The geophysical validation by independent means provides fundamental end-to-end information on the overall data quality. A comprehensive validation program is an inherent component of Operational Earth Observation missions. The program includes both, validation using ground-based measurements as well as inter-comparisons with other spaceborne missions. They give information on instrument characterization and calibration as well as on the performance of its processing algorithms and thus final geophysical products. Ground observations enable estimation of product accuracies, and development of regional atmospheric models and independent time series, as well as generic algorithms. The mission inter-comparisons are important to derive instrument on-orbit characterization not available from ground point collocations, such as the consistency of long time series spanning several independent and possibly not overlapping missions which are needed for climate research. ESA is committed to performing the validation of its products, including atmospheric chemistry and physics data. Detailed results for ENVISAT’s GOMOS, MIPAS, SCIAMACHY and MERIS instruments will be presented. Several targeted activities are supported that cover the validation by ground based instruments, lidars, balloons, Radio Occultation and other satellite instrumentation, as well as the inter-calibration of lidar systems in the CEOS context. Long term strategies are being developed that shall allow the validation of independent missions and are covering types of geophysical measurements rather than individual instruments or missions. In this context, CEOS/WGCV is proposing a community derived process that establishes an international quality assurance framework to facilitate harmonisation and interoperability of EO data, QA4EO.

The Earth System behaves as a highly coupled system comprising physical, chemical, biological, and anthropogenic components and processes with complex interactions and feedbacks between them. Climate change is arguably the greatest challenge to balance in the Earth system. The Paris Agreement (UNFCCC, 2016) recognised the need to reduce the risks from and impacts of climate change and called for the increase in the global average temperature to be held well below 2 °C above pre-industrial levels, with the ideal aim being to limit it to 1.5 °C. The UN Framework Convention on Climate Change (UNFCCC) agreement is based on the evidence for, and likely causes of, climate change synthesised by the Intergovernmental Panel on Climate Change (IPCC) and is supported, for climate, by the Global Climate Observing System (GCOS). GCOS has defined a set of Essential Climate Variables (ECV), established the requirements for their systematic observation, and the development of data archives, needed to support the study the climate system. The Climate Change Initiative (CCI), represents the contribution by the European Space Agency (ESA) to GCOS. CCI is a programme designed to bring together European expertise in Earth Observation with that from the climate research community to address those ECV that can be generated using satellite observations. Specifically the objective is ‘to realise the full potential of the long-term global Earth Observation archives that ESA together with its Member States have established over the last thirty years, as a significant and timely contribution to the ECV databases required by the United Nations Framework Convention on Climate Change (UNFCCC)’. In doing so the intended legacy of the programme is to put in place mechanisms capable of providing long term, fully traceable, and transparent access to its records. This paper provides an overview of the CCI Programme and highlights a few of its achievements to date.

Climate Data Records of soil moisture are fundamental for improving our understanding of long-term dynamics in the coupled water, energy, and carbon cycles over land. To respond to this need, in 2012 the European Space Agency (ESA) released the first multi-decadal, global satellite-observed soil moisture (SM) dataset as part of its Climate Change Initiative (CCI) program. This product, named ESA CCI SM, combines various single-sensor active and passive microwave soil moisture products into three harmonised products: a merged ACTIVE, a merged PASSIVE, and a COMBINED active+passive microwave product. Compared to the first product release, the latest version of ESA CCI SM includes a large number of enhancements, incorporates various new satellite sensors, and extends its temporal coverage to the period 1978–2015. In this study, we first provide a comprehensive overview of the characteristics, evolution, and performance of the ESA CCI SM products. Based on original research and a review of existing literature we show that the product quality has steadily increased with each successive release and that the merged products generally outperform the single-sensor input products. Although ESA CCI SM generally agrees well with the spatial and temporal patterns estimated by land surface models and observed in-situ, we identify surface conditions (e.g., dense vegetation, organic soils) for which it still has large uncertainties. Second, capitalising on the results of >100 research studies that made use of the ESA CCI SM data we provide a synopsis of how it has contributed to improved process understanding in the following Earth system domains: climate variability and change, land-atmosphere interactions, global biogeochemical cycles and ecology, hydrological and land surface modelling, drought applications, and meteorology. While in some disciplines the use of ESA CCI SM is already widespread (e.g. in the evaluation of model soil moisture states) in others (e.g. in numerical weather prediction or flood forecasting) it is still in its infancy. The latter is partly related to current shortcomings of the product, e.g., the lack of near-real-time availability and data gaps in time and space. This study discloses the discrepancies between current ESA CCI SM product characteristics and the preferred characteristics of long-term satellite soil moisture products as outlined by the Global Climate Observing System (GCOS), and provides important directions for future ESA CCI SM product improvements to bridge these gaps.

Since 17 th January 2001 a gyro-less Attitude and Orbit Control System (AOCS) is used to pilot the ERS-2 satellite. The scope of this new AOCS is to increase the mission safety, after the lost of 5 of the 6 gyroscopes. With this new AOCS configuration (named as: Zero Gyro Mode – ZGM) the satellite attitude is degraded in particular for the yaw angle. The antenna mispointing could not be corrected in the existing ground processor, which assumed a very high accuracy in the satellite attitude (± 0.2o). As consequence the backscattering coefficients derived from the returned echoes are not calibrated anymore. For that reason a complete review of the Scatterometer processor became necessary to insure the continuity of the Scat ERS-2 mission with the nominal high data quality.The scope of this paper is to present the impact of attitude error angles in the Scatterometer data quality. The paper also addresses the main reasons of that degradation (large frequency shift of the returned echoes, bandwidth of the on board receiver) and gives the first input for a review of the ERS-2 Scatterometer ground processing.