The ability to acquire new insight into Earth-system processes poses significant scientific and technical challenges. The European Space Agency (ESA) is taking on these challenges within its line of scientific user-driven research missions the Earth Explorers. To date ESA has developed and launched five such Earth Explorer research missions (GOCE, SMOS, CryoSat, Swarm, and Aeolus), and is currently developing a further three (EarthCARE, Biomass and FLEX).
This series of pioneering Earth Explorer satellite missions represents the epitome of Europe’s technical endeavour in conceiving and realising new Earth-observing capabilities. Each of these research missions offers new innovative measurement techniques to explore and understand different aspects of the Earth system. Meanwhile, the guiding principle remains to define, develop and operate these Earth Explorer missions in close cooperation with the scientific community to address the most critical Earth-science questions in as comprehensive and effective a manner as possible.
Two Earth Explorer 9 candidates, FORUM and SKIM, selected by ESA member states in 2017 currently compete to become the ninth Earth Explorer mission, each addressing critical Earth science challenges. The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) will uniquely measure Earth’s outgoing radiation in the 15 – 100 micron range of the electromagnetic spectrum, whilst the Sea-surface Kinematics Multiscale monitoring (SKIM) measures surface currents and the dynamics of the upper ocean.
These two Earth Explorer Fast-Track missions are presently being prepared for development over a compressed timescale, owing to their higher initial technical/scientific maturity and more focused research objectives. Upon completion of the ongoing feasibility assessment in 2019, a User Consultation meeting will take place in Cambridge, UK, on which basis the Advisory Committee on Earth Observation will advise and recommend one mission for implementation, with a launch foreseen in the 2025 timeframe.
This presentation will give an insight into to the process of Fast Track mission process: from the Call for full Mission Concept Proposals, the Proposal evaluation process, the Advisory Committee on Earth Observation Selection recommendation though to the Programme Board decision for full implementation of a single Earth Explorer 9 mission.

It has long been acknowledged that the far infrared (far-ir: defined here as wavelengths > 15 μm) plays a pivotal role in determining our planet’s energy budget. Under clear-sky conditions the region contributes of the order a quarter to one-third of the Earth’s Greenhouse Effect. Under all-sky conditions its influence on the Earth’s emitted energy is even larger, with the typically colder emitting temperatures of mid and high level cloud shifting the peak of the Planck function to longer wavelengths such that, in the global mean, approximately half of the Earth’s emission to space occurs within the far-ir.

The dominant role of the far-ir in determining the Earth’s Outgoing Longwave Radiation (OLR) is in part due to the strong water vapour rotation band at wavelengths > 16.5 μm. This in turn means that radiative emission in the far-ir is particularly sensitive to water vapour in the climatically important upper troposphere/lower stratosphere (UTLS) region. Similarly, clear-sky longwave radiative cooling through the mid and upper troposphere is dominated by the contribution from the far-ir.

While for much of the globe this water vapour absorption means that far-ir surface emission cannot be sensed from space, in very dry, clear-sky conditions experienced in the Arctic and Antarctica this is no longer the case. Here the outgoing longwave radiation becomes sensitive to surface emission within the far-ir through a number of micro-windows which become progressively more transmissive as water vapour concentration reduces. Very recent studies show that this previously unexplored phenomenon may play a critical role in both reducing persistent climate model biases and determining the pace of Arctic and Antarctic climate change.

Moreover, cirrus clouds, typically poorly constrained by observations yet crucial players in determining current and future climate, have emitting temperatures that place the peak of their radiative emission within the far-ir. Our ability to correctly simulate the interaction of the radiation spectrum with cirri relies on the capability of optical models to adequately represent their scattering and absorption properties. These are crucially dependent on the complex ice-crystal shapes and their size distributions within the clouds. Recent advances in cirrus cloud optical modelling have attempted to capture the bulk microphysical properties of cirri spanning the entire electromagnetic spectrum. However, while there are many satellite observations of the reflected visible and emitted near- and mid-infrared radiation in the presence of cirrus clouds that can be exploited to test these developments, there are no such observations that span the far-ir. This represents a major barrier to improving our confidence in our ability to understand and monitor cirrus properties and their interaction with the Earth’s outgoing longwave energy, particularly since the contrast in ice and water refractive indices between the far-ir and mid-infrared implies that unique information relating to cloud classification and microphysics can be leveraged from measurements of the far-ir spectrum.

Despite the sensitivity of the far-ir to these key climate drivers, to date there has been no dedicated satellite mission to observe the complete, radiatively important far-ir spectrum. Those space-based data which do extend into the far-ir, from the InfraRed Interferometer Spectrometer (IRIS-D) mission, only reach 25 μm and are limited in temporal extent from April 1970 to February 1971. Moreover, there are questions regarding their suitability for quantitative climate related studies. From a purely energetics viewpoint the absence of a dedicated far-ir mission constitutes the major missing piece of our climate observing system.

Recognising this, the European Space Agency (ESA) has selected the Far infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission as one of two candidates for their Earth Explorer 9 programme. In this talk we will discuss the scientific motivation for the mission, presenting results from a number of studies which have helped to shape the design concept. In particular, we will demonstrate how FORUM will advance Earth system science by observing, for the first time, with high accuracy, the far-infrared spectrum of the Earth, a region which plays a fundamental role in determining our planet’s energy balance.

The primary goal of the FORUM mission is to measure the Earth’s top-of-atmosphere emission spectrum in the 100 to 1600 cm-¹ spectral region covering the observational gap of the Far IR (100-667 cm-¹), which has never been observed from space in its entirety. This measurement will provide an improved understanding of the climate system by supplying, for the first time, most of the spectral features of the Far IR contribution to the Earth radiation budget, particularly focusing on the water vapour contribution related to the continuum absorption in the rotational band, cirrus cloud and ice/snow surface emissivity.

This goal is achieved by measuring the spectrum with a nominal resolution of 0.3 cm-¹, and with an absolute calibration accuracy of 0.1 K, benchmarked against international standards. A precision of 0.4 mW/m² sr cm-¹ on the radiance will also allow the retrieval of the atmospheric state and cloud parameters with improved accuracy.

A demonstration of the FORUM capabilities in improving the characterisation of the atmospheric radiative properties using the Far IR is shown by means of ground-based observations performed at high-altitude sites where the air is sufficiently transparent to sound the Far IR spectral features of the atmospheric emission.

To this aim, two instruments are used: the REFIR-PAD (Radiation Explorer in the Far InfraRed – Prototype for Applications and Development) instrument, which has been deployed on the Antarctic Plateau, where it has been acquiring downwelling longwave radiation spectra continuously since 2012, and the FORUM prototype, which will be deployed at the Zugspitze summit at the end of 2018 - early 2019.
Both instruments are Fourier transform spectrometers with an optical configuration of the interferometer similar to FORUM. They cover the Far IR spectral region down to 100cm-¹ and can be operated under all sky non-precipitating conditions.

In this presentation, a review of the measurement requirements of the FORUM mission will be provided with a description of the measurement results obtained during the high-altitude Antarctic and Zugspitze campaigns.

SKIM is an ESA Earth Explorer-9 candidate mission designed to measure directly and simultaneously, for the first time, directional ocean surface current vector (OSCV) and ocean wave spectra. SKIM builds on satellite altimetry, including a nadir altimeter beam, and the first ocean wave spectrometer SWIM on CFOSAT. Using rotating beams across a 330 km swath, SKIM will explore beyond the 200 km - 15 days resolution of ocean currents that can be diagnosed today from sea level at mid latitudes.

In particular SKIM will fill two important blind spots: in the tropics and in the marginal ice zone, and expand the effective space and time resolution of the altimeter constellation by a factor 2 or more. The novel direct measurement of surface currents in the top two meters will produce the first maps of the equatorial upwellings that are critical for understanding and forecasting the heat budget at the equator with far-reaching weather and climate consequences, for example on the African monsoon. OSCV maps will also allow the first monitoring of the highly dynamic currents at the ice edge. Adding this new and fundamental variable to Earth Observation capability together with high fidelity measurements of wave spectra will allow scientists to address a wide range of questions, including:
How OSCV and waves influence upper ocean mixing and large scale circulation?
How do OSCV and waves influence the dynamics of the ice edge in the Arctic and Antarctic?
What are the roles of eddies, wind-driven flows and waves in setting the surface concentration of marine litter and shaping marine ecosystems?

This presentation will explain how SKIM measurements will be used to address these scientific challenges using examples from mission and instrument simulator outputs.

SKIM is an ESA Earth Explorer-9 candidate mission designed to measure directly and simultaneously, for the first time, the magnitude and direction of Ocean Surface Current Vector (OSCV) and Waves. SKIM will characterise and quantify the impact of these measurements on upper ocean and lower atmosphere processes and at the global and regional scale. It will reduce uncertainty in our understanding of OSCV and wave regime shifts, atmosphere-ocean exchange of heat, gas (CO2), biology and momentum, poorly measured equatorial current systems, upper ocean heat budget, kinetic energy fluxes and vertical motion in the upper ocean, the role of OSCV and waves in the rapidly changing Arctic and Antarctic regions, better mapping of marine pollutant drift (plastics, oil).

The SKIM candidate mission would implement a SKIM Ka-band (35.75 GHz) Radar (SKaR) instrument which is a unique multi-beam, Doppler-enabled, conically scanning Ka-band Radar wave scatterometer. SKaR includes a nadir beam and 7 off-nadir beams (incidence angles of 12°and 6°) that are rotated about the nadir point. Each off-nadir beam footprint projected onto the sea surface is an ellipse with a width of ~6 km. The range resolution of SKaR will be ≤ 4 m at an incidence angle of 12°. Beams are fired in sequentially with only one beam active at any moment. The mean Line of Sight (LOS) radial velocity is determined from the phase difference between two consecutive echoes measured by each off-nadir rotating beam (pulsed-pair processing). The incidence angles are chosen to maximise the swath width while provide a sufficient density of measurements across the swath as they rotate.

SKIM will have a swath of ~300 km and provide coverage of the global ocean and inland seas up to 82° N and S and a revisit time of ≤10 days at the equator. In this configuration as the satellite flies forwards a spiral-like beam ground track pattern emerges as each beam sweeps across the instrument swath with many crossover points between different beams. OSC vectors are derived by combining individual beam LOS radial-velocity measurements (after compensation for the vertical velocities associated with waves) at the same location but in different LOS azimuth direction and slightly different times. Nadir altimetry using un-focussed SAR along-track altimetry (non-ambiguous – PRF=32kHz) is used to generate range-Doppler maps to estimate the Sea Surface Height, sigma0 and the significant wave height similar to conventional nadir altimetry (e.g. Sentinel-3, Sentinel-6). The random range noise for the nadir beam shall be ≤1.0 cm at a significant wave height of 2 m (1 σ) for a 1 second average. Wave Doppler scatterometry using off-nadir beams is used to generate directional ocean wave spectra from backscatter modulation and incoherently averaged power waveforms. The wave velocity spectrum is derived from the phase modulation spectrum. An experimental delta-K algorithm is also included to study wave dispersion relationships/power spectra and explore their relationship to ocean surface currents.

A range of data products will be generated by the mission including nadir and off-nadir L1B radar measurements, nadir L2 altimetry products, L2 beam-specific LOS and wave spectrum measurements, single and multi-pass gridded L2 OSC and 2D wave spectra.

This presentation will review the scientific and technical aspects of the EE9 SKIM Candidate Mission highlighting the scientific measurement approach and proposed technical implementation of the candidate mission.