CERES Replacement “Libera” SI Traceable Measurement Spectral Calibration Concept using Direct Solar Views by High Resolution Earth Telescopes

Better predictions of global warming can be enabled by tuning legacy and current computer simulations to Earth Radiation Budget (ERB) measurements. Since the 1970’s, such orbital results exist, and the next generation instruments called “Libera” are in design. Climate communities have requested that ERB observing system calibration accuracy obtain significantly better SI traceability and stability improvements. This is to prevent untracked instrument calibration drifts, that could lead to false conclusions on climate change. Based on experience from previous ERB missions, the concept presented here utilizes solar calibration for cloud size Earth measurement resolution, at ≪1% accuracy. However it neglects shown to be unsuccessful calibration technology like solar diffusers and on-board lights, as used by ERBE, ScaRaB, CERES, GERB & other Libera designs etc. New spectral characterizing concepts are therefore introduced. This allows in-flight wavelength dependent calibration of Earth observing Libera telescopes using direct solar views, through narrow-band filters continuously characterized on-orbit.

Toward a Climate and Calibration Observatory in space: NASA CLARREO Pathfinder and ESA TRUTHS

<p>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. </p><p>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. </p><p>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. </p><p>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.</p><p>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.</p><p>References</p><p>[1]  https://clarreo-pathfinder.larc.nasa.gov/</p><p>[2] https://www.npl.co.uk/earth-observation/truths</p>

Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS): An Element of a Space-Based Climate and Calibration Observatory

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.

The Moon as a Climate-Quality Radiometric Calibration Reference

On-orbit calibration requirements for a space-based climate observing system include long-term sensor response stability and reliable inter-calibration of multiple sensors, both contemporaneous and in succession. The difficulties with achieving these for reflected solar wavelength instruments are well known. The Moon can be considered a diffuse reflector of sunlight, and its exceptional photometric stability has enabled development of a lunar radiometric reference, manifest as a model that is queried for the specific conditions of Moon observations. The lunar irradiance model developed by the Robotic Lunar Observatory (ROLO) project has adequate precision for sensor response temporal trending, but a climate-quality lunar reference will require at least an order of magnitude improvement in absolute accuracy. To redevelop the lunar calibration reference with sub-percent uncertainty and SI traceability requires collecting new, high-accuracy Moon characterization measurements. This paper describes specifications for such measurements, along with a conceptual framework for reconstructing the lunar reference using them. Three currently active NASA-sponsored projects have objectives to acquire measurements that can support a climate-quality lunar reference: air-LUSI, dedicated lunar spectral irradiance measurements from the NASA ER-2 high altitude aircraft; ARCSTONE, dedicated lunar spectral reflectance measurements from a small satellite; and Moon viewing opportunities by CLARREO Pathfinder from the International Space Station.

SI-Traceability and Measurement Uncertainty of the Atmospheric Infrared Sounder Version 5 Level 1B Radiances

The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on 4 May 2002. The AIRS is designed to measure atmospheric temperature and water vapor profiles and has demonstrated exceptional radiometric and spectral accuracy and stability in orbit. The International System of Units (SI)-traceability of the derived radiances is achieved by transferring the calibration from the Large Area Blackbody (LABB) with SI traceable temperature sensors, to the On-Board Calibrator (OBC) blackbody during preflight testing. The AIRS views the OBC blackbody and four full aperture space views every scan. A recent analysis of pre-flight and on-board data has improved our understanding of the measurement uncertainty of the Version 5 AIRS L1B radiance product. For temperatures greater than 260 K, the measurement uncertainty is better than 250 mK 1-sigma for most channels. SI-traceability and quantification of the radiometric measurement uncertainty is critical to reducing biases in reanalysis products and radiative transfer models (RTMs) that use AIRS data, as well as establishing the suitability of AIRS as a benchmark for radiances established in the early 2000s.

Enabling and demonstrating SI traceability of ECVs and climate data records: the role of a national metrology institute

<p>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.</p><p> </p><p>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:</p><ul><li>both bespoke and tailored standards together with methods for the calibration of remote-sensing instruments (including pre-flight calibration of satellite sensors),</li> <li>field measurements in the worlds Forests, Oceans, Deserts and the atmosphere</li> <li>development of metrological methods to assess and describe uncertainties, end to end (sensor to user-relevant information)</li> <li>most recently, extending to the development of a satellite to establish SI traceability from orbit as part of the ESA EarthWatch programme.</li> </ul><p> </p><p>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.</p><p> </p><p>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:</p><ul><li>the ESA Fiducial Reference Measurement (FRM) projects,</li> <li>establishment of SI-traceable reference test-sites for satellite calibration and validation</li> <li>novel infrastructure to calibrate and characterise optical satellite sensors</li> <li>and efforts to harmonise their in-flight radiometric gain.</li> </ul><p> </p><p>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.</p>