1. Introduction
This abstract presents the SaTellite RAdar sounder for earTh sUbsurface Sensing (STRATUS) ‎[1], which is a proposed satellite mission for Earth Observation (EO) with onboard an instrument capable of probing the Earth's subsurface. STRATUS is based on a radar sounder (RS) with the unique capability to obtain continuous and large-scale subsurface measurements, with homogeneous and consistent quality. It aims to address the main scientific challenges in the current Living Planet Programme in two of the least characterized and crucial terrestrial frontiers: globally on the polar ice sheets, i.e., Greenland and Antarctica (primary objective), and regionally on the arid areas and deserts (secondary objective). They jointly represent ~20% of the Earth's continental surface ‎[2] and exhibit different responses to climatic stresses that remain largely unquantified as the available subsurface measurements are too sparse to effectively capture a large number of surface observations of both the ice sheets dynamic and the geographical expansion of deserts (Figure 1). Thus, STRATUS is a groundbreaking exploratory mission providing new fundamental data and has an expected high and genuine scientific return enabling us to tackle the deficiencies mentioned above in assessing the signature of climate change.
Over the past decades, ground penetrating radars (GPR) and RS instruments operated on the ground or airborne platforms have been used to perform local and regional measurements of the ice sheets' subsurface characteristics and localized experimental surveys in arid areas. However, these acquisitions are spatially and temporally limited, and the quality of the data among each mapping campaign varies dramatically depending on the surveying and technical constraints. STRATUS will efficiently address these limitations by taking subsurface measurements to obtain 1) full coverage of polar regions and coverage of large regional arid areas, 2) homogeneous quality (in terms of spatial and vertical resolution and spatio-temporal distribution), and 3) acquisitions over regions of interest in two seasons (for analysis of seasonal changes). These goals are well beyond the capabilities of any airborne or ground-based survey. They would result in a revolutionary scientific and socio-economic return in understanding the dynamical evolution of the cryosphere and the arid areas and their future under climate change.

2. Scientific Objectives
The primary objective (Figure 2) of the STRATUS mission is to provide a dedicated baseline for future monitoring of the Earth's polar regions by:
- Mapping the basal interface topography and ice-sheet thickness (important for understanding ice-sheet dynamics).
- Investigating the near-surface reflectivity and dielectric properties of ice-sheets/shelves
- Mapping the structure of internal englacial layering (important for improving past and future climatic models).
- Determining basal boundary conditions and processes (for understanding the stability of the ice sheets and their contribution to sea-level rise).
- Estimating the ice-sheet thermal regime.
- Mapping the subglacial hydrologic systems: subglacial lakes and channels.
- Estimating the thickness of floating ice, and marine and meteoric ice, and detecting the grounding zone.
- Mapping the channels at the base of ice-shelves.
As a secondary objective (Figure 2), STRATUS will provide the first time-coherent snapshot of the distribution and depth of water reservoirs in arid areas at unprecedented resolution and coverage by:
- Mapping the depth of the water table in shallow (50-100 m) aquifers.
- Mapping buried structural elements, such as faults, that can act as groundwater conduits.

3. Technical Concept, Innovation, and Challenges
The measurements for accomplishing the science objectives are performed by a nadir-looking radar sounder instrument operating in the frequency band between 40-50 MHz. The relatively low frequency of the electromagnetic wave allows the penetration of the transmitted signals into the subsurface where the reflections due to dielectric, thermal and mechanical discontinuities generate an echo to be measured by the instrument receiver. This echo is used for generating radargrams representing subsurface profiles in the along-track direction of the satellite. The STRATUS penetration efficiency is particularly high in ice and sand due to the transparency of these materials at low frequencies.
STRATUS exploits a small array of satellites deployed in an orbital formation flying for synthesizing a very large antenna aperture ‎[3]. The small satellites formation flying solution can drastically reduce the impact of surface clutter (which hinders data interpretation) and increase the signal-to-noise ratio. Moreover, it enables advanced radar processing strategies that can be used for clutter removal or feature extraction.
The following STRATUS configuration is proposed:
- Distributed radar sounder in VHF Band (40-50 MHz, frequency allocation in progress at ITU).
- Maximum penetration depth: up to about 4 km in ice and hundreds of meters in arid areas.
- Horizontal resolution: better than 1.3 km in along-track and 7.5 km in across-track direction.
- Vertical resolution: better than 10 m (in the subsurface).
- Orbit type: polar (90° inclination) with a height of 500 km.
- Mission duration: 3 years (possibility of extension to 5 years).
The STRATUS formation flying is composed by five satellites (Figure 3). One satellite (master) transmits and receives radar signals, while the other four ones are passive receivers that measure radar echoes received from the subsurface target along with the master. All the satellites have polarimetric capabilities. The radar parameters (Table 1) are a trade-off between the number of deployed sensors and the achievable performance considering the scientific requirements. It is indeed possible to increase the performance (SNR and clutter suppression) by increasing the number of sensors and/or implementing a transmission capability in the slave sensors. The radar design of the instrument is mostly inherited from the consolidated and large heritage of both terrestrial airborne and planetary orbital radar sounders (e,g., MARSIS and SHARAD currently operating on Mars).

4. Feasibility assessment
The main technical challenges of the STRATUS mission are (i) the effects of the ionosphere on the radar signal (ii) the capability of maintaining radar signals phase coherence between satellites and avoiding collisions with each other, (iii) off-nadir surface and firn clutter, and (iv) low SNR due to the presence of cosmic noise. Radar signal distortions in the STRATUS band resulting from ionospheric propagation can be significantly mitigated by compensation or adjusting the observing time and period ‎[4]‎[5]. Faraday rotation is mitigated by transmitting circularly polarized waves. The most critical ionospheric effects are signal phase and amplitude scintillations which are difficult to compensate but they have an acceptable rate of occurrence in polar regions ‎[4]. To maintain the radar signals phase coherence between satellites and avoid collisions, the relative position knowledge posterior must be known with about 0.5 m accuracy for properly synthesizing the antenna pattern. This can be achieved by accurate knowledge of orbital information and by exploiting positioning technology such as differential GNSS and inter-satellite link subsystems mounted on each satellite. Regarding collision avoidance, two suitable formation flight orbital configurations, namely pendulum and helix, can guarantee the required formation safety. The prescribed inter-satellite distances for the antenna pattern synthesis are in the order of tens of meters. However, it is feasible to relax the requirement on inter-satellite distance by applying suitable phase compensations to the signals in post-processing.
The effect of firn-clutter and volume-scattering is negligible at 45 MHz central frequency ‎[6]. The detectability of englacial layering and the basal interface subjected to galactic noise levels has been assessed through extensive simulations and performance assessment to determine the required SNR budget at the surface for achieving STRATUS science objectives ‎[7]. The analysis shows that layering and basal interface in the grounding zone are easily detectable (approaching the performance of the airborne systems) even with SNR budget as low as 65 dB, whereas in the interior of the ice sheets, about 85 dB SNR budget ensures a detection performance comparable (80%) to that of the airborne data.

References
[1] Bruzzone, L., et. al. (2021). STRATUS: A new mission concept for monitoring the subsurface of polar and arid regions. In 2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS (pp. 661-664). IEEE.
[2] WCRP, The World Climate Research Program. https://www.wcrp-climate.org/
[3] Carrer, L., et. al. (2019). Distributed radar sounder: a novel concept for subsurface investigations using sensors in formation flight. IEEE Transactions on Geoscience and Remote Sensing, 57(12), 9791-9809.
[4] Freeman, A., et. al. (2017). Radar sounding through the Earth's ionosphere at 45 MHz. IEEE Transactions on Geoscience and Remote Sensing, 55(10), 5833-5842.
[5] Scuccato, T., et. al. (2018). Compensating Earth Ionosphere Phase Distortion in Spaceborne VHF Radar Sounders for Subsurface Investigations. IEEE Geoscience and Remote Sensing Letters, 15(11), 1672-1676.
[6] Culberg, R., & Schroeder, D. M. (2020). Firn clutter constraints on the design and performance of orbital radar ice sounders. IEEE Transactions on Geoscience and Remote Sensing, 58(9), 6344-6361.
[7] Thakur, S., et. al. (2021). An Approach to the Assessment of Detectability of Subsurface Targets in Polar Ice from Satellite Radar Sounders. IEEE Transactions on Geoscience and Remote Sensing.

Lidar is the optimum technology for measuring bare Earth elevation through vegetation and the structure of vegetation. Airborne laser scanning (ALS) is widely used for topographic mapping and vegetation monitoring, but has high cost per unit area so is often limited in coverage and infrequently updated. A new generation of satellite lidars were launched in 2018 (Dubayah et al 2020, Markus et al 2017, Stoffelen et al 2006) and have proven the utility of spaceborne lidar for measuring forests, the atmosphere and ice. However, the energy requirements of lasers lead to very limited coverage compared to other satellite technologies. GEDI, the densest sampling lidar mission, will directly measure only 2-4% of the Earth’s surface during its mission. This sparse sampling causes uncertainty in data products, preventing robust change detection whilst also making the data unusable in many common applications of ALS, such as flood modelling, quantifying the properties of small parcels of land and urban mapping.

This study aims to calculate how spaceborne lidar technology could be scaled to create a system with continuous coverage, enabling applications that previously relied on ALS data. First the lidar and orbital equations are combined to estimate how wide a swath a lidar satellite can achieve for a given satellite power, mirror area, lidar instrument and measurement accuracy. This is combined with a global cloud map to estimate how many satellites would be needed in a constellation to measure the Earth’s land surface within a given temporal resolution; a Global Lidar System (GLS). This would provide a wall-to-wall bare Earth topographic map (rather than the top of canopy maps provided by radar and photogrammetry) and quantify the structure above, enabling precise flood mapping and biomass change detection.

Using the parameters for currently in orbit technology shows that a 30 m resolution global map could be made within 5 years using 12 ICESat-2 sized lidar satellites. Whilst this would provide unique data, it is likely to be prohibitively expensive. In order to realise a GLS more cost-effectively, potential ways to increase the lidar coverage per unit cost through technological developments have been explored. These include:

• Instrument: Laser and detector efficiencies improved with new photonics
• Platform and optics: Maximise payload power and telescope area per unit cost
• Signal processing: Reduce energy requirements with signal processing

Instrument and signal processing: All satellite lidars have used solid-state lasers, either Nd:YAG or Nd:YVO4, which have wall-plug efficiencies of around 5-8%. Diode lasers offer far higher efficiencies (approx. 25%) but at the expense of a lower peak power. The lower peak power prevents diode lasers being used as traditional single pulse lidars, such as GEDI and ICESat-2, instead needing to have their energy spread over either long pulses, or multiple low energy pulses. For a diode laser to be used in a satellite lidar, they must either be used as a train of small pulses (pulse-train), or in a pulse compressed lidar (PCL) mode, where a long, chirped pulse is emitted. The PCL return is then cross-correlated with the chirp to create a waveform with a shorter effective pulse shape.

A satellite lidar simulator (Hancock et al 2019) was used to investigate the use of these two novel lidar modes and both were benchmarked against traditional solid-state lidars in both full-waveform (such as GEDI) and photon counting (such as ICESat-2) mode. This has shown that diode laser lidar could be used from space and can provide greater coverage than full-waveform systems, but a photon-counting system with around 30 times the energy of ICESat-2 would provide even greater coverage. The primary limitation on diode laser performance, and so coverage is background noise, due to the longer integration times needed to collect sufficient energy. Therefore increasing the diode laser wavelength stability would allow narrower band-pass filters to be used, reducing noise and making diode laser coverage in pulse-train mode better than a solid-state, photon-counting system.

Platform and optics: In addition to the laser source and signal processing, a range of platform sizes, from 12U cubesats up to 500 kg satellites, and fixed and deployable primary mirrors, were considered. The platform and mirror control lidar coverage through the payload power and mirror collecting area, with the swath width and so coverage directly proportional to these. Maximising the payload power times mirror area per unit cost (cost of platform, launch and primary mirror) gives the most cost-effective constellation configuration. Collecting estimates for costs, payload powers and mirror sizes from satellite manufacturers showed that fewer larger platforms would provide a more cost-effective solution than a larger number of smaller satellites. Comparing fixed mirrors with the more expensive deployable optics showed that deployable optics were more cost effective due to their larger collecting area.

It is concluded that a Global Lidar System with wall-to-wall coverage is possible with current technology, but that developments in photonics and deployable optics will allow such a system to be realised more cost-effectively, once their technology readiness levels have been increased.

References

Dubayah, R., Blair, J.B., Goetz, S., Fatoyinbo, L., Hansen, M., Healey, S., Hofton, M., Hurtt, G., Kellner, J., Luthcke, S. and Armston, J., 2020. The Global Ecosystem Dynamics Investigation: High-resolution laser ranging of the Earth’s forests and topography. Science of remote sensing, 1, p.100002.

Hancock, S., Armston, J., Hofton, M., Sun, X., Tang, H., Duncanson, L.I., Kellner, J.R. and Dubayah, R., 2019. The GEDI simulator: A large‐footprint waveform lidar simulator for calibration and validation of spaceborne missions. Earth and Space Science, 6(2), pp.294-310.

Markus, T., Neumann, T., Martino, A., Abdalati, W., Brunt, K., Csatho, B., Farrell, S., Fricker, H., Gardner, A., Harding, D. and Jasinski, M., 2017. The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2): science requirements, concept, and implementation. Remote sensing of environment, 190, pp.260-273.

Stoffelen, A., Marseille, G.J., Bouttier, F., Vasiljevic, D., De Haan, S. and Cardinali, C., 2006. ADM‐Aeolus Doppler wind lidar observing system simulation experiment. Quarterly Journal of the Royal Meteorological Society: A journal of the atmospheric sciences, applied meteorology and physical oceanography, 132(619), pp.1927-1947.

Surface air-pressure is one of the most important parameters used in Numerical Weather Prediction (NWP) models. These models are critical for weather forecasting, and global surface air-pressure measurements will reduce uncertainties dramatically. There are limited numbers of in-situ measurements by weather stations for surface air pressure. The network of such stations is very heterogeneously distributed, and has been shrinking over the last decades due to high costs of installation and maintenance.
In a warming climate it is expected that the frequency and intensity of extreme weather events, capable of widespread devastation, will increase and thus there is an urgent need for improved weather prediction capabilities. Key to achieving this goal is the timely forecasting of related weather and climate-driving parameters, of which surface pressure is a critically important feature. Regular, accurate and global-scale measurement of surface air-pressure presents a substantial technical challenge, which can only be achieved by remote sensing from space. In an initial ESA project, the feasibility of a spaceborne air pressure mission was assessed. Future user requirements with regards to accuracy, spatial resolution and temporal availability of the measurements, were identified. Possible technologies were considered and the best technique was found to be a differential absorption radar which measures received signals either on the left wing or the right wing of the oxygen band. Retrieval of atmospheric pressure from satellite observations requires very accurate knowledge of the atmospheric mixing ratio of the spectral feature to be targeted. In practice, molecular oxygen offers the only viable candidate, since its mixing ratio is effectively constant over the full range of atmospheric conditions and altitude. Previous authors have designed a microwave pressure sounder based on differential absorption in the 60GHz absorption band of oxygen. They showed that measurements at several frequencies in the wing of the 60GHz band, combined with measurements closer to the water vapour line at 22GHz, had the potential of achieving ±1-2hPa accuracy in surface pressure determination with a spatial resolution of 200km. The radar design was based on measurements using six channels, which made the system complicated. A novel design of the surface-air-pressure pulsed radar with measurements in three channels in the right wing of the oxygen band reduces instrument complexity. The novel design by RAL Space of the BARometric Differential Absorption Radar (BARODAR) instrument accompanied with a new Millimetre Wave Radiometer operating at 183GHz band will measure atmospheric surface-pressure with unprecedented accuracy of ±0.2-1hpa and spatial resolution of at least 5x5km. The mission and instrument concept design, the scientific and operational requirements of different applications, technical challenges and solutions will be discussed. The first bench model of the system was built by RAL Space and an update on the design and the proof of concept measurements will be presented.

SCIENTIFIC MOTIVATION

The outer frontier of our Earth, where the atmosphere gradually transitions to space, is the least well know region of the planet. The altitude range between 50km and 250km—i.e., the upper mesosphere and lower thermosphere / ionosphere (MLT)—is too high up to be reached by balloons or aircraft, but too low for persistent satellite orbits due to residual drag, primarily due to the most abundant species at this height: atomic oxygen. Atomic oxygen also drives the chemistry and photochemistry of the MLT, but despite its importance it is poorly known. The reason for this is that existing optical remote sensing techniques in the infrared (IR) and at visible and ultraviolet light (UV-Vis) only offer a very indirect method to estimate the amount of atomic oxygen in the MLT. There is no global and long-term dataset for atomic oxygen so far.

Understanding atomic oxygen is considered the “holy grail” of upper atmospheric science. It is the missing keystone measurement, without which current remote sensing techniques must rely on—often contradictory—model assumptions. To resolve this conundrum, the Keystone mission was proposed as a candidate for ESA Earth Explorer 11 (and previously for EE-9 and EE-10 under the former name LOCUS).

The Keystone mission is built around a heterodyne radiometer at THz frequencies, which are wavelengths between the IR and the UV-Vis. Heterodyne instruments offer a very high spectral resolution, which allows the abundance of a gas to be directly retrieved from the shape of the spectral emission lines by its molecules.

Such instruments have existed at longer wavelengths for a long time, but because of technological limitations we were never able to build them at the THz frequencies that are required to measure atomic oxygen, and other key gases in the upper atmosphere (e.g., NO, OH, and HO2). Recent progress in quantum cascade laser (QCL) technology has since opened the possibility to build a highly integrated THz remote sensing radiometer. A QCL-pumped THz radiometer is compact, simple (compared to conventional technologies), and thus uniquely suited for deployment in space on a satellite platform.

Keystone not only closes the THz gap in heterodyne remote sensing, but by measuring atomic oxygen also closes a long-standing gap in the understanding of our planet. Combined with co-located IR and UV-Visible measurements, the Keystone mission provides the missing keystone to an overarching understanding of the least well-known region of the planet.


MISSION OBJECTIVES & BENEFITS

Thermosphere Ionosphere Interactions: Both the neutral upper atmosphere (thermosphere) and the ionosphere are in a constant state of flux, subject to natural fluctuation in incoming energy (from breaking atmospheric gravity waves, as well as from solar radiation). The thermosphere and ionosphere are linked in their response to external stimuli, but these links are poorly understood because the main physical parameters of the atmosphere—density and temperature—are not routinely measured, due to technological limitations. By filling this gap, Keystone will revolutionise our understanding of the chemical and photochemical processes that govern the upper atmosphere.

Thermal Balance and Climate: Direct abundance measurement of atomic oxygen [O] will allow us to understand the physical processes behind the observed upper-atmospheric cooling rates and help us understand if and how they are linked to anthropogenically increased levels in [CO2] in the atmosphere. Crucially, when combined with infrared heat flux measurements, abundance measurements of atomic oxygen will finally reveal the true O-CO2 quenching rates, thus resolving an old conundrum, and adding significant new value to decades of existing infrared measurements.

Space Weather: Abundance measurements of [NO] and [OH], [H2O] during space weather events will help understand the impact that space weather has on the physics and chemistry of the upper and middle atmosphere, and thus on our climate. Combined with next generation solar weather missions this provides a powerful package to study the sun-atmosphere system.

Improved NWP and climate models: Atmospheric models have become crucial to our society, with the economy and transport heavily depending on reliable weather forecast, and government policies depending on the predictions of climate change from global climate models. The performance of climate and weather prediction models is significantly improved by including the MLT region, but measurements are needed to validate them. Keystone provides missing composition measurements to be assimilate into atmospheric models. Keystone also allows mesospheric winds to be retrieved from the Doppler shift in the narrow upper atmospheric THz lines (as demonstrated by MLS and SMILES).

Space Situational Awareness: The models that compute orbit trajectories of LEO satellites (and space debris!) are crucially dependant on the getting atmospheric density and temperature right at the mesopause (90-100km). Keystone measurements provide more accurate input for this than the current model assumptions.


MISSION IMPLEMENTATION CONCEPT

Keystone is a limb-sounding mission comprising a novel terahertz (THz) frequency heterodyne spectrometer, exploiting quantum cascade laser (QCL) local oscillators (LOs) in space for the first time. It is accompanied by an IR filter radiometer, as well as an UV-Vis spectrometer. Keystone will be deployed in a polar, sun synchronous, low Earth orbit, scanning the atmospheric limb between 50 km and 250 km at ≤ 2 km vertical sampling and a longitudinal spacing of ≤ 1000 km (9°). Atmospheric emission spectra are simultaneously recorded in five THz bands, five to seven IR channels. The measured THz spectra and IR/UV-Visible radiances will be downloaded to ground stations each orbit and converted to temperature and mixing ratio profiles by an optimal estimation retrieval scheme. The nominal mode of operation is daily global sampling. Alternative modes are foreseen for e.g., space weather event campaigns (finer vertical sampling of [NO] over a reduced altitude range). A mission-duration of two years will enable inter-annual variability to be observed as well as seasonal and shorter-term variability. The mission is compatible with a small satellite platform and a large selection of launchers, including Vega.


DATA PRODUCTS

THz instrument: The THz radiometer measures vertical abundance profiles up to 250km (depending on species) of key upper atmospheric gases: [O], [OH], [NO], [CO], [H2O], [O3], [HO2] and temperature. The comprehensive composition mapping will allow us to study the photochemistry and thermal structure of the thermosphere and ionosphere, and how it reacts to external forcing, e.g., from climate change, space weather or gravity waves from large scale weather cells.

IR Instrument: The IR radiometer will measure the heat emitted by greenhouse gases from collisions with [O]. Combining this with [O] abundance measurements from the THz instrument will establish the kinetic quenching rates and result in more accurate upper atmospheric temperatures (climate change record!). It also provides abundances of the greenhouse gase [CO2] which—because [O] is known from the THz measurements—will be more accurate than from IR alone. This will show the impact of anthropogenic climate change on the upper atmosphere.

UV-Vis Instrument: The UV-Vis spectrometer measures direct and indirect airglow emissions from oxygen species ([O], [O+], [O2], [O3]) as well as several metals and metal ions. Combining this with [O] abundance measurements from the THz instrument will improve and validate our photochemical models. Many metals and metal ions to study ionospheric processes are also being measured.

The ocean is Earth’s main air conditioning system, absorbing about 90% of the excess heat of the present global warming. How this works is not completely understood, as it depends on a complex machinery of horizontal transport and vertical exchanges that bring water masses in contact with the atmosphere. Exploring how energy is transformed in the ocean is one way to understand it and help future climate projections. Winds are a major source of the energy required to make that ocean engine work. Ad hoc exchange coefficients that represent these processes are one of the key sources of uncertainty in numerical models used for weather forecasting and climate projections. The ocean kinetic energy budget is poorly constrained by observations: only 10 to 50% of the surface kinetic energy can be estimated with existing techniques (altimetry, gravimetry and scatterometry) and models, and there is no global measurement of the air-sea kinetic energy flux. The combination of warming, long-term absorption of CO2 resulting in ocean acidification, and ocean deoxygenation is putting considerable stress on the ocean. Combined with intensified fishing and pollution, ocean health is already seriously degraded with losses in structure and function, impacting services to humanity. Emerging threats, such as ocean heat waves, red tides, proliferation of invasive species, require new knowledge and a monitoring of the ocean breathing capacity to enable predictions of their impact on the marine system and the goods and services that it provides.

STREAM is a mission designed to explore how the ocean works and breathes. If implemented, STREAM will jointly measure motions across the air-sea interface: the Ocean Vector Wind (within 1.5 m/s) and, for the first time, the Total Surface Current Vector (TSCV, within 15 cm/s for each component for a single orbit) and its near-surface profile or “shear” (within 5 cm/s difference from 1 to 15 m depth), all resolved at a spatial resolution of 25 km or better over more than 95% of the global ocean, with a mean revisit time less than 36 hours at the Equator. This will be the first ever global measurement of the “wind work” that is the main source of kinetic energy for the oceans, and its decomposition into slowly evolving currents, and fast oscillation that radiate energy away and promote a mixing that limits the insulation of the vast heat and carbon reservoir that is the ocean interior. STREAM measurements of the TSCV combined with other data will also reveal the horizontal surface transport of heat, freshwater, carbon, plankton, nutrients, etc. that shape the weather and climate, and the ocean ecosystems, also constraining the vertical exchanges that connects the surface mixed layer to the deeper ocean. These requirements have led to a space segment design with a single STREAM platform orbiting at 510 km altitude in a sun-synchronous orbit that crosses the equator at 2PM (ascending node). STREAM carries a main instrument STREAM-R that is a conically scanning Ka-band radar, with an on-ground incidence angle of 45° and 900 km wide swath, using a 3 m diameter reflector rotating at 35 rpm. It will measure total surface currents and wind stress with 5 km wide footprints that can be combined in measurement cells of 25 km by 25 km or less.

A companion STREAM-O is a high-resolution (5 m) optical imager with 5 beams obtained in push-frame mode, implemented with 3 CCD detectors shifted along the focal plane and overlapping over a continuous 10 km wide strip. The nominal time separations of 1, 4, 9 and 10 s will provide measurements of the phase speed of the surface waves, from which profiles of current velocity in the depth range between 1 to 40 m may be estimated. These profiles will serve in the context of a Fiducial Reference Measurement system of near-surface currents, linking STREAM-R measurements to in situ subsurface data. Profiles will also contribute to the evaluation of horizontal transport and vertical mixing. STREAM-O imagery will be the first program of routine optical monitoring of the open ocean at very high resolution, even though using a sparse coverage. STREAM will complement the Copernicus Sentinel 1 radar “wave mode” for the exploration of small-scale processes at the air-sea interface, including in the presence of sea ice. STREAM-R may also make possible estimates of Sea Ice Drift. Combined with STREAM-O imagery of ice floes and waves, STREAM would allow a detailed investigation of the marginal ice zone, an important transition area between the pack ice and the open ocean, important for vertical exchanges.