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The status and most recent developments of the operational L2 Cloud product will be presented in this contribution for the ongoing Sentinel-5 Precursor and upcoming Sentinel-4 missions. These Copernicus missions are focused on atmospheric composition, operate in the UV/VIS/NIR/(SWIR) spectral region and comprise the retrieval of trace gases, greenhouse gases, aerosol and cloud properties. A good knowledge about the latter, i.e. the presence and characteristics of clouds, is a pre-requisite for an accurate retrieval of the aforementioned trace gases and greenhouse gases. Additionally, clouds are by themselves an interesting indicator to measure and monitor because of their contribution to the radiation budget, and hence, impact on climatological applications. The algorithms for retrieving the operational cloud products from TROPOMI onboard Sentinel-5 Precursor and the UVN spectrometer onboard Sentinel-4 are called OCRA (Optical Cloud Recognition Algorithm) and ROCINN (Retrieval of Cloud Information using Neural Networks) and both have their heritage with GOME/ERS-2 and GOME-2 MetOp-A/B/C, where they have already been successfully implemented in an operational environment. OCRA applies a broad band color space approach to the measured reflectance in order to retrieve a radiometric cloud fraction that is used as an a priori input to ROCINN, which retrieves cloud top height, cloud optical thickness and cloud albedo from measurements of sun-normalized radiances in the NIR in and around the oxygen A-band. The cloud parameters retrieved by ROCINN are provided for two different cloud models. The Clouds-as-Layers (CAL) model treats clouds as layers of scattering water droplets, which is physically more realistic than the second model, Clouds-as-Reflecting Boundaries (CRB), which treats a cloud as a simple Lambertian reflector.In addition, this contribution will cover initial results of applying the OCRA algorithm to the recently launched Korean geostationary GEMS instrument. Applying the algorithm to GEMS does provide a great opportunity to test the performance under a geostationary configuration and to transfer lessons learned in a synergetic way to the Sentinel-4 development. Also, initial comparisons of the S5P and GEMS cloud products will be shown.

The Sentinel-4 payload is a multi-spectral camera system which is designed to monitor atmospheric conditions over Europe. The German Aerospace Center (DLR) in Berlin, Germany conducted the verification campaign of the Focal Plane Subsystem (FPS) on behalf of Airbus Defense and Space GmbH, Ottobrunn, Germany. The FPS consists, inter alia, of two Focal Plane Assemblies (FPAs), one for the UV-VIS spectral range (305 nm … 500 nm), the second for NIR (750 nm … 775 nm). In this publication, we will present in detail the opto-mechanical laboratory set-up of the verification campaign of the Sentinel-4 Qualification Model (QM) which will also be used for the upcoming Flight Model (FM) verification. The test campaign consists mainly of radiometric tests performed with an integrating sphere as homogenous light source. The FPAs have mainly to be operated at 215 K ± 5 K, making it necessary to exploit a thermal vacuum chamber (TVC) for the test accomplishment. This publication focuses on the challenge to remotely illuminate both Sentinel-4 detectors as well as a reference detector homogeneously over a distance of approximately 1 m from outside the TVC. Furthermore selected test analyses and results will be presented, showing that the Sentinel-4 FPS meets specifications.

The Advanced Ultraviolet Radiation and Ozone Retrieval for Applications (AURORA) is a three-year project supported by the European Union during the 2016-2019 period. The main goal of AURORA is to develop and test the coupling of a data fusion algorithm with state-of-the-art data assimilation systems in an end-to-end infrastructure that could be used as prototype for the exploitation of future Copernicus Sentinel-4 (S4) and Sentinel-5 (S5) atmospheric data. A simplified Observing System Simulated Experiment approach has been adopted in the project. Using MERRA-2 reanalysis as virtual truth, synthetic measurements have been generated to simulate the future S4 and S5 data in three spectral ranges (i.e. Ultraviolet, Visible and Thermal Infrared). Level 2 ozone products have then been derived for both platforms and the three spectral ranges. A data fusion technique, referred to as the Complete Data Fusion (CDF), has been applied as an a-posteriori method to combine the Level-2 products into a comprehensive and concise description of that atmospheric state. Both the Level-2 and fused ozone products have been assimilated in two state-of-the-art data assimilation systems: the ECMWF IFS and KNMI TM5. An incremental assimilation experiment design has been defined to assess the potential value of (1) assimilating fused data instead of the Level-2 retrievals, (2) using fused data from one geostationary platform in addition to polar orbiting-based fused data, and (3) exploiting cross-platform fused data instead of jointly assimilating fused products from multiple platforms. A collaboration with the American TEMPO and Korean GEMS geostationary satellite communities has been established in the AURORA project with the aim of evaluating the value of increasing the geostationary coverage from one (S4 only) to three. Off-line tropospheric ozone and surface UV products are also generated from the data assimilation ozone analyses and forecasts. A validation function has been developed to assess the quality of all the AURORA outputs against both independent observations and the MERRA-2 reanalysis used to derive the simulated measurements. This contribution will present the scientific developments undertaken within the AURORA project, and summarize its results.

We present an Observing System Simulation Experiment (OSSE) dedicated to the evaluation of the added value of the Sentinel-4 and Sentinel-5P missions for tropospheric nitrogen dioxide (NO2). Sentinel-4 is a geostationary (GEO) mission covering the European continent, providing observations with high temporal resolution (hourly). Sentinel-5P is a low Earth orbit (LEO) mission providing daily observations with a global coverage. The OSSE experiment has been carefully designed, with separate models for the simulation of observations and for the assimilation experiments and with conservative estimates of the total observation uncertainties. In the experiment we simulate Sentinel-4 and Sentinel-5P tropospheric NO2 columns and surface ozone concentrations at 7 by 7 km resolution over Europe for two 3-month summer and winter periods. The synthetic observations are based on a nature run (NR) from a chemistry transport model (MOCAGE) and error estimates using instrument characteristics. We assimilate the simulated observations into a chemistry transport model (LOTOS-EUROS) independent of the NR to evaluate their impact on modelled NO2 tropospheric columns and surface concentrations. The results are compared to an operational system where only ground-based ozone observations are ingested. Both instruments have an added value to analysed NO2 columns and surface values, reflected in decreased biases and improved correlations. The Sentinel-4 NO2 observations with hourly temporal resolution benefit modelled NO2 analyses throughout the entire day where the daily Sentinel-5P NO2 observations have a slightly lower impact that lasts up to 3–6 h after overpass. The evaluated benefits may be even higher in reality as the applied error estimates were shown to be higher than actual errors in the now operational Sentinel-5P NO2 products. We show that an accurate representation of the NO2 profile is crucial for the benefit of the column observations on surface values. The results support the need for having a combination of GEO and LEO missions for NO2 analyses in view of the complementary benefits of hourly temporal resolution (GEO, Sentinel-4) and global coverage (LEO, Sentinel-5P).

Remote sensing observations of the atmosphere are performed with instruments operating on space-borne and airborne platforms, as well as from ground-based stations and vertical profiles of atmospheric variables are often obtained with an inversion procedure from the observed radiances. When an instrument or several instruments observe more times the same portion of atmosphere, the information obtained from the different measurements can be combined in order to obtain a single vertical profile of improved quality with respect to that of the profiles retrieved from the single observations. The most comprehensive way to combine different measurements of the same quantity is considered to be the simultaneous retrieval, in which all the observations are used as inputs of a single retrieval algorithm that produces a single profile. However, recently a new method, referred to as Complete Data Fusion, was proposed that, with simple implementation requirements, provides products of quality equivalent to that of the simultaneous retrieval products. The use of the Complete Data Fusion highlighted a problem that we believe to be common to simultaneous retrieval and data fusion. The measurements that we wish to fuse often present some inconsistencies due to three causes: (i) the profiles to be fused (in the following referred to as fusing profiles) are represented on different vertical grids, (ii) a variability is present in the observed species and the fusing profiles refer to different times and space locations and (iii) the fusing profiles are affected by different forward model errors. These inconsistencies may spoil the quality of the fused profile. In order to apply the Complete Data Fusion method to inconsistent measurements without a degradation of the product, it is necessary to add to the error covariance matrix of each fusing profile a covariance matrix that takes into account the inconsistencies. Therefore, the main problem in the fusion of inconsistent measurements is the realistic estimate of these inconsistency covariance matrices. The value of the cost function, which is minimized in the Complete Data Fusion, depends on the inconsistency covariance matrices and can be used to establish some constraints on their amplitude. To this purpose, we have analytically calculated the expected value and the variance of the cost function and used these quantities to define a procedure that estimates the inconsistency covariance matrices. Modelling the inconsistency covariance matrices with one parameter, we determine the value of this parameter that makes the reduced cost function equal to its expected value and use the variance to assign an error to this determination. The use of the Complete Data Fusion will be particularly relevant for the analysis of the future atmospheric Sentinel missions of the Copernicus program. The amount of data that will be available from these missions will pose technical challenges to most applications and the Complete Data Fusion can be used to reduce the number of products while maintaining the information content of the full datasets. For this reason, we test the proposed procedure on simulated measurements of ozone profiles as they could be obtained by the Infrared Sounder operating in the thermal infrared on board the Meteosat Third Generation satellite in the framework of the Sentinel 4 mission. In this context, the procedure is used to estimate the coincidence covariance matrices that take into account the variability of ozone when the fusing profiles refer to different times and space locations. The merits of this new procedure in the case of this specific dataset are presented and discussed.

The Institute for Optical Sensor Systems was involved in many international space projects in recent years. These include, for example, the fokal plane array (FPA) of the hyperspectral sensors ENMAP or Sentinel-4, but also the FPA for the high resolution FPA for Kompsat-3. An important requirement of the customer is the measurement of the detector MTF for different wavelengths. A measuring station under clean room conditions and evaluation algorithms was developed for these measurements. The measurement setup consist of a collimator with slit target in focus for illumination at infinity, a gimbal mounted detector facing an auxiliary lens in front, a halogen lamp with monochromator or filter, as well as optical and electrical ground support equipment. Different targets and therefore also different measurement and data evaluation opportunities are possible with this setup. Examples are slit, edge, pin hole but also a Siemens star. The article describes the measurement setup, the different measuring and evaluation procedures and exemplary results for Sentinel-4 detector.

The exploitation of complementary and redundant information on atmospheric targets simultaneously observed by multiple independent data sources, such as the atmospheric Sentinels, has intrinsic potential to extend the quality of geophysical products beyond the limits already set by the challenging requirements of the Copernicus program for the atmosphere. Synergistic use of measurements from the Sentinel-5p satellite, launched on October 13th, 2017, from the Sentinel-4 and Sentinel-5 and their contributing missions MTG and MSG, can take advantage of different spectral coverage from the UV, to the Visible, to the Thermal Infrared region and observation geometry on Low Earth Orbit and Geostationary platforms. The use of simultaneous retrieval or data fusion techniques is fit for purpose of merging datasets from different measurements and combination with state-of-the-art modeling capabilities is offered by data assimilation systems. The Horizon 2020 project AURORA (Advanced Ultraviolet Radiation and Ozone Retrieval for Applications) is now investigating a novel approach based on sequential application of the innovative Complete Data Fusion technique and of data assimilation models (TM5, IFS and C-IFS) to synthetic data of Sentinel-4 and Sentinel-5 to obtain a unique product for the vertical profile of atmospheric ozone. First results from assimilation experiments conducted using the TM5 data assimilation systems applied to Sentinel-4 and Sentinel-5 standard retrieval products and to the corresponding fused profiles will be presented and discussed. An estimate of the impact of the advanced quality attained for the ozone profiles will be given for derived products such as tropospheric ozone column and ultraviolet surface radiation of immediate relevance for applications. The presentation will be finally focusing on the progress status of the activities for development of two applications - HappySun and AirPortal - using UV surface radiation and Ozone products from AURORA for demonstration purposes.

The Sentinel 4 instrument is an imaging spectrometer, developed by Airbus under ESA contract in the frame of the joint European Union (EU)/ESA COPERNICUS program with the objective of monitoring trace gas concentrations. Sentinel 4 will provide accurate measurements of key atmospheric constituents such as ozone, nitrogen dioxide, sulfur dioxide, formaldehyde, as well as aerosol and cloud properties. Sentinel 4 is unique in being the first geostationary UVN mission. The SENTINEL 4 space segment will be integrated on EUMETSAT's Meteosat Third Generation Sounder satellite (MTG-S). Sentinel 4 will provide coverage of Europe and adjacent regions. The Sentinel 4 instrument comprises as a major element two Focal Plane Subsystems (FPS) covering the wavelength ranges 305 nm to 500 nm (UVVIS) and 750 nm to 775 nm (NIR) respectively. The paper describes the Focal Plane Subsystems, comprising the detectors, the optical bench and the control electronics. Further the design and development approach will be presented as well as first measurement results of FPS Qualification Model.

The Sentinel-4 instrument is an imaging spectrometer, developed by Airbus under ESA contract in the frame of the joint European Union (EU)/ESA COPERNICUS program. SENTINEL-4 will provide accurate measurements of trace gases from geostationary orbit, including key atmospheric constituents such as ozone, nitrogen dioxide, sulfur dioxide, formaldehyde, as well as aerosol and cloud properties. Key to achieving these atmospheric measurements are the two CCD detectors, covering the wavelengths in the ranges 305 nm to 500 nm (UVVIS) and 750 to 775 nm (NIR) respectively. The paper describes the architecture, and operation of these two CCD detectors, which have an unusually high full-well capacity and a very specific architecture and read-out sequence to match the requirements of the Sentinel- 4 instrument. The key performance aspects and their verification through measurement are presented, with a focus on an unusual, bi-modal dark signal generation rate observed during test.

The Sentinel-4 payload is a multi-spectral camera system, designed to monitor atmospheric conditions over Europe from a geostationary orbit. The German Aerospace Center, DLR Berlin, conducted the verification campaign of the Focal Plane Subsystem (FPS) during the second half of 2016. The FPS consists, of two Focal Plane Assemblies (FPAs), two Front End Electronics (FEEs), one Front End Support Electronic (FSE) and one Instrument Control Unit (ICU). The FPAs are designed for two spectral ranges: UV-VIS (305 nm - 500 nm) and NIR (750 nm - 775 nm). In this publication, we will present in detail the set-up of the verification campaign of the Sentinel-4 Qualification Model (QM). This set up will also be used for the upcoming Flight Model (FM) verification, planned for early 2018. The FPAs have to be operated at 215 K ± 5 K, making it necessary to exploit a thermal vacuum chamber (TVC) for the test accomplishment. The test campaign consists mainly of radiometric tests. This publication focuses on the challenge to remotely illuminate both Sentinel-4 detectors as well as a reference detector homogeneously over a distance of approximately 1 m from outside the TVC. Selected test analyses and results will be presented.