The overall goal of MetEOC-4 is, building on outputs of previous projects (EMRP/EMPIR and others funded by the EU and European Space Station (ESA) for example, to develop metrology tools and frameworks capable of underpinning a global climate observing system. Efforts are focused on the following objectives, that are designed to capitalise on and synergise with other international initiatives, and serve implementation requirements of forthcoming European climate-focused sensors and related Essential Climate Variables. Develop a robust metrological chains (infrastructure and methods), that traces the SI a new generation of highly accurate, cost-effective sensors, for a space-based climate observing system, suitable for pre- and in-flight measurements, prioritising needs emerging from current mission studies. Develop SI traceable measurement methods with associated uncertainties for bio-geophysical parameters at pixel level, accounting for scene specific characteristics including the means to optimally parameterise, validate, and assess uncertainties of retrieval algorithms. These methods will consider harmonisation of sampling methods including optical and Synthetic Aperture Radar (SAR) based techniques. Develop satellite derived SI traceable measurement methods (including uncertainty assessment, associated validation and interoperability) for greenhouse gases emissions and natural carbon sinks, including robust monitoring of implemented policies, designed to reduce anthropogenic carbon emissions (in accordance with the Paris Agreement of 2015 and Vienna 2018). Develop instrumentation and standards for traceable climate quality measurements, including temperature of the Mesopause and thermal infrared sky radiance, from surface-based networks such as those operated for WMO and UN e.g. NDMC. Facilitate take up of the technology and measurement infrastructure developed in the project by the measurement supply chain (accredited laboratories, instrument manufacturers), standards developing organisations and end users (environmental monitoring and regulation bodies such as the WMO and Group of Earth Observations (GEO). The project is broken down into 4 technical work packages: Work package 1: EO Sensor performance and interoperability The latest pre-flight techniques require development to address challenges of new missions where high-accuracy SI-traceability (demands of climate) are critical. MeteEOC-4 involves development of methods and metrological infrastructures to support these needs. For example: The ESA FORUM mission, needs spectral scales extended from 50 µm to 100 µm and uncertainties equivalent to <0.1 K. TRUTHS, prototyped in MetEOC, is now an ESA Earth Watch mission, aiming to replicate NMI capabilities in space and thus requires pre-flight calibration to match. In-flight standards to maintain traceability for limb-sounders on stratospheric balloons will also be developed. Work package 2: Remote sensing of the Atmosphere Operationalisation of CEOS Cal/Val test-sites, including RadCalNet, requires extending from ‘bright targets’ to biophysical surfaces requiring challenging spectral/spatial corrections. Work package 2 is devising metrology needed to underpin this capability. Long-time-base datasets together with operational temporal‑continuity need scene/pixel dependent uncertainty characterisation – thus techniques using machine learning will be explored. Furthermore, transformational algorithms such as radiative transfer codes will be metrologically evaluated in physical and virtual environments. Work package 3: Validating Carbon stocktake: GHG emissions and biosphere Work package 3 initiates development of strategies and methods to establish traceability and evaluate associated uncertainties of retrieved GHG inventories at power-station/city-scale. It continues work to quantify the uncertainty of Carbon stored in sinks such as forest biomass and ocean phytoplankton. This includes retrieval algorithms and ‘ground-truthing’. The optical domain provides critical insight on health and classification of the biosystem, and this is complemented by Synthetic Aperture Radar (SAR) where clouds are transparent. This project will expand validation methods to combine drone-based SAR and optical observations assigning, for the first time, uncertainties based on combined observation techniques. Work package 4: Surface based ‘radiation’ networks The artefact-based WRR (World Radiometric Reference) is close to being replaced by an SI standard, CSAR, having taken part in two WMO comparisons – a major achievement of EMRP/EMPIR. However, for full acceptance, work to operationalise and remove associated uncertainties in measurements of window transmittance and diffraction is required. Other scales, e.g. WISG (World Infrared Standard Group) received attention in MetEOC-3, offering an initial design of a calibration source. This source needs full characterisation before its use calibrating the WISG radiometers. This project extends the capabilities to allow spectral and spatially resolved measurements of infrared sky radiance to be determined. This will provide information to understand the observed discrepancies between different pyrgeometer types and unexplained dependence with atmospheric opacity. A new pyrgeometer taking account of these effects will be designed and built.