This paper will present the status of the on-going and planned mission concept studies in the period 2023 to 2025 in the frame of the ESA’s Future Earth Observation Programme (FutureEO).The European Space Agency (ESA) is conducting preparatory activities on innovative Earth Observation (EO) missions to foster better scientific understanding of the Earth system through research missions, respond to the requirements of the operational users via operational missions and assess the potential of "Newspace" approach with smallsat Earth Observation concepts to boost innovation through the Φsat missions.ESA’s program of EO research missions comprises Earth Explorers, Scouts and Mission of Opportunity. Earth Explorer missions objective is to advance Earth science by providing answers to key scientific questions on the Earth system. Up to date, five Earth Explorers have been launched and another five are in different stages of development. ESA’s Earth Explorer 11 mission candidates (CAIRT, Nitrosat, SEASTAR and WIVERN) are undergoing Phase 0 activities while preparatory activities to study mission concepts for the Earth Explorer 12 are expected to start in 2024.Missions of Opportunity are research missions implemented under cooperation frameworks with Space Agencies outside Europe and Canada. Next Generation Gravity Mission (NGGM) is a candidate Mission of Opportunity for ESA-NASA cooperation in the framework of MAGIC (MAss change and Geosciences International Constellation).Scouts are small budget research mission based on small satellites or CubeSats, implemented in three years and complementing Earth Explorers. Two mission concepts, TANGO and NanoMagSat, are currently undergoing risk retirement activities.Preparatory activities for operational mission are underway for the next generation of Sentinels in the frame of the Copernicus programme (e,g. Sentinel-2 Next Generation and Sentinel-3 Next Generation Optical, which are currently in phase 0) in close coordination with the European Commission). Preparatory activities are planned in the coming years for the next generation of meteorological missions (Meteosat 4thGeneration and EPS/MetOp 3rdGeneration) in close coordination with EUMETSAT.Φsat missions are CubeSat missions aimed to demonstrate radical and new technologies or remote sensing concepts in space, such as Φsat -2, which is currently under development and is focused on onboard Artificial Intelligence applications.

Geosynchronous synthetic aperture radar (SAR) missions offer the advantage of conducting multiple sub-daily interferometric acquisitions using a single platform. However, this comes at the expense of increased fuel and power budgets, due to the significant distance to Earth’s surface. Consequently, such missions are suited for delivering SAR products with relatively lower spatial resolutions, typically in the range of hundreds of meters, over large localized areas spanning more than one million square kilometers. Alternatively, a constellation of small SAR satellites operating in low Earth orbits (LEO) can also deliver similar interferometric products. In LEO, the reduced free-space propagation losses and simpler orbit insertion can be exploited to realize and operate a larger constellation of LEO-SAR satellites. This paper provides an equivalence framework between the two alternatives in terms of interferometric lags and coverage. It further outlines an example mission concept, equivalent to Hydroterra, one of ESA’s Earth Explorer 10 candidate missions.

The Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA), together with NASA's Soil Moisture Active Passive (SMAP) mission, is providing a wealth of information to the user community for a wide range of applications. Although both missions are still operational, they have significantly exceeded their design life time. For this reason, ESA is looking at future mission concepts, which would adequately address the requirements of the passive L-band community beyond SMOS and SMAP. This article proposes one mission concept, TriHex, which has been found capable of achieving high spatial resolution, radiometric resolution, and accuracy, approaching the user needs. This is possible by the combination of aperture synthesis, formation flying, the use of general circular orbits, and alias-free imaging.

This This paper reviews the choice of polarisation axes in aperture synthesis arrays for remote sensing of the Earth, in particular when using dual polarisation receivers. In this case, as it is shown, there is the freedom of adopting rotated polarisation axes across the array without signal loss. This is not possible in systems having dual polarised antennas connected to single channel receivers, as then, polarisation time multiplexing requires the same antenna polarisation at any time in all elements of the array if signal loss is to be avoided. The flexibility allowed by the first type of instrument leads to a simpler antenna design and system architecture, providing a clear advantage over other polarisation schemes. The review of the choice of the polarisation axes is performed in the frame of the development of an advanced antenna for a future L-band aperture synthesis radiometer, but the analysis is general and applicable to any instrument of this kind. This is a study undertaken by the European Space Agency aiming at a possible future high-resolution follow-on mission to the currently flying Soil Moisture and Ocean Salinity (SMOS) mission.

WBSCAT is a polarimetric scatterometer developed for the European Space Agency for measuring microwave signatures of snow in support of the SnowLab and SnowLab-NG projects. SnowLab-NG includes feasibility studies for ESA candidate satellite missions operating at L-Band such as ROSE-L and Hydroterra. The WBSCAT instrument is based on a vector network analyzer to cover the 1–40 GHz frequency range, including L-Band. Using ground-based data collected in the near-field to model data collected by satellites is challenging when there is volume scattering from a thick layer such as snowpack. WBSCAT has been constructed to use near-field aperture synthesis to permit focusing of backscatter data. Focusing mitigates the effects of near-field operation by synthesizing a narrow antenna beam that illuminates a smaller volume of the snowpack. Using aperture synthesis, an improvement in azimuth resolution by a factor of 6 is possible with WBSCAT at L-band. Initial results from phase calibration of a calibration target response show the potential for this approach.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission [1] has been in orbit for over 11 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions keeps being fully operational. This II-year long lifetime of SMOS, so far, has enabled the calibration and Level-1 processor team to improve the calibration procedures and the image reconstruction resulting in a new version of the Level-1 data processor, v724. To present the main performance features of this new version and the improvement in the calibration procedures constitute the main objective and content of this presentation.

Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiometers. After the lessons learnt from SMOS, ESA is preparing the next generation of L-band interferometric radiometers with RFI mitigation integrated into the cross-correlators. This work presents a preliminary design and results of a pre-correlation RFI mitigation algorithm tailored for interferometric radiometers. The results show that the correlation error introduced by the RFI is reduced on average to the half, with peaks of 20 dB of mitigation.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is still in good health, providing valuable L-band observations of the Earth surface [1]. A number of products are obtained from these, including thin sea ice [2], frost/thaw soils [3], high winds [4], ocean surface wind [5] and Sun brightness temperature [6], besides the main mission measurements of soil moisture and sea surface salinity [7] [8]. This paper deals with the description and early results of some technology activities conducted by ESA applying the lessons learnt by SMOS and in preparation of an advanced L-band radiometer mission.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission [1] has been in orbit for over 9 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions is working well. The data products are generated using version v620 of the Level-1 operational processor, a version which entered into operation in Spring 2015. During last year a comprehensive data set was processed using a new processor version v720 and the assessment of the results is expected to be completed by mid 2019. In parallel to this evaluation of v720, the following version v730 of the Level-1 processor of SMOS has been already produced. This latter version is intended for investigating the capability to reduce Radio Frequency Interferences (RFI) by applying image processing techniques. This paper describes the major features and status of the two mentioned versions of the SMOS Level-1 processor, and importantly, aims at updating the remote sensing community on those aspects of the SMOS mission.

WBSCAT is a new terrestrial 1-40 GHz polarimetric scatterometer. This instrument, built for the European Space Agency, with additional support from ETH WSL, is currently an element of the ESA SnowLab project for continuous microwave measurements of snowpack in Davos-Laret Switzerland. WBSCAT is based on a compact Vector Network Analyzer (VNA), combined with calibration standards and low-noise amplifiers to increase sensitivity. A pan/tilt positioner provides the angular and spatial diversity required for measurement of radar cross-section, 3D tomographic imaging, and measurements of interferometric coherence. The instrument can apply angular diversity and aperture synthesis to increase radiometric accuracy and suppress clutter. In Davos-Laret, WBSCAT is suspended from a 2.2-meter linear rail positioner that provides additional spatial diversity for high-resolution 3D tomographic imaging.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission [1] has been in orbit for over 8 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions is working well. The data for this whole period has been and is being processed with the operational version of the current Level-l processor (version v620). Also a representative part of the same data set has been processed with a working version of a new processor (v720) which is now in preparation so that homogenous records of brightness temperatures have been made available. These rich and long data records have allowed learning important lessons from the in-flight experience, and shall eventually lead into the consolidation of the new Level-l processor version (v720) with its corresponding auxiliary calibration and configuration files. Once the improvements are confirmed the new processor version shall be recommended for the operational chain.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission has been in orbit for over 7 years, with its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) functioning well. This 7 year period has provided a wealth of information which has enabled us to understand and consolidate the performance of the payload in great detail. More importantly, we know now the things that work well, those that need improvement, and how the instrument could be enhanced if we were to build it again. This paper presents the lessons learnt from SMOS after 7 years in orbit.

ESA's Soil Moisture and Ocean Salinity (SMOS) mission has been in orbit for over 6 years, and its Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) in two dimensions keeps working well. The data for almost this whole period has been reprocessed with the new fully polarimetric version (v620) of the Level-1 processor which also includes refined calibration schema for the antenna losses. This reprocessing has allowed the assessment of an improved performance benchmark, a better understanding of the observations, and the preparation of a new version (v700) of the Level-1 processor with further potential.

ESA has initiated several studies to enhance the performance of next generation of C-Band SAR. This paper provides an overview of the on-going demonstration activity for critical technology in C-band as required for a next generation Sentinel-1 SAR instrument with enhanced performance in terms of coverage and resolution. The new instrument will use digital beam forming (DBF) capabilities to achieve the performance improvement by a new combined analogue/digital beam forming unit called TRIM (Transmit/Receive Interface Module). The paper reports on the status of ongoing developments at subsystem level including required technological elements as European GaN and deep sub-micron silicon semiconductor technology.

The Soil Moisture and Ocean Salinity (SMOS) mission is a European Space Agency (ESA) mission aimed at the global monitoring of surface soil moisture and ocean salinity from radiometric L-band observations. This work described in this letter is devoted to the reduction of the systematic error in the reconstruction of brightness temperature maps from SMOS interferometric measurements. Despite the fact that the image reconstruction method currently used was proposed and implemented in the ESA L1 processor for reducing this error, residual offset and ripples still persist. This is particularly penalizing for oceanographic applications with SMOS data. A new approach for reducing this residual error is presented here and illustrated with brightness temperature maps retrieved over the Pacific ocean.

During the past ten years a number of research groups around the world have been investigating the use of digital beamforming for spaceborne SAR instruments. The separate optimization of the effective transmit and receive aperture sizes together with the splitting of the receive side into multiple receive sub-apertures which can be combined on board by means of digital beamforming allows for a number of new SAR modes. With this it is for example possible to overcome the principle swath width and azimuth resolution limitation known in classical SAR instrument designs. ESA has performed and initiated a number of systems studies to identify the critical technologies needed for this type of instrument. Based on these system studies a number of technology development activities have been started. This paper gives an overview on the studies and technology developments performed in this context.

This paper reviews the reconfiguration capabilities of a real time compact optronic SAR processor in order to adapt for changes in altitude and look angles of a SAR system in flight, either airborne or space-based. The optronic processor can be reconfigured instantaneously making it suitable for all application requiring real-time SAR imagery despite modification of flight and orbit parameters. Moreover the system can be fully calibrated during flight or in-orbit providing outstanding ruggedness to environmental constraints. SAR image sets may thus be produced immediately on-demand without bottleneck; that is, the optronic SAR processor processing rate is matched to the SAR system raw data generation rate. These capabilities, combined with the fine ground sampling distances in both azimuth and range directions could provide benefits for such applications as landslide and flood monitoring, snow and ice coverage, glacier monitoring and real-time onboard tracking of events.

This paper introduces a real-time compact optronic SAR processor that has the capability to generate ENVISAT/ASAR image swaths of 100 km × 100 km in 10 seconds exhibiting slant plane sampling distances of 4 meters in azimuth and 1 meter in range. It may be instantaneously reconfigured to process data from any of the 7 ASAR image swath modes. In this respect, numerous SAR image sets may be produced immediately on-demand without bottleneck. A rapid SAR processor that also provides fine ground sampling distances in both azimuth and range directions could provide benefits for such applications as ship detection, landslide and flood monitoring, snow and ice coverage and glacier monitoring.

This paper describes a practical method to calibrate the microwave imaging radiometer with aperture synthesis (MIRAS) in orbit using external targets. Together with the in-orbit validation of the antenna patterns, the method ensures the generation of valid and calibrated visibilities from raw measurements with minimum impact from instrument errors, particularly antenna errors. It has been possible to devise this strategy only after the derivation of the Corbella equation, which correctly describes how MIRAS works. The proposed in-orbit calibration and validation approach is fully based on this equation. In a markedly different way to the point target response method used to calibrate other imaging systems, the technique put forward is essentially based on measuring a ldquoflatrdquo target such as the cold sky near the Galactic poles. This is why it has been called the ldquoflat target transformationrdquo.

This article is concerned with the apodization windows to be applied to brightness temperature maps reconstructed from complex visibilities provided by the MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) instrument on board the SMOS (Soil Moisture and Ocean Salinity space mission) spacecraft in order to achieve a close to uniform pixel at the Earth's surface level

The SMOS data product steps from raw data to brightness temperature maps are described. Originally ordered in the sequence of snapshots, the data are rearranged for the level 1c Brightness Temperature maps according to their location in a Discrete Global Grid (DGG). A number of candidate DGGs are introduced and their suitability is evaluated.