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This work reports on a novel C-band monostatic UAV-radar system deployed over two forested wetlands in arctic Sweden, near to the Abisko research station. A Videodrone X4S drone acted as the carrying body, allowing programmable and repeatable flight paths. The radar system is multi-polarized (VV, VH, HV, HH), using one transmitter and optionally one or two receivers. The radar operates in a sawtooth FMCW mode, monotonically stepping in frequency across 5.2 GHz to 5.6 GHz. The choice of sweep time (1 to 8 ms) and number of data points (128 to 2048) are programmable and selected before a flight. The radar is triggered in flight manually from the ground using a Wi-Fi link, and which then repeatedly loops over a pre-set number of 10 s scans. Here, the choice of a drone speed of 5 m s-1 meant that each scan covered a 50 m flight line. There is a re-setting time of 0.3 s between scans. The wetlands are covered by a sparse forest, primarily of birch typically 3 to 7 m tall. We used the tomographic profiling (TP) scheme to collect high-resolution maps of the vertical scattering through the forest canopy. Such information is not available from the coarser satellite imagery, which provides no information on the vertical distribution of the backscatter, not even on the relative strengths of the ground and canopy returns. As the TP scheme has the antennas forward facing, only a narrow image transect beneath the flight path is collected. A synthetic aperture technique is used both to sharpen the real beam in the along track direction, and additionally steer it in angle. Thus, post-measurement, a single flight can be processed to capture the incidence angle response of the whole scene at a single incidence angle, selectable over a ~40 degree range. The results show how the forest backscatter response changes from one dominated by a ground return close to nadir viewing, to one dominated by the canopy above 20 degrees incidence angle. Comparisons and comment will also be provided of the differing responses with polarisation.

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.

A new framework that enables evaluation of the in situ ocean color radiometry measurement uncertainty is presented. The study was conducted on the multispectral data from a permanent mooring deployed in clear open ocean water. The uncertainty is evaluated for each component of the measurement equation and data processing step that leads to deriving the remote sensing reflectance. The Monte Carlo method was selected to handle the data complexity such as correlation and nonlinearity in an efficient manner. The results are presented for a prescreened dataset that is suitable for system vicarious calibration applications. The framework provides uncertainty value per measurement taking into consideration environmental conditions present during acquisition. A summary value is calculated from the statistics of the individual uncertainties per each spectral channel. This summary value is below 4% (k = 1) for the blue and green spectral range. For the red spectral channels, the summary uncertainty value increases to approximately 5%. The presented method helps to understand the significance of various uncertainty components and to provide a way of identifying major contributors. This can be used for efficient system performance improvement in the future.

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%.

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.

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

Environmental observations of essential climate variables (ECVs) and related quantities made by satellites and in situ observational networks are used for a wide range of societal applications. To identify a small climate trend from an observational record that is also sensitive to changes in weather, to seasonal effects and to geophysical processes, it is essential that observations have a stable basis that holds for multiple decades, whilst still allowing for changes in the observation instrumentation and operational procedures. To achieve this, all aspects of data collection and handling must be underpinned by robust quality assurance. The resultant data should also be linked to a common reference, with well-understood uncertainty analysis, so that observations are interoperable and coherent; in other words, measurements by different organisations, different instruments and different techniques should be able to be meaningfully combined and compared. Metrology, the science of measurement, can provide a critical role in enabling robust, interoperable and stable observational records and can aid users in judging the fitness-for-purpose of such records. In addition to Global Climate Observing System (GCOS) monitoring principles, metrology’s value, and the role of National Metrology Institutes (NMI) in observations, has been recognised in initiatives such as the Quality Assurance Framework for Earth Observation (QA4EO) established by the Committee on Earth Observation Satellites (CEOS) and in the implementation plans of the World Meteorological Organization’s (WMO’s), Global Atmosphere Watch and the European Ocean Observing System. The European Association for National Metrology Institutes (EURAMET) has recently created the “European Metrology Network (EMN) for Climate and Ocean Observation” to support further engagement of the expert communities with metrologists at national metrology insitutes and to encourage Europe’s metrologists to coordinate their research in response to community needs. The EMN has a scope that covers metrological support for in situ and remote sensing observations of atmosphere, land and ocean ECVs (and related parameters) for climate applications. It also covers the additional economic and ecological applications of ocean Essential Ocean Variable (EOV) observations. It is the European contribution to a global effort to further enhance metrological best practice into such observations through targeted research efforts. In late 2019 and early 2020 the EMN carried out a survey to identify the need for metrology within the observational communities and held a webinar workshop to prioritise the identified needs. Here we present the results of the survey and discuss the role that metrology can play in the climate observing system of the future.

In recent years, the increasing range of applications of Earth Observation data products and availability of low-cost satellites has resulted in an increasing number of commercial satellite systems. These services may provide complementary capabilities to those of Space Agencies. Adoption of these data products for many applications requires that they meet an assured level of quality that is fit for the given purpose. For the most efficient exploitation of EO data, therefore, assessment of data quality, calibration and validation are indispensable tasks, forming the basis for reliable scientific conclusions. In this context, the European Space Agency has established the Earthnet Data Assessment Pilot (EDAP) project, which aims to enable maximum exploitation of growing data availability by performing early data assessment for various missions that fall into one of the following instrument domains number of missions, in the Optical, SAR and atmospheric domains. These assessments are intended to evaluate and report the quality of a satellite mission with respect to what is “fit for purpose” within the context of the its stated performance and application. This activity compliments similar activities from other international partners, including NASA. Such quality information is often communicated to users in an ill-defined or incomplete manner. We show the development of a generic satellite mission quality assessment framework, developed within EDAP, which is designed provide a thorough review of all important aspects of mission quality. The assessment results are conveye d ata top level to the user as a quality assessment matrix diagram. The framework itself is based on the principles of CEOS QA4EO (Quality Assurance for Earth Observation) and builds on the experience of several European projects that worked towards practically implementing them. In a wider context, such a framework has potential for more general use in both institutional and commercial Earth Observation – helping mission providers to understand the information their users need and empowering users to make informed decisions about which data is fit for their purpose. As such, there is potential for international collaboration, between space agencies, to synergise quality assessment approaches and to work towards the development of a common standard.

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.

This paper describes spectral stray light measurements of diode array spectrometers and, in particular, the dependence of stray light and bandpass properties on the illumination beam geometry. The effects of underfilling and overfilling the nominal field of view of the instrument are presented. Our measurements showed that the line spread function of a commercially available miniature spectrometer module having a fiber optic entrance with 0.22 nominal numerical aperture can depend on fiber illumination cone apex angle up to $40^\circ$ , which is well above the $25^\circ$ nominal acceptance cone apex angle of the fiber. The bandpass width and amount of stray light inside the spectrometer increased proportionally with the illumination cone angle. Spectral stray light measurements with different setups gave consistent and repeatable results for similar illumination geometries. Care should be taken to ensure that illumination geometry during spectral stray light characterization of a spectrometer matches the conditions during target measurements for which the spectral stray light corrections are applied. The impact of illumination geometry during stray light characterization depends on the target signal and particular application. Photochemical reflectance index (PRI) and normalized difference vegetation index (NDVI) of a hemiboreal birch forest are used as contrasting examples in this paper. Compared to natural variability, the relative impact on PRI exceeds that of NDVI by 10 times.

Background A warmer climate has consequences for the timing of phenological events, as temperature is a key factor controlling plant development and flowering. In this study, we analyse the effects of the long-term climate change and an extreme weather event on the first flowering day (FFD) of five spring-flowering wild plant species in the United Kingdom. Citizen science data from the UK Woodland Trust were obtained for five species: Tussilago farfara (coltsfoot), Anemone nemorosa (wood anemone), Hyacinthoides non-scripta (bluebell), Cardamine pratensis (cuckooflower) and Alliaria petiolate (garlic mustard). Results Out of the 351 site-specific time series (≥ 15-years of FFD records), 74.6% showed significant negative response rates, i.e. earlier flowering in warmer years, ranging from − 5.6 to − 7.7 days °C−1. 23.7% of the series had non-significant negative response rates, and 1.7% had non-significant positive response rates. For cuckooflower, the response rate was increasingly more negative with decreasing latitudes. The winter of 2007 reflects an extreme weather event, about 2 °C warmer compared to 2006, where the 2006 winter temperatures were similar to the 1961–1990 baseline average. The FFD of each species was compared between 2006 and 2007. The results showed that the mean FFD of all species significantly advanced between 13 and 18 days during the extreme warmer winter of 2007, confirming that FFD is affected by temperature. Conclusion Given that all species in the study significantly respond to ambient near-surface temperatures, they are suitable as climate-change indicators. However, the responses to a + 2 °C warmer winter were both more and less pronounced than expected from an analysis of ≥ 15-year time series. This may reflect non-linear responses, species-specific thresholds and cumulative temperature effects. It also indicates that knowledge on extreme weather events is needed for detailed projections of potential climate change effects.

For over 40 years ESA’s Earthnet Programme has played a significant role as part of ESA’s mandatory activities, being a major contributor to the Global Earth Observation System of Systems (GEOSS). This role involved providing the framework for integrating non-ESA missions, i.e. Third Party Missions (TPM), into the overall ESA Earth Observation (EO) strategy. Complementary to ESA-owned EO missions, the programme allows European users access to a large portfolio of TPM and is particularly important for promoting the international use of EO data. In line with the Earthnet Programme objectives first established in 1977, ESA aims to foster cooperation and collaboration with not only other national space agencies, but also commercial mission providers. In recent years the availability of low cost small satellites and the innovation of constellations resulted in an increased number of commercial companies who have established business models to provide information services fed by their own satellite systems. These New Space players are now portraying an important role in the EO international strategy. Some of these new missions are potential candidates for Earthnet TPMs, and ESA have therefore set up a project to assess the quality and the suitability of these missions and also to establish dialogues with the various mission providers in order to improve the overall coherence of the EO system. This project is known as the Earthnet Data Assessment Pilot (EDAP). The EDAP consortium, headed by Telespazio VEGA UK is aimed at the provision of various clusters of expertise to perform an early data quality assessment of existing or future EO missions from national or commercial providers, which may potentially become TPMs within ESA’s Earthnet Programme. Complementary to this support is a focus on the generation of methodologies and guidelines for training and capacity building with each mission provider in regards to performing efficient data quality assessments in preparation for future missions. This work presents how the EDAP activities are organized and executed, and will also provide details of the various missions included within each of the instrument-specific domains covering Optical Sensors, Synthetic Aperture Radar (SAR) and Atmospheric missions. Important multi-mission aspects will also be presented for studies that will require inputs from several missions, possibly spanning multiple instrument domains; such studies contribute to interoperability across existing and future missions and help foster synergies between these missions.

EURAMET, the association responsible for coordinating the integrated metrology community in Europe, has recently established European Metrology Networks (EMNs) to support its aim to bring high-quality scientific research and coordinated metrological services to meet the rapidly advancing needs of different stakeholders for metrology. The EMN for Climate and Ocean Observation is one of the initial six EMNs and has been set up as a European coordination body to support the global effort to bring rigorous metrological discipline into the observations that underpin our understanding of the climate and the oceans. This paper describes the network, its activity so far, and its plans.

A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellites in the visible/near-IR has been set up in Gobabeb, Namibia in July 2017. The station location has been decided after a global analysis of satellite data assessing surface spatial homogeneity, cloud coverage, temporal variability, atmospheric turbidity etc. which has been compared with insitu data (surface reflectance, BRDF etc.) from a field campaign which took place in late 2015. The instrumentation automatically measures atmospheric (aerosol optical thickness etc.) and surface conditions (BRDF) from 414 nm to 1600 nm in 12 narrow spectral bands. The data processing is performed at CNES and includes the calibration of the photometer itself using in-situ measurements, and the simulation of the top-of-atmosphere radiance seen by any optical sensor over-passing the site, thus calibrating the sensor. This paper describes the instrument location, measurement protocol, calibration principle and calibration results of satellites Sentinel2A and Sentine12B.

The Radiometric Uncertainty Tool (RUT) is a software tool integrated as part of the Sentinel Application Platform (SNAP). The tool estimates the radiometric uncertainty associated with each pixel in the top-of-atmosphere reflectance factor images of the Sentinel-2 (S2) Level-1C products provided by the European Space Agency. The tool and the uncertainty analysis are under continuous evolution. This paper summarises the main features and improvements considered for a second version of the RUT. This second version has under-gone a set of software improvements such as: metadata associated to the uncertainty image, integration of the S2 L1C masks, and the feasibility of parallel processing. Furthermore, it includes upgrades in the uncertainty analysis as distinguishing between S2A and S2B satellites or the estimate of the uncertainty of a mean of pixels in a region-of-interest. Finally, the second version also includes an improved user guide and documentation.

Measurements of the Hemispherical Directional Reflectance Factor (HDRF) of snow surfaces were performed at Dome C, Antarctica, during the Australis Summer 2011–2012 to support the inter-comparison and inter-calibration of satellite optical sensors. HDRF data were collected with the Gonio Radiometric Spectrometer System (GRASS) which performs hyper-spectral measurements of radiance from the same target surface with independent collectors at a number of viewing and azimuth angles in the 0–60° and 0–360° angular ranges, respectively. The radiance collectors, installed on a hemispheric frame and connected to a spectrometer through fibre optics, have an 8° full cone of acceptance and a viewing footprint varying from 0.049 m2 at nadir to 0.142 m2 at viewing angles of 60°. These relatively small footprints allow for the characterisation of small-scale heterogeneities in the HDRF of observed surfaces. HDRF measurements representative of the Dome C snow surfaces were made at eight different sites along a transect approximately 100 m long. All the sites exhibit similar HDRF distributions with inter-site differences explained by small-scale inhomogeneities of the surface. The measured HDRF display marked forward scattering with anisotropy increasing with wavelength in the 400–1600 nm spectral region. These data complement those from previous measurements performed in the same area with a different technique. Agreement between the two data sets is shown by differences generally lower than 4% between HDRF distributions derived from a previous study and the spatially averaged HDRF from the various sites along a transect presented in this work.

Sound policymaking requires high confidence in climate predictions verified against decadal change observations with robustly known accuracy. Yet, our ability to monitor and predict the future of the climate is inadequate as we currently do not possess sufficient accuracy in our observing capability to confidently observe the small but critical climate change signals that are expected to occur over decadal time scales. These signals are fundamental to assessing the accuracy of climate change projections made by models and for the unambiguous attribution of climate change.

The Radiometric Calibration Network (RadCalNet, www.radcalnet.org) routinely provides top-of-atmosphere (TOA) reflectance data from instrumented ground sites. The data represents the nadir view of the ground for different sites that cover areas ranging from 50 m × 50 m to 1 km x 1 km. The smaller sites can only be used with high resolution sensors (≤ 30 m), but the larger sites, such as Railroad Valley (RRV) in Nevada can also be used for the validation or vicarious calibration of medium resolution sensors (> 250 m spatial resolution). Prior to utilising RadCalNet data in this manner, this paper describes the application of a high and a medium resolution sensor to assess potential biases between the RadCalNet data and satellite data at two different spatial resolutions. Results are shown for initial comparisons over RRV for the high resolution Sentinel-2 MultiSpectral Instrument (S2-MSI) and the medium resolution Sentinel-3 Ocean and Land Colour Instrument (S3-OLCI), and indicate the potential for RadCalNet to validate and vicariously calibrate sensors with differing spatial resolutions. The comparison analysis includes taking into account the temporal differences between the Sentinel-2 and Sentinel-3 overpasses and the time of RadCalNet data collection, as well as the spectral response functions (SRF) of the bands for both instruments. The comparison against the RRV site has shown there are significant biases between the RadCalNet data and S2-MSI and S3-OLCI for non-nadir viewing geometries that may be due to directional viewing and illumination effects and the non-Lambertian character of the RadCalNet RRV site.

This paper describes a method to calibrate photodiode array spectrometers in the spectral radiance mode using a fixed-point blackbody as a reference source. Fixed-point blackbodies are characterized by their excellent emissivity, uniformity, and stability, which make them superior to both conventional standard lamps and variable temperature blackbodies. The temperature of these fixed points is accurately determined being traceable to either the International Temperature Scale (ITS-90) or thermodynamically through radiometric standards. The potential advantage of the fixed-point traceability chain is that it can be universally reproduced without recourse to any hierarchical calibrations or standards. The paper presents the calibration system and discusses the limitations of such an approach. The method used obtained an uncertainty of around 1.4 % ( $$k = 2$$ ) associated with radiance responsivity across the spectral region from 550 nm to 1050 nm, which is comparable to what is readily achieved with a lamp-tile or lamp-illuminated spherical source.

Vicarious calibration approaches using in situ measurements saw first use in the early 1980s and have since improved to keep pace with the evolution of the radiometric requirements of the sensors that are being calibrated. The advantage of in situ measurements for vicarious calibration is that they can be carried out with traceable and quantifiable accuracy, making them ideal for interconsistency studies of on-orbit sensors. The recent development of automated sites to collect the in situ data has led to an increase in the available number of datasets for sensor calibration. The current work describes the Radiometric Calibration Network (RadCalNet) that is an effort to provide automated surface and atmosphere in situ data as part of a network including multiple sites for the purpose of optical imager radiometric calibration in the visible to shortwave infrared spectral range. The key goals of RadCalNet are to standardize protocols for collecting data, process to top-of-atmosphere reflectance, and provide uncertainty budgets for automated sites traceable to the international system of units. RadCalNet is the result of efforts by the RadCalNet Working Group under the umbrella of the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV) and the Infrared Visible Optical Sensors (IVOS). Four radiometric calibration instrumented sites located in the USA, France, China, and Namibia are presented here that were used as initial sites for prototyping and demonstrating RadCalNet. All four sites rely on collection of data for assessing the surface reflectance as well as atmospheric data over that site. The data are converted to top-of-atmosphere reflectance within RadCalNet and provided through a web portal to allow users to either radiometrically calibrate or verify the calibration of their sensors of interest. Top-of-atmosphere reflectance data with associated uncertainties are available at 10 nm intervals over the 400 nm to 1000 nm spectral range at 30 min intervals for a nadir-viewing geometry. An example is shown demonstrating how top-of-atmosphere data from RadCalNet can be used to determine the interconsistency between two sensors.

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission will provide a calibration laboratory in orbit for the purpose of accurately measuring and attributing climate change. CLARREO measurements establish new climate change benchmarks with high absolute radiometric accuracy and high statistical confidence across a wide range of essential climate variables. CLARREO's inherently high absolute accuracy will be verified and traceable on orbit to Système Internationale (SI) units. The benchmarks established by CLARREO will be critical for assessing changes in the Earth system and climate model predictive capabilities for decades into the future as society works to meet the challenge of optimizing strategies for mitigating and adapting to climate change. The CLARREO benchmarks are derived from measurements of the Earth's thermal infrared spectrum (5–50 μm), the spectrum of solar radiation reflected by the Earth and its atmosphere (320–2300 nm), and radio occultation refractivity from which accurate temperature profiles are derived. The mission has the ability to provide new spectral fingerprints of climate change, as well as to provide the first orbiting radiometer with accuracy sufficient to serve as the reference transfer standard for other space sensors, in essence serving as a “NIST [National Institute of Standards and Technology] in orbit.” CLARREO will greatly improve the accuracy and relevance of a wide range of space-borne instruments for decadal climate change. Finally, CLARREO has developed new metrics and methods for determining the accuracy requirements of climate observations for a wide range of climate variables and uncertainty sources. These methods should be useful for improving our understanding of observing requirements for most climate change observations.

Intercalibration of satellite instruments is critical for detection and quantification of changes in the Earth's environment, weather forecasting, understanding climate processes, and monitoring climate and land cover change. These applications use data from many satellites; for the data to be interoperable, the instruments must be cross-calibrated. To meet the stringent needs of such applications, instruments must provide reliable, accurate, and consistent measurements over time. Robust techniques are required to ensure that observations from different instruments can be normalized to a common scale that the community agrees on. The long-term reliability of this process needs to be sustained in accordance with established reference standards and best practices. Furthermore, establishing physical meaning to the information through robust Systeme International d'unites traceable calibration and validation (Cal/Val) is essential to fully understand the parameters under observation. The processes of calibration, correction, stability monitoring, and quality assurance need to be underpinned and evidenced by comparison with “peer instruments” and, ideally, highly calibrated in-orbit reference instruments. Intercalibration between instruments is a central pillar of the Cal/Val strategies of many national and international satellite remote sensing organizations. Intercalibration techniques as outlined in this paper not only provide a practical means of identifying and correcting relative biases in radiometric calibration between instruments but also enable potential data gaps between measurement records in a critical time series to be bridged. Use of a robust set of internationally agreed upon and coordinated intercalibration techniques will lead to significant improvement in the consistency between satellite instruments and facilitate accurate monitoring of the Earth's climate at uncertainty levels needed to detect and attribute the mechanisms of change. This paper summarizes the state-of-the-art of postlaunch radiometric calibration of remote sensing satellite instruments through intercalibration.

The response of a variety of Earth observation satellite sensors from multiple platforms, as well as different sensors on the same platform, were cross-compared over Dome C in Antarctica. A single unified methodology was employed to remove geometric, temporal, and atmospheric biases between different sensors with a range of spatial resolutions, bandwidths, and overpass times. The result assumes no “correct” baseline value, only relative responses. The resulting responses of the sensors, namely, AATSR, MERIS, and VEGETATION 2, are here presented and, where overlap occurs, compared with the results from other groups. In addition to applying the Quality Assurance for Earth Observation protocols for the quality assurance of environmental monitoring satellites, this paper discusses the intercalibration of several sensors used to establish the technical basis of the ESA GlobAlbedo project.

This forty papers in this special issue focus on how intercalibration and comparison between sensors can provide an effective and convenient means of verifying their postlaunch performance and correcting their measurement differences.

The World Radiometric Reference (WRR) is an artefact based reference for Direct Solar Irradiance (DSI) measurements. The WRR is realized by a group of electrical substitution radiometers, the World Standard Group (WSG). In recent years, a relative difference of about -0.3% between the International System of Units (SI) scale and the WRR scale was observed with the SI scale being lower. The Cryogenic Solar Absolute Radiometer (CSAR) aims for i) providing direct traceability of DSI measurements to the SI system, ii) reducing the overall uncertainty of DSI measurements towards 0.01% and for iii) replacing the WSG in future. The latest SI-WRR intercomparisons performed with CSAR revealed a relative difference of -0.29% ± 0.064% (k = 1) between the SI and the WRR scale, a result that agrees well with previous findings. The uncertainty of corrections for the window transmittance results currently in the largest contribution to the total uncertainty for the CSAR measurements. The formal transition from the WRR t...

TRUTHS (Traceable Radiometry Underpinning Terrestrial-and Helio- Studies) is a proposed small satellite mission to enable a space-based climate observing system capable of delivering data of the quality needed to provide the information needed by policy makers to make robust mitigation and adaptation decisions. This is achieved by embedding trust and confidence in the data and derived information (tied to international standards) from both its own measurements and by upgrading the performance and interoperability of other EO platforms, such as the Sentinels by in-flight reference calibration. TRUTHS would provide measurements of incoming (total and spectrally resolved) and global reflected spectrally and spatially (50 m) solar radiation at the 0.3% uncertainty level. The calibration scheme components and the route to SI-traceable Earth-reflected solar spectral radiance and solar spectral irradiance are described.

A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellite sensors is currently under development, focussing on the visible and near infra-red parts of the spectrum. The site will become a European contribution to the Committee on Earth Observation Satellites (CEOS) initiative RadCalNet (Radiometric Calibration Network). This paper describes the site characterisation that was carried out, both to define the exact location of permanent monitoring instrumentation, and to provide an initial detailed assessment of the site's properties. The characterisation involved a range of tests, using an ASD FieldSpec spectroradiometer, and the Gonio Radiometric Spectrometer System, a hyperspectral, multi-angle HDRF instrument. Teams from NPL and CNES each performed parts of this characterisation, working together to acquire the data and generate the final outcome

Data from Earth Observation (EO) satellites are increasingly used to monitor the environment, understand variability and change, inform evaluations of climate model simulations and forecasts and manage natural resources. Policy makers are progressively relying on information derived from EO data to make decisions on mitigating and adapting to climate change. These decisions should be evidence based, which requires complete confidence in satellite-derived products as well as the in situ measurements used to calibrate, validate or complement these data. This paper provides an overview of the components required to develop robust quality assurance procedures for terrestrial essential climate variable (ECV) data products derived from Earth Observation (EO) satellite datasets.

We report the fourth World Radiometric Reference (WRR)-to-SI comparison. At the National Physical Laboratory we compared three transfer pyrheliometer instruments in power mode with the SI radiometric scale. Compared with the three previous comparisons, we improved the experiment by operating the transfer instruments in vacuum. At the Total solar irradiance Radiometer Facility (TRF) located at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, we repeated the power comparison of one of the transfer instruments. The TRF also allowed the comparison and characterization of this instrument in irradiance mode. Using the WRR comparisons performed in Davos, we find that the WRR is 0.34% higher than the SI scale. Comparing irradiance mode calibrations with power mode calibrations reveals that previous estimates of stray light of PMO6-type radiometers were very low. The instrument calibrated at TRF was integrated in the space experiment PREMOS on the French satellite PICARD and carries the first vacuum irradiance calibration to space.

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) and the network of National Metrology Institutes were developed to address such requirements. However, ensuring and maintaining SI traceability of sufficient accuracy in instruments orbiting the Earth presents a significant new challenge to the metrology community. This paper highlights some key measurands and applications driving the uncertainty demand of the climate community in the solar reflective domain, e.g. solar irradiances and reflectances/radiances of the Earth. It discusses how meeting these uncertainties facilitate significant improvement in the forecasting abilities of climate models. After discussing the current state of the art, it describes a new satellite mission, called 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 . Keywords: climate change; Earth observation; satellites; radiometry; solar irradiance

This paper describes a thorough investigation of the NPL Vega BB3500 furnace to improve the quality and duration of the melt plateaux of a Re–C eutectic cell. The results of this investigation have shown that the position of the cell within the furnace is the most critical factor for optimizing fixed-point performance. It was found that better performance could be obtained from a furnace with uniform ring resistance than with highly non-uniform resistance profiles, but this effect was relatively small. When high resistance (hot) rings were deliberately introduced on either side of the cell, at a distance corresponding to several rings beyond the position of the cell, considerable improvement in the plateaux was observed. Nevertheless, the cell (or the cell performance or even plateaux) is very sensitive to the positioning of these hot rings. A new cell holder was designed and introduced. Raising the cell on supporting rings provided a radiative gap that increased the thermal uniformity of the cell itself. Carbon–carbon composite material (CC sheet) wrapped loosely around the cell further improved the uniformity of the cell wall and hence the plateau quality. Following these changes, the plateau duration was increased by a factor of 5. (Some figures in this article are in colour only in the electronic version)

Below the freezing point of silver, radiation thermometers are generally calibrated by implementing the multi-point interpolation method using blackbody measurements at three or more calibration points, rather than the ITS-90 extrapolation technique. The interpolation method eliminates the need to measure the spectral responsivity and provides greater accuracy at the longer wavelengths required below the silver point. This article identifies all the sources of uncertainty associated with the interpolation method, in particular, those related to the reference blackbody temperatures (either variable-temperature or fixed-point blackbodies) and to the measured thermometer signals at these points. Estimates are given of the ‘normal’ and ‘best’ uncertainties currently achievable. A model of the thermometer response is used to propagate all the uncertainties at the reference points and provide a total uncertainty at any temperature within the calibration range. The multi-point method has the effect of constraining the total uncertainty over this range, unlike the ITS-90 technique for which the uncertainties propagate as T 2. This article is a joint effort of the working group on radiation thermometry of the Consultative Committee for Thermometry (CCT), summarizing the knowledge and experience of all experts in this field.

A small palm-sized, reference spectrometer, mounted on a remote-controlled model helicopter is being developed and tested by the National Physical Laboratory (NPL) in conjunction with City University, London. The developed system will be used as a key element for field vicarious calibration of optical earth observation systems in the visible-near infrared (VNIR) region. The spectrometer is hand held, low weight, and uses a photodiode array. It has good stray light rejection and wide spectral coverage, allowing simultaneous measurements from 400 to 900 nm. The spectrometer is traceable to NPL’s primary standard cryogenic radiometer via a high-temperature metal-carbon eutectic fixed-point blackbody. Once the fixed-point temperature has been determined (using filter radiometry), the eutectic provides a high emissivity and high stability source of known spectral radiance over the emitted spectral range. All wavelength channels of the spectrometer can be calibrated simultaneously using the eutectic transition without the need for additional instrumentation. The spectrometer itself has been characterized for stray light performance and wavelength accuracy. Its long-term and transportation stability has been proven in an experiment that determined the “World’s Bluest Sky”—a process that involved 56 flights, covering 100,000 km in 72 days. This vicarious calibration methodology using a eutectic standard is presented alongside the preliminary results of an evaluation study of the spectrometer characteristics.

For accurate determinations of thermodynamic temperature, NPL has developed its absolute radiation thermometer (ART), which is calibrated traceably against a cryogenic radiometer. This article reviews some of the potential sources of systematic uncertainty present in the calibration and use of ART. In particular, this article is concerned with the evaluation of the size-of-source effect and the lens transmittance, as well as potential differences in the responsivity of a transfer trap detector when calibrated in terms of radiant power and used in irradiance mode.

The diffuse and regular reflectances of five optically absorbing coatings frequently used in optical systems, were measured over the 0.32–14.3 μm wavelength range, before and after exposure to heat and intense optical radiation. The measured coatings included Nextel Velvet Black, an anodised coating and NPL Super Black. The anodised coating exhibited substantial variations in its diffuse and regular reflectance values after thermal and simulated solar ageing. Solar and thermal ageing of the Nextel Velvet Black resulted in increases of its reflectance. However, thermal ageing tended to decrease the reflectance of the other paint samples examined. Thermal and solar ageing of the NPL Super Black resulted in only minor changes in its reflectance characteristics. All measurements are traceable to the UK National Standards.

The candela, the SI (systeme internationale) unit for optical radiation, has been one of the base units since the inception of the system. The latest definition was in 1979, when it was linked to the derived unit, the watt. Advances in optical technology and the needs of the communication sector suggest that it is timely that consideration be given to redefining the candela in terms of fundamental quantum optical entities, i.e. photons. Validation of this approach will require comparison against the most accurate conventional technique, cryogenic radiometry. A definition in terms of photon number and the requirements for demonstrating equivalence with existing techniques is discussed, together with new possibilities which would result from further improvements in accuracy. Work being carried out at the National Physical Laboratory (NPL) towards these goals is described, drawing on developments of photon-counting calibration techniques and low temperature measurements, and research into single photon sourc...

A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellite sensors is currently under development, focussing on the visible and near infra-red parts of the spectrum. The site will become a European contribution to the Committee on Earth Observation Satellites (CEOS) initiative RadCalNet (Radiometric Calibration Network). The exact location of the permanent monitoring instrumentation will be defined following the initial site characterisation. The new ESA/CNES RadCalNet site will have a robust uncertainty budget and its data fully SI traceable through detailed characterisation and calibration by NPL of the instruments and artefacts to be used on the site. This includes a CIMEL sun photometer (the permanent instrumentation) an ASD FieldSpec spectroradiometer, Gonio Radiometric Spectrometer System (GRASS), and reference reflectance standards.

TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio-Studies) is a proposed small satellite mission to enable a space-based climate observing system capable of delivering data of the quality needed to provide the information needed by policy makers to make robust mitigation and adaptation decisions. This is achieved by embedding trust and confidence in the data and derived information (tied to international standards) from both its own measurements and by upgrading the performance and interoperability of other EO platforms, such as the Sentinels by in-flight reference calibration. TRUTHS would provide measurements of incoming (total and spectrally resolved) and global reflected spectrally and spatially (50 m) solar radiation at the 0.3% uncertainty level. These fundamental climate data products can be convolved into the building blocks for many ECVs and EO applications as envisaged by the 2015 ESA science strategy; in a cost effective manner. We describe the scientific drivers for the TRUTHS mission and how the requirements for the climate benchmarking and cross-calibration reference sensor are both complementary and simply implemented, with a small additional complexity on top of heritage calibration schemes. The calibration scheme components and the route to SI-traceable Earth-reflected solar spectral radiance and solar spectral irradiance are described.

In the framework of the European Union Copernicus programme, the European Space Agency (ESA) has launched the Sentinel-2 (S2) Earth Observation (EO) mission which provides optical high spatial resolution imagery. Here is presented a tool, S2-RUT, (Sentinel-2 Radiometric Uncertainty Tool) allowing estimation of the radiometric uncertainties associated to each pixel using as input the top-of-atmosphere (TOA) reflectance images provided by ESA. The Sentinel-2 radiometric analysis focuses on the review of the pre- and post-launch characterisations in order to specify the uncertainty contributors at a pixel level and allow changes to be proposed in the uncertainty contributors where necessary. The identified uncertainty contributors are combined using a metrological Guide to Expression of Uncertainty in Measurement’ (GUM) model that is validated by comparing the results to a multivariate Monte Carlo Method (MCM). Specific contributors of the TOA reflectance are initially characterised and its future integration in the tool is discussed. The software implementation of the S2-RUT tool relies on the flexibility of the JPEG2000 standard using partial decoding. Auxiliary information for the uncertainty calculation is extracted from the metadata and quality masks integrated in the L1C product. In addition, using the detector footprint mask it is possible to account for parameters dependent on the neighbouring pixels and/or detector module. The L1C uncertainty is coded using 1 byte with an extra optional byte for complementary information. The resulting images and the metadata are directly appended to the original L1C product.

This paper describes a set of tools, algorithms and methodologies that have been developed and used in order to estimate the radiometric uncertainty achievable for a Sentinel 2/3 like sensor through in-flight cross-calibration using a well-calibrated hyperspectral SI traceable reference sensor such as the proposed TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) mission. This first study considers the criticality of the instrumental and observational characteristics in a fully iterative manner and uses the Libya-4 Pseudo Invariant Calibration Site (PICS) as a demonstration. The main uncertainty contributions have been studied: spectral response, reflectance heterogeneity, sun angular effect and aerosol (spatial and temporal) effects. The tool contributes to optimizing the design of TRUTHS; but will find immediate use in support of existing sensor-to-sensor cross-calibration activities being carried out under the auspices of Committee on Earth Observation Satellites (CEOS).

We have developed a multi-band ground viewing radiometer based on light-emitting diodes (LEDs) to create a self-calibrating sensing system for land-based measurements. The system is intended for in-situ data collection needed for vicarious calibration, using the reflectance-based method, for earth observing satellites. An autonomous prototype radiometer, which is part of a sensor web of 5 radiometers which could be geographically distributed over hundreds of meters, has been realized. Temporally continuous in-situ measurement of the land reflectance in 4 spectral bands, from 350 nm to 900 nm, are acquired. The system has the ability to regularly re-calibrate autonomously and in the field, communicating results to a remote home base.

The CCPR K1-a key comparison of spectral irradiance (from 250 nm to 2500 nm) was carried out to meet the requirements of the Mutual Recognition Arrangement by 13 participating national metrology institutes. Because of the fragile nature of the tungsten halogen lamps used as comparison artefacts the comparison was arranged as a star comparison with many more lamps than participants. The National Physical Laboratory (UK) piloted the comparison and, by measuring all lamps, provided a link between participants' measurements. The comparison was analysed using a model-based method, which ensured that all participants, including the pilot, were treated equitably. This paper presents the comparison philosophy, methodology, analysis and results.

The performance of two single pixel, large area quantum well infrared photodetectors (QWIPs) were compared with that of a commercially available 2 mm diameter photovoltaic (PV) HgCdTe detector. Parameters which were compared include the absolute spectral responsivity, noise equivalent power (NEP), spatial uniformity of response, non-linearity and stability. Both detector types were shown to have sufficiently high spectral responsivity and D* values to satisfy a number of applications in infrared radiometry when operated at 77 K. However, the spatial non-uniformity of response of both QWIPs examined was unacceptably high. Furthermore the spatial uniformity of response of the same detectors was shown to be strongly dependent on the state of polarisation of the incident radiation as well as also exhibiting some dependency on wavelength. The spatial uniformity of response of the PV HgCdTe detector was shown to be very poor due to the very low shunt resistance of this detector. However, it exhibited no polarisation dependency and only a slight dependency on wavelength for wavelengths below 5 μm. Neither detector type could be considered for high accuracy radiometric applications in their current form.

In this paper we report the observation of drifts in the responsivity of cryogenically cooled InSb detector-based infrared filter radiometers which have very strong wavelength dependence. These drifts can result in the increase or decrease of the response of the filter radiometers by over 5%. The origin of these variations was investigated and was shown to arise due to a thin film of ice formed on the multi-layer bandpass filter used to define the spectral response of the filter radiometer. The thin layer of ice interacts with the characteristics of the filter (which itself consists of a number of thin layers) and modifies the filter spectral transmission thus modifying the response of the filter radiometer of which the filter is part of. These observations are particularly relevant to space instruments which use infrared filter radiometers for earth observation. Debris from the spacecraft engines is known to accumulate on cold surfaces of instruments carried on board. The deposition of this debris on cold filters can modify the spectral response of the instruments, which use these filters to define a spectral response.

An intercomparison of radiation temperature measurements was performed at VNIIOFI during October 2000 using a pyrolytic graphite blackbody operating over the temperature range from 1600?K to 3300?K. A pyrometer and two photometers from VNIIOFI, a pyrometer and four broadband glass filter detectors from PTB, and two narrow-band interference filter based radiometers and a broadband glass filter radiometer from NPL were used to perform the temperature measurements in either radiance or irradiance mode. Across almost the entire temperature range the VNIIOFI, NPL and PTB instruments showed results within the combined standard measurement uncertainties.

The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube Array (VANTA) on aluminium substrates is reported for the first time. The coating on aluminium was realised using a process that employs top down thermal radiation to assist growth, enabling deposition at temperatures below the substrate’s melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths in the 2.5 µm to 15 µm range. VANTA-coated aluminium substrates were subjected to space qualification testing (mass loss, outgassing, shock, vibration and temperature cycling) before their optical properties were re-assessed. Within measurement uncertainty, no changes to hemispherical reflectance were detected, confirming that NanoTube Black coatings on aluminium are good candidates for Earth Observation (EO) applications.

We present the development of a novel semiconductor chip-based spectrometer for calibration of Earth observation instruments. The chip follows the Solo spectroscopy approach utilising an array of microdisk resonators evanescently coupled to a central waveguide. Each resonator is tuned to select out a specific wavelength from the incoming spectrum, and forms a p-i-n junction in which current is generated when light of the correct wavelength is present. In this paper we discuss important design aspects including the choice of semiconductor material, design of semiconductor quantum well structures for optical absorption, and design and optimisation of the waveguide and resonators.

The performance of spectral irradiance transfer-standard lamps has long been a concern in the radiometry and photometry communities. The adoption of a modified FEL-type lamp has recently been proposed as the most suitable transfer standard for spectral irradiance measurements. While data have been published on the performance of these lamps when operated continuously and in a laboratory environment, however, little information is available on their performance under typical usage. The results are presented of real-life testing of over thirty FEL lamps both pre-selected, according to published procedures, and direct from the manufacturer. When operated vertically all of the lamps tested suffered terminal coil collapse after between 140 h and 250 h burn time. The reproducibility of the FEL lamps after transportation is also investigated.

The design of a new detector-stabilized spectral irradiance source is presented, together with an evaluation of its performance. This new source consists of a tungsten-halogen lamp that is current-controlled using feedback from an array of filtered photodiodes. The bandpass and central wavelength of these filtered detectors may be selected specifically to meet the required application of the source. The lamp is fully enclosed (except for the exit aperture) in an air-cooled, light-tight housing that minimizes unwanted light pollution. The lamp current is stabilized using feedback from one photodiode; the recording of the non-stabilization channels allows a real-time check on the effective colour temperature of the lamp, and provides an instant diagnosis of the state of calibration of the lamp. The calibration data for each lamp (in its associated housing) are stored on a microchip within the lamp housing, which allows the lamp to be transported independently of its associated control electronics for calibration purposes. The performance of the lamp indicates ageing rates of better than 2 × 10−5 h−1, across the ultraviolet-A (UV-A) and visible spectral regions.