Five ESA Earth Explorer (EE) research missions have been prepared, developed, launched and successfully operated over the last decade, each providing new observations and scientific insights to the science community which are revolutionising our understanding of the Earth system. Meanwhile four new Earth Explorers EarthCARE (EE6), Biomass (EE7), FLEX (EE8) and FORUM (EE9) are undergoing development, and a further five mission concepts: candidate Harmony (EE10); and four new EE11 candidates - are presently undergoing competitive preparatory phase study activities.

Today ESA’s Future Earth Observation (FutureEO) Programme supports science-driven Research missions motivated by the EO EUROPE 2040 Earth Observation Strategy and the EO Living Planet Challenges, with the primary objective to deliver cutting-edge Earth Explorer missions, ground-breaking Earth observation capabilities, and excellence in Earth science. The categories of user-driven Research missions are: Earth Explorers, Missions of Opportunity and Scout Missions.

With a renewed strategic commitment to provide regular opportunities for new research mission proposals, in May 2020 ESA released a new Call for proposals for Earth Explorer 11 mission ideas. Upon completion of the scientific, technical and programmatic evaluation of all proposals coming from Europe's satellite remote sensing community, in June 2021 the four Earth Explorer 11 mission candidates CAIRT, Nitrosat, SEASTAR, and WIVERN were selected by ESA Member States to undergo competitive preparatory study activities. Each of these new science-driven Earth Explorer missions employs pioneering new observing techniques made possible by novel technology, enabling European scientists to make new Earth system discoveries and to fulfil the scientific ambitions and needs of the European Earth observation user community through the next decade.

The presentation will give an insight into the background and the journey of the four Earth Explorer mission proposals from submission to the present day, and will chart the next steps which the Candidates will take, en-route to selection of the Earth Explorer 11 mission for implementation in 2025. This talk sets the scene for the four mission presentations before placing the spotlight on each of the candidates: CAIRT, Nitrosat, SEASTAR, and WIVERN.

The nitrogen cycle has been heavily perturbed due to ever growing agriculture, industry, transport and domestic production. It is believed that we now have reached a point where the nitrogen biochemical flow has exceeded its planetary boundary for a safe operating zone. This goes together with a cascade of impacts on human health and ecosystems. To better understand and address these impacts, there is a critical need to quantify the global nitrogen cycle and monitor its perturbations on all scales, down to the urban or agricultural source. The Nitrosat concept, which was preselected recently in the framework of ESA’s Earth Explorer 11 call and is entering Phase0 activities, has for overarching objective to simultaneously identify the emission contributions of NH3 and NO2 from farming activities, industrial complexes, transport, fires and urban areas. The specific Nitrosat science goals are to: Quantify the emissions of NH3 and NO2 on the landscape scales, to expose individual sources and characterize the temporal patterns of their emissions. Quantify the relative contribution of agriculture, in its diversity of sectors and practices, to the total emissions of reactive nitrogen. Quantify the contribution of reactive nitrogen to air pollution and its impact on human health. Constrain the atmospheric dispersion and surface deposition of reactive nitrogen and its impacts on ecosystems and climate; and contribute to monitoring policy progress to reduce nitrogen deposition in Natura 2000 areas in Europe. Reduce uncertainties in the contribution of reactive nitrogen to climate forcing, atmospheric chemistry and interactions between biogeochemical cycles. To achieve these objectives, Nitrosat would consist of an infrared Imaging Fourier Transform Spectrometer and a Visible Imaging Pushbroom Spectrometer. These imaging spectrometers will measure NH3 and NO2 (respectively) at 500 m, which is the required spatial scale to differentiate, identify and quantify the main point and area sources in a single satellite overpass. Source regions would be probed from once a week to once a month to reveal the seasonal patterns. Combined with air quality models, assimilation and inverse modelling, these measurements would allow assessing the processes that are relevant for the human disruption of the nitrogen cycle and their resulting effects, in much more detail than what will be achieved with the satellite missions that are planned in the next decade. In this way, Nitrosat would enable informed evaluations of future policies on nitrogen emission control. This presentation will detail the mission concept, provide first results from the Phase 0 scientific studies and from supporting aircraft campaigns.

To improve our knowledge of the coupling of atmospheric circulation, composition and regional climate change, and to provide the urgently needed observations of the on-going changes and processes involved, we have proposed the Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT), selected for Phase 0 as one of four candidates for Earth Explorer 11. There is growing evidence that the global atmosphere is changing throughout its entire depth from the surface to the fringes of space due to anthropogenic emissions of greenhouse gases, pollutants, aerosol precursors, and the recovery from ozone-depleting substances. Changes in atmospheric composition are closely coupled with changes in circulation and together affect surface climate, weather and air quality. CAIRT will be the first limb-sounder with imaging Fourier-transform infrared technology in space. By observing simultaneously the atmosphere from the troposphere to the lower thermosphere (about 5 to 115 km altitude), CAIRT will provide global observations of ozone, temperature, water vapour, as well as key halogen and nitrogen compounds. Observing nitrogen oxides from the stratosphere up to the lower thermosphere will help to better constrain the coupling with the upper atmosphere, solar variability and space weather. Observation of long-lived tracers (such as N2O, CH4, SF6, CF4) will provide information critical on transport, mixing and circulation changes. CAIRT will deliver essentially a complete budget of stratospheric sulfur (by observations of OCS, SO2, and H2SO4-aerosols), as well as observations of ammonia and ammonium nitrate aerosols. Biomass burning and other pollution plumes, and their impact on ozone chemistry in the UTLS region, will be detected from observations of HCN, CO and a further wealth of volatile organic compounds. The potential to measure water vapour isotopologues will help to constrain water vapour and cloud processes and interactions at the Earth’s surface. The high-resolution measurements of temperature will provide the momentum flux, phase speed and direction of atmospheric gravity waves. CAIRT thus will provide comprehensive information on the driving of the large-scale circulation by different types of waves. Tomographic retrievals will provide temperature and trace gas profiles at a much higher horizontal resolution and coverage than achieved from space so far. Flying in loose formation with the Second Generation Meteorological Operational Satellite (MetOp-SG) will enable combined retrievals with observations by the New Generation Infrared Atmospheric Sounding Interferometer (IASI-NG) and Sentinel-5, resulting in consistent atmospheric profile information from the surface up to the lower thermosphere. Our presentation will give an overview of the proposed CAIRT mission, the science to be addressed and first results from the ongoing Phase 0 science study and campaign activities.

The proposed EE11 mission WIVERN (a WInd VElocity Radar Nephoscope) will launch a conically scanning 94GHz Doppler radar in a 500 km orbit with a sample swath width of 800km and should provide global in-cloud winds line-of-sight winds from the Doppler shift of the radar returns and estimates of cloud properties and precipitation from the reflectivity both with a vertical resolution of about 650m. The successful Aeolus mission has demonstrated that assimilating Doppler line-of-sight lidar winds from the clear sky and from cloud tops leads to a significant reduction in forecast error, so we envisage that assimilating the in-cloud winds from WIVERN would lead to further improvement in the forecasts. The WIVERN radar footprint on the ground is of size about 1km2 and if the antenna scans with a period of 5 seconds during which time the satellite moves 35km along track, then the radar footprint will trace out a cycloid on the surface of the Earth and visit every box of size 30 by 30km on the Earth’s surface at latitudes below 80deg on average once a day. These winds would be representative of the large-scale flow and be suitable for data assimilation. Changes of line-of-sight winds on the km scale should provide a statistical measure of the vertical convective motions that should be useful for validating the representation of convection in NWP and climate models. The global profiles of reflectivity should provide information on ice water content and precipitation rates.

Doppler observations from a moving satellite are challenging. The Doppler shift is usually detected by the “pulse-pair” technique, that is to say the phase change in the radar return from two successive transmitted pulses. A 94GHz frequency is necessary to have a 1km sized footprint on the ground. The wavelength is only 3.2mm, so a phase change of +/- 180degs will be detected if the particle moves 800um in the time between the two pulses, so to detect a maximum unambiguous line-of sight velocity of 40m/s, the pulse separation must be just 20us, or only 3km in range. To avoid the problem of identifying the returns from two pulses with 3km separation, we adopt the “polarisation diversity pulse pair” (PDPP) approach whereby one pulse is H polarised and the other V polarised. The pulses are effectively “labelled” H and V and, provided there is no cross talk between the H and V pulses on transmission, reflection and reception , the high line-of-sight winds encountered in the atmosphere can be reliably measured. For the highest wind speeds there will be a single fold, but experience with ground-based radars is that there are reliable techniques to unfold a single fold. The PDPP technique with 94GHz radars has been implemented on an aircraft and on the ground. Extensive observations with these two systems confirm the accuracy of the technique. This performance has also been confirmed with a comprehensive end-to-end WIVERN simulator that has been developed.

Using the well- established Doppler theory and its validation using wind observations with the new PDPP technique, we can predict that, for WIVERN, the precision of the wind should be better than 2 m/s for reflectivities about -20dBZ and the 20km horizontal integration needed to acquire winds representative of the large scale flow. Analysis of the global climatology of reflectivity derived from CloudSat indicates that WIVERN should acquire over one million line-of-sight winds per day.

Small-scale (below 10km) dynamical features at the surface of the ocean are ubiquitous and play an important role in vertical exchanges of heat, gases and freshwater between the surface layer and the ocean interior, horizontal transport and dispersion pathways and interactions between the atmosphere, ocean, land and cryosphere. By modulating the transport of nutrients, they also impact significantly on marine biogeochemistry, the growth of phytoplankton and the marine food chain. The characteristic swirls, filaments and eddies are seen frequently in high resolution images of sea surface temperature and ocean colour. However, direct measurements of dynamics at such short scales (known as ocean submesoscales) are very scarce, and no existing satellite can measure these scales with the resolution and precision that would be needed for their full characterisation.

Synoptic 2D observations of dynamics at these scales are needed to improve knowledge of the relevant ocean surface processes and benefit ocean and climate modelling. Numerical models predict that ocean dynamics change dramatically around 1-10km scales, with enhanced air-sea coupling through wind/wave/current interactions and enhanced upper ocean mixing & vertical transport. Observations of submesoscale dynamics would improve the representation of these phenomena in models and improve forecasts and projections at multiple spatial and temporal scales, including basin-wide and climate scales. The need for new high-resolution observations is particularly critical for total surface current vectors and wind vectors in coastal and shelf seas and marginal ice zones (MIZs). New satellite data are urgently needed to validate and improve numerical models of ocean and atmospheric circulation, ocean waves and sea ice that currently ignore or misrepresent these small-scale phenomena.

SEASTAR is an Earth Explorer 11 candidate mission that addresses the scientific need for observing small-scale ocean dynamics by proposing to measure, for the first time, 2-D fields of total surface current vectors and wind vectors at 1 km resolution over a wide swath with high accuracy. A key objective of SEASTAR is to characterise, for the first time, the magnitude, spatial characteristics, regional extent and temporal variability on daily, seasonal to multi-annual time scales, over all coastal seas, shelf seas and MIZs. As such SEASTAR should allow to investigate the relations between small-scale dynamics, air-sea interactions, vertical processes and marine productivity, using synergy with high-resolution satellite data from optical, thermal and microwave sensors. Most importantly, new observations from SEASTAR would enable the validation of high-resolution and coupled models and support the development of new parameterisations to improve operational forecasts and reduce uncertainties in climate projections.

SEASTAR is currently being investigated under EE11 Phase 0 science and industrial activities. SEASTAR is a three-beam along-track SAR interferometer that observes the motion of the ocean surface by exploiting the doppler shift between two SAR images of the same scene taken within a short time-lag. SEASTAR has a three-beam configuration that includes two pairs of interferometric antennas looking 45° forward and 45° backward of broadside, and a dual-polarisation broadside beam that provides a third look in azimuth.

The presentation will outline the key elements of the mission and the latest status of the mission concept evolution, with the technical solutions and trade-offs that are being considered. The talk will also report on ESA-funded science activities to consolidate user requirements and the current assessment of the Scientific Readiness Level of the mission. Other planned activities, including flight campaigns with the OSCAR airborne demonstrator and numerical end-to-end simulator developments, are expected to provide a comprehensive assessment of the suitability and benefits of the mission in preparation for the selection by ACEO and ESA in late 2023 of two of the four EE11 candidates to enter Phase A.