To-date the European Space Agency (ESA) has developed and launched five Earth Explorer research missions (GOCE, SMOS, CryoSat, Swarm, and Aeolus), comprising 8 individual satellites across a spectrum of different scientific domains. Each mission employs an exciting combination of cutting-edge technology, novel observing techniques and constellations, enabling European scientists to make new Earth system discoveries and to reach beyond contemporary scientific frontiers.
Today ESA’s Earth Observation (EO) Envelope Programme supports three types of user-driven Research missions that address objectives of the ESA Earth Observation Science Strategy, each implemented under the responsibility of ESA:
i) Earth Explorer Core missions are research missions designed to address broad or complex scientific challenges. Mission ideas are selected from proposals submitted in response to a Call for Mission Ideas;
ii) Earth Explorer Fast-Track missions are smaller than the Core mission category and with shorter development cycle due to their higher initial technical/scientific maturity and more focused research objectives. Candidate mission concepts are selected from proposals in reply to a Call for full Mission Concept Proposals;
iii) Opportunity Missions respond to proposals for cooperation with entities outside Participating States, including flight opportunities for satellites or instruments, to address key objectives of the ESA Earth Observation Science Strategy.
Beyond the Earth Explorer missions in orbit, there are presently 3 further missions in development: EarthCARE, Biomass and FLEX, each of which are planned for launch in the coming years.
As part of ESA’s continuing commitment to realise cutting-edge satellite missions to advance scientific understanding of our planet and to show how new technologies can be used in space, Calls for Earth Explorer Fast Track and Core missions were held in 2016 and 2017, respectively. As a result of the selection process two new candidate Earth Explorer 9 Fast-track missions, FORUM and SKIM were selected in November 2017, and 3 news candidate Earth Explorer 10 mission ideas: STEREOID, Daedalus and G-CLASS were selected in September 2018.
The presentation will give an introduction to the Earth Explorer programme, past highlights and achievements, together with an insight into The Earth Explorer Call framework, the selection process (Evaluation), and the Advisory Committee on Earth Observation Selection recommendation and Programme Board Selection decision.

The Daedalus mission will investigate the key electrodynamics processes that determine the structure and composition of the upper layers of the atmosphere, the gateway between the Earth’s atmosphere and space. In particular, altitude ranges between 100 and 200 km remain to this date the most under-sampled regions of the Earth's atmosphere. An innovative mission design allows Daedalus to explore below 150 km in an unprecedented manner and access electrodynamics processes at lower altitudes. Furthermore, an innovation of the Daedalus mission concept is that it includes the release of sub-satellites at low altitudes: combined with the main spacecraft, these sub-satellites will provide multi-point measurements throughout the lower thermosphere - ionosphere region, down to altitudes below 120 km. Daedalus will perform in-situ measurements of neutral winds and ion drifts, neutral and ion composition, electric and magnetic fields, energetic particles, neutral and plasma density and temperature. These measurements will quantify the amount of energy deposited in the upper atmosphere via Joule heating and Energetic Particle Precipitation. Estimates of energy deposition currently vary by orders of magnitude, hindering current efforts to expand global climate models into the thermosphere. Together with its importance for global energy balance processes, this altitude range is critical in understanding the mechanisms that produce Geomagnetically Induced Currents (GICs) and ionospheric scintillation of Global Navigation Satellite System (GNSS) signals, as well as for resolving uncertainties in orbital calculations, vehicle re-entry and space debris lifetimes.

STEREOID (Stereo Thermo-Optically Enhanced Radar for Earth, Ocean, Ice, and land Dynamics) is one of the three mission proposals selected as Earth Explorer 10 candidates. If implemented, STEREOID will dramatically augment the capabilities of the Sentinel-1 mission by flying two identical sub-500 kg- class spacecraft carrying a receive-only radar instrument as main payload that will flying in a re-configurable formation with Sentinel-1D, which will be used as illuminator. STEREOID is conceived as a multipurpose mission, that will exploit its geometric diversity to help precisely quantify small scale motion and deformation fields of the ocean surface, glaciers and ice sheets, and solid Earth, aiming at providing modellers with data required to better understand dynamic processes in these three domains. The radar measurements will be enhanced and supported by a medium resolution dual VNIR and TIR payload. The mission will be organized in several phases, associated to distinct formation configurations. In our preliminary concept, the mission will start and end with a cross-track interferometric configuration, with both STEREOIDs flying in close formation, aimed at quantifying volume changes of ice masses and unstable grounds. In between, the two STEREOIDs would fly an extended period of time in a so-called StereoSAR configuration, with one spacecraft about 300 km ahead and the second one the same distance behind Sentinel-1. This configuration will maximize the sensitivity to surface motion vectors. STEREOID blends elements from the StereoSAR, SESAME, and PICOSAR mission concepts.

STEREOID has scientific objectives relating to the cryosphere, oceans, and solid Earth. For the cryosphere, STEREOID will fill major gaps in observations of glacier mass balance, and provide data necessary to model and understand the dynamics of sea ice and Marginal Ice Zone processes.

For the oceans, the mission will focus on processes with scales smaller than 10 km, in particular in coastal regions. STEREOID will aim at providing high resolution and information rich data sets that are needed to validate and drive the development of numerical models coupling sea, ice and the atmosphere. In addition, STEREOID will provide an unprecedented opportunity to study extreme weather events, such as Tropical Cyclones, by providing high-resolution estimates of winds and the gradients of surface currents within the phenomena.

For solid Earth, STEREOID will extend differential InSAR techniques to provide 3-D deformation fields related to co-seismic, inter-seismic, and post-seismic deformation; tectonic rifting; volcanism; and landslides. In addition, during the cross-track interferometric phases, STEREOID will allow quantifying topographic changes associated to, for example, volcanic eruptions and landslides.

STEREOID will also allow the exploration, for the first time, of the information content present in true multistatic data.

The main payload of the STEREOIDs will be a passive SAR receiver. In our current concept, it will feature two small antennas, spaced approximately 4.5 m in the flight direction. The echoes received from both antennas will be stored and processed on-ground. This solution allows following the TOPS azimuth steering a posteriori, while providing an adequately shaped two-way antenna pattern, which is necessary both for ambiguity suppression and for Doppler-Centroid estimation. In addition, the two channels can be treated as a short-baseline ATI system. Despite the limited baseline, under favourable conditions, ATI processing may significantly outperform Doppler Centroid Anomaly estimates.

The water cycle is fundamental to human society and to life on Earth. Despite its importance, there are still aspects which are poorly understood and for which we only have limited measurements. It is increasingly recognised that improved temporal sampling is needed for future space missions, so that processes on timescales of minutes to hours can be observed directly. From space, this could be achieved using a large constellation of satellites in conventional low orbits, or a few satellites in geosynchronous orbit (GEO). A single GEO satellite can view Europe and Africa continuously, and is the concept for the G-CLASS proposal. Of the imaging bands available, microwave imagers (radar) are particularly good for water both at the surface (radar backscatter measurements) and in the atmosphere (using radar interferometry), and can provide all-weather imaging 24 hours per day. Using synthetic aperture radar it is possible to achieve resolution as good as 20 m from GEO. To improve understanding of the diurnal water cycle and related societal concerns such as floods, landslides and water resources, we therefore propose a GEO synthetic aperture radar mission: G-CLASS.

Specific water cycle science objectives for the mission are:

1. Improve prediction capability for intense storms and related impacts (flood, landslides),
2. Improve understanding of the diurnal water cycle, especially soil moisture in dry regions and snow-melt / re-freeze in mountain areas,

As weather prediction capability improves we need commensurate high resolution observations (in space and time) to initialise and validate the predictions. A GEO radar with its continuous viewing capability can potentially provide these. Diurnal forcing of soil moisture and snow state through the day are important for hydrology, and affect water resource management and agriculture. The radar measurements also enable general ground motion to be monitored, thus enabling a third science objective at no extra cost to the mission:

3. Enable near-real-time prediction of ground motion and response management (landslides, earthquakes, volcanoes).

The baseline mission concept uses a Vega-C launch followed by an orbit-raising phase to reach GEO. A standard small-GEO satellite (mass around 2 tonne, few kW electrical power) is suitable, with a C-band radar (300-400 W RF), compact polarimetry and a 7 m diameter deployable reflector. Beam-steering is achieved by slewing the satellite: the whole Earth disk is only 16° across so small manoeuvres are used and require no propellant. Coverage is controlled by pointing and is therefore almost independent of the orbit.

G-CLASS will be the first mission to provide continuous observations of key diurnal processes. It will simultaneously observe the land surface and overlying atmosphere, and powerfully complements existing EO missions. The temporal resolution opens up significant new opportunities – only some of which we can currently predict. G-CLASS has great potential for new science, especially of the diurnal water cycle, and enables applications with important societal impact.