these polar-region match-ups.

TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio-Studies) is an operational climate mission, aiming to enhance up to an order-of-magnitude our ability to estimate the Earth radiation budget through direct measurements of spectrally resolved solar reflected Earth radiances and Sun irradiances becoming a ‘gold standard’ reference in support of climate emergency research and operational applications. It aims to establish a “metrology laboratory in space” by creating a fiducial, SI-traceable reference data set to cross-calibrate other sensors and improve the quality of their data.TRUTHS main objective is to establish a reference baseline measurement (benchmark) of the state of the planet, against which past and future observations can be compared, in order to:allow climate model improvements and forecast testing, and provide observational evidence of climate change, including mitigation strategies in the shortest time possible. TRUTHS will primarily measure the incoming and outgoing energy from the climate system with an accuracy needed to detect climate trends in the shortest possible time.The datasets needed to meet this objective have many additional applications, such as:SI traceable measurements of the incoming and reflected solar spectrum, to address direct science questions. Operational products for removing radiometric biases in other satellite instruments by cross-calibration with TRUTHS data, improving accuracy and enabling inter-operability including improvement of retrieval algorithms. Transferring radiometric reference values to existing Cal/Val infrastructure, e.g. RadCalNet, Pseudo-Invariant Calibration sites, In-situ ocean colour reference observations; selected surface reflectance test-sites (fluxnet ..), both nadir and multi-angular; to the Moon. The mission comprises an “agile” satellite capable to point and image the Earth, the Moon and the Sun in a polar orbit hosting the Hyperspectral Imaging Spectrometer (HIS) capable to provide an accurate, continuously calibrated, dataset of spectrally resolved solar and lunar irradiance and Top of Atmosphere (ToA) Earth-reflected radiance in the near-UV/Visible/NIR/SWIR (320 nm to 2400 nm) waveband with a spectral sampling between 2 and 6 nm and a spatial sampling of 50 m. The payload utilises a novel SI traceable on-board calibration system, the Cryogenic Solar Absolute Radiometer (CSAR), as part of an innovative On-Board Calibration System (OBCS), allowing the HIS observations to achieve its unprecedented in-space accuracy, targeting an expanded radiometric uncertainty tied to international SI standards of 0.3% (k=2).TRUTHS is implemented by the European Space Agency (ESA) as a UK led Earth Watch mission in collaboration with Switzerland, Czech Republic, Greece, Romania and Spain. The mission was conceived by the UK national metrology institute, NPL, in response to challenges highlighted by the worlds space agencies, through bodies such as CEOS, in relation to interoperability and accuracy. The mission is being developed by an industrial consortium led by Airbus Defence and Space UK.The TRUTHS mission targets a launch in 2030 with a minimal life-time of 5 years, and design goal to reach 8 years, of in-orbit operations. It will become part of a future fleet of SI-Traceable Satellites (SITSATs) currently being developed by different space agencies, including CLARREO-Pathfinder (NASA) and CSRB (CMA), and together with FORUM (ESA) and IASI-NG (EUMETSAT) will provide spectrally resolved Earth radiance information from the UV to the Far-Infrared in the coming decade.

In recent years, the concept of a Fiducial Reference Measurement (FRM) has been developed to highlight the need for precise and well-characterised measurements tailored explicitly to the post-launch calibration and validation (Cal/Val) of Earth observation satellite missions. The confidence that stems from robust, unambiguous uncertainty assessment of space observations is fundamental to assessing the changes in the Earth system and climate model prediction and delivering the essential evidence-based input for policy makers and society striving to mitigate and adapt to climate change. The underlying concept of an FRM has long been a core element of a Cal/Val program, providing a ‘trustable’ reference against which performance can be anchored or assessed. The ‘FRM’ label was created to embody into such a reference a set of key criteria. These criteria included the establishment of documented evidence of uncertainty with respect to a community-agreed reference (ideally SI-traceable) and specific tailoring to the needs of a satellite mission. It therefore facilitates comparison and interoperability between products and missions in a cost-efficient manner. Committee on Earth Observation Satellites (CEOS) Working Group Cal/Val (WGCV) is now putting in place a framework to assess the maturity and compliance of a ‘Cal/Val reference measurement’ in terms of a set of community-agreed criteria which define it to be of CEOS-FRM quality. The assessment process is based on a maturity matrix that provides a visual assessment of the state of any FRM against each of a set of given criteria, making visible where it is mature and where evolution and effort are still needed. This paper provides the overarching definition of what constitutes an FRM and introduces the new CEOS-FRM assessment framework.

Laboratory in London, UK, 9-11 September 2019 and sponsored by the UK Space Agency.The workshop was organized under the auspices of the Global Space-based Inter-Calibration System (GSICS) and the Committee on Earth Observation Satellites -Working Group on Calibration and Validation (CEOS-WGCV).The goal of the workshop was to quantify the benefits and resulting requirements of a space-based climate observing system and produce this Workshop Report summarizing current measurement capabilities, climate-based needs, and future plans for implementation.The international workshop included about 100 attendees and spanned users, satellite instrument designer/builders, metrologists, and space agencies with expertise across a wide range of applications and technologies.

Presented is a new quality assurance (QA) framework for Earth Observation missions that has been developed as a joint initiative between ESA and NASA. It aims ensure the rigorous assessment of all aspects of relevant aspects of mission quality, verifying claimed mission performance and, where applicable, reviewing the extent to which the mission follows community best practice in a manner that is “fit for purpose”. The QA framework has potential for more general use in both institutional and commercial Earth Observation – helping mission providers to understand the information their users' needs and empowering users to make informed decisions about which data is fit for their purpose.

CubeSats are currently gaining significant traction in Earth Observation, with increasingly advanced instrumentation such as hyperspectral imaging. However, the challenges of bringing such instrumentation to CubeSats are great; the platform suffers from severe physical, operational and budgetary constraints. Adopting a holistic design methodology may hold the key to allowing science-grade Earth Observation to be achieved from a CubeSat. Presented here is CHAFF (CubeSat Hyperspectral Application For Farming), a low-cost hyperspectral imager prototype, capable of taking 1024 spectral bands between 460 nm - 820 nm. CHAFF has been constructed using commercial off-the-shelf optics, in order to produce a design commensurate with the typical resources of a university CubeSat mission. CHAFF has been calibrated at the National Physical Laboratory, in order to assess the performance of the COTS optics. An impressive spectral resolution of 3.46 nm at 546 nm has been achieved, and 74.95% of CHAFF's pixels exhibit a linearity deviation of < 2%.

The number, range and criticality of applications of Earth viewing optical sensors is increasing rapidly. Not only from national/international space agencies but also through the launch of commercial constellations such as those of planet and the concept of Analysis Ready Data (ARD) reducing the skill needed for utilisation of the data. However, no one organisation can provide all the tools necessary, and the need for a coordinated holistic earth observing system has never been greater. Achieving this vision has led to international initiatives coordinated by bodies such as the Committee on Earth Observation Satellites (CEOS and Global Space Inter-Calibration System (GISCS) of WMO to establish strategies to facilitate interoperability and the understanding and removal of bias through post-launch Calibration and Validation.In parallel, the societal challenge resulting from climate change has been a major stimulus for significantly improved accuracy and trust of satellite data. Instrumental biases and uncertainty must be sufficiently small to minimise the multi-decadal timescales needed to detect small trends and attribute their cause, enabling them to become unequivocally accepted as evidence.Although there have been many advances in the pre-flight SI-traceable calibration of optical sensors, in the last decade, unpredictable degradation in performance from both launch and operational environment remains a major difficulty. Even with on-board calibration systems, uncertainties of less than a few percent are rarely achieved and maintained and the evidential link to SI-traceability is weak. For many climate observations the target uncertainty needs to be improved ten-fold.However, this decade will hopefully see the launch of two missions providing spectrally resolved observations of the Earth at optical wavelengths, CLARREO Pathfinder on the International Space Station from NASA [1] and TRUTHS from ESA [2] to change this paradigm. Both payloads are explicitly designed to achieve uncertainties close to the ideal observing system, commensurate with the needs of climate, with robust SI-Traceability evidenced in space. Not only can they make high accuracy climate quality observations of the Earth and in the case of TRUTHS also the Sun, but they will also transfer their SI-traceable uncertainty to other sensors. In this way creating the concept of a ‘metrology laboratory in space’, providing a ‘gold standard’ reference to anchor and improve the calibration of other sensors. The two missions achieve their traceability in orbit through differing methods but will use synergistic approaches for establishing in-flight cross-calibrations. This paper will describe these strategies and illustrate the benefit through examples where improved accuracy has the most impact on the Earth observing system.The complementarity and international value of these missions has ensured a strong partnership during early development phases of the full CLARREO mission and that of the NPL conceived TRUTHS. Following a proposal by the UK Space Agency and subsequent adoption into the ESA EarthWatch program this partnership is further strengthened with the ESA team and a vision that together the two missions can lay the foundation of a framework for a future sustainable international climate and calibration observatory to the benefit of the global Earth Observing community.References[1] https://clarreo-pathfinder.larc.nasa.gov/[2] https://www.npl.co.uk/earth-observation/truths

Reliable characterization and radiometric calibration of satellite sensors are critical to their optimal performance on-orbit. The uses of satellite sensor data, with their increased use in long-term environmental monitoring and climate studies mean that the performance and data quality provided by a single sensor can no longer be considered in isolation but needs to be considered as a part of the international Earth Observation (EO) infrastructure and referenced to common standard, the SI. The drive for improved performance, together with the desire for inter-operability between sensors creates increased demands on the pre-flight characterization and radiometric calibration of sensors. Sensor pre-flight characterization and calibration facilities, or optical ground support equipment (OGSE) test sensor performance over a few broad categories including geometric performance/image quality together with spectral and radiometric calibration. The specific requirements of the sensor have historically created a drive for a bespoke OGSE. For large-scale multi-sensor series programs, a bespoke solution may remain the preferred solution. However, for single/few unit explorer missions, the expense & post-use redundancy of a bespoke OGSE system may be prohibitive. NPL together with M Squared lasers has developed a universal OGSE facility, the Spectroscopically Tunable Absolute Radiometric calibration & characterization OGSE (STAR-CC-OGSE), a versatile facility for the radiometric calibration and characterization of satellite sensors. The system is provided fully characterized, calibrated and performance verified, with an easy to use software interface that allows fully automated remote operation

The need for SI traceability to ensure integrity and trust in the Essential Climate Variables (ECVs) and the services and information derived from them, is well established. However, the means to achieve and demonstrate this in a universally-consistent manner globally and between variables, particularly for the complex bio-geophysical variables that make up many of the ECVs, is challenging. National Physical Laboratory (NPL), the UK national metrology institute, has, over the last three decades, established a comprehensive research programme to extend traditional underpinning laboratory-based capabilities to meet the needs of a wide range of Earth Observation and climate applications. These have included: * both bespoke and tailored standards together with methods for the calibration of remote-sensing instruments (including pre-flight calibration of satellite sensors), * field measurements in the worlds Forests, Oceans, Deserts and the atmosphere * development of metrological methods to assess and describe uncertainties, end to end (sensor to user-relevant information) * most recently, extending to the development of a satellite to establish SI traceability from orbit as part of the ESA EarthWatch programme. To build the necessary skills, capacity and trust within the community, NPL has established a close dialogue with EO/climate community experts and built international partnerships through active participation in international bodies such as CEOS & GEO. This has led to a close working relationship with ESA and other European national and international space agencies to provide metrological support across a wide range of projects. This paper will discuss the criticality of SI traceability to providing trust in globally-relevant environmental & climate datasets and illustrate how it is being achieved through case studies, such as: * the ESA Fiducial Reference Measurement (FRM) projects, * establishment of SI-traceable reference test-sites for satellite calibration and validation * novel infrastructure to calibrate and characterise optical satellite sensors * and efforts to harmonise their in-flight radiometric gain. NPL plays a lead role in the recently created European Metrology Network for Climate and Ocean and is keen to continue to ensure its efforts and research program address the priorities of the EO and climate community and will welcome input on future research directions.

The Earth’s climate is undoubtedly changing; however, the time scale, consequences, and causal attribution remain the subject of significant debate and uncertainty. Detection of subtle indicators from a background of natural variability requires measurements over a time-base of decades. This places severe demands on the instrumentation used, requiring measurements of sufficient accuracy and sensitivity that can allow reliable judgements to be made decades apart. The International System of Units (SI) was developed to address such requirements, providing a reference framework tied to invariant constants of nature. 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. This paper describes a new satellite mission, called Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS), which enables, for the first time, high-accuracy SI traceability to be established in orbit. The direct use of a ‘primary standard’ and replication of the terrestrial traceability chain extends the SI into space, in effect realizing a ‘metrology laboratory in space’ providing and enabling SI-traceable measurements of unequivocal accuracy in the solar reflective domain—an enabling element of an international space-based climate observing system. TRUTHS will not only provide a benchmark of the radiation state of the planet (incoming and outgoing) from which to compare change in the shortest time possible, but also facilitate an upgrade in performance of the Earth Observing system as a whole, through ‘in-orbit’ reference calibration.

We present the results from Verification of Reference Irradiance and Radiance Sources Laboratory Calibration Experiment Campaign. Ten international laboratories took part in the measurements. The spectral irradiance comparison included the measurements of the 1000 W tungsten halogen filament lamps in the spectral range of 350 nm–900 nm in the pilot laboratory. The radiance comparison took a form of round robin where each participant in turn received two transfer radiometers and did the radiance calibration in their own laboratory. The transfer radiometers have seven spectral bands covering the wavelength range from 400 nm–700 nm. The irradiance comparison results showed an agreement between all lamps within ±1.5%. The radiance comparison results presented higher than expected discrepancies at the level of ±4%. Additional investigation to determine the causes for these discrepancies identified them as a combination of the size-of-source effect and instrument effective field of view that affected some of the results.