ESA is currently conducting studies in phase 0 and phase A defining four new SAR missions. Each of the mission studies are conducted by two separate industrial teams in parallel. The paper will provide an overview on the programmatic context, the mission requirements and explain the different mission concepts which are investigated.
In the frame of Copernicus program of the European Union, ESA is working on the definition of the overall system architecture for the Copernicus space component and its evolution on basis of user requirements, coordinated by the Commission. In this context ESA is studying the Next Generation is Sentinel 1 in phase 0. The objective is to ensure long-term continuity of the C-band SAR measurements and to improve in particular the revisit capacity.
Besides this extension of the measurement capabilities of the current series of Sentinel satellites with the definition of the second generation, there have been also some emerging needs for new types of observations identified which would have to be realized through an expansion of the Copernicus Sentinel fleet. Six new mission concepts were identified as so-called High Priority Candidate Missions. All the six missions are currently studied in phase A. Amongst these 6 missions there is also one SAR mission. The ROSE-L (Radar Observing System for Europe at L-band) mission aims to complement the current C-band measurements with additional L-band observations.
Besides the Copernicus missions which are driven by requirements and needs of the operational user services, ESA is developing the Earth Explorer series in the frame of the Earth Observation Envelope Program. The purpose of the Earth Explorer missions is to conduct research in the field of Earth observation and to demonstrate the potential of new innovative Earth Observation techniques which are relevant to the scientific and application oriented users.
As response to the call for Earth Explorer-10 mission ideas number of innovative proposals have been received from the scientific community. After evaluation of the proposals, three mission ideas were selected that were found to provide the highest scientific return and the best response and solution to the posed scientific and societal challenges within the programmatic constraints. Amongst the three missions selected to be studied first in phase 0 there are two SAR related missions, namely STEREOID and G-CLASS:H2O. At the end of the phase 0 study, two mission concepts will be down selected to continue to be studied in phase A. Only one of the candidate missions will finally be implemented and fly as ESA’s Earth Explorer 10.
STEREOID aims at observing small-scale motion and deformation fields of the ocean surface, glaciers and ice sheets, sea ice and solid Earth. The mission concept consists of two bi-static passive SAR receive only satellites flying in distant formation with one Sentinel-1 satellite.
G-CLASS aims to observe key processes of the daily water cycle by placing a single SAR satellite in geosynchronous orbit. The main science goals are to improve the prediction capability of intense rainfall and related flooding and landslides, to improve the understanding of the diurnal water cycle and to establish the real time prediction of ground motion. The SAR payload operates in C-band and uses a large deployable reflector antenna and a transmitter based on TWT technology.
Both SAR related EE-10 mission concepts are not only scientifically very interesting but also very innovative with respect to the SAR observation concept. STEREOID would be the first constellation of small passive receive only SAR satellites and G-CLASS would be the first SAR operating from geo-synchronous orbit.

Among the upcoming Earth Observation Missions, the High Resolution Wide Swath Mission (HRWS) is intended to continue and enhance the German X-Band Radar Domain. The multi-static HRWS Mission is the evolution of the very successful TerraSAR-X und TanDEM-X Missions and will provide the scientific community, the national institutions and the international commercial sector further with high resolution Synthetic Aperture Radar (SAR) data.
The German Aerospace Center (DLR) contracted the company Airbus Defence and Space GmbH (ADS) to perform a feasibility study (Phase 0/A). Based on this analysis and on extensive users’ needs assessments a very innovative Mission concept has being proposed. As a result the HRWS Mission will be using digital beamforming technology as well as the newly available chirp bandwidth of 1200 MHz and will be therefore many times more powerful than the two predecessor satellites.
The actual baseline concept consists of the formation of one active satellite and three small satellite companions. With such constellation it will be the first in-space operation of the highly innovative Mirror-SAR concept: Companions act passively as a mirror for actively transmitted SAR pulses from the main satellite. This multi-static concept can achieve a resolution of up to 25 cm as well as several hundred kilometers wide strips and full polarimetry. The multi-static approach projected for HRWS will allow on demand digital elevation models and height change maps collected from just one pass. Consequently, the HRWS Mission will serve a wide range of needs like:
• Monitoring of surface movements:
Infrastructure monitoring, landslide, volcanic or earthquake research
• Maritime surveillance:
Ship detection, oil and ice monitoring, wind speed, ocean currents
• Thematic mapping:
Agriculture, forestry, environment, mapping for industry, trade and logistics
• Ecosystem monitoring:
Mapping of vegetation and surface water, permafrost, glacier monitoring
• Production of digital elevation models:
Improved global DEM, regional and local updates
• Military Reconnaissance and Image Intelligence
• Multi-static applications: Volume and height measurement, stock pile monitoring

The up-to-date status of the Mission will be finally presented together with future plans and promising perspectives.

The NRC 2017 Earth Decadal Survey recommended a new NASA “Designated” program element to address a set of five high-value Targeted Observables during the next decadal period. In response to that recommendation, and based on guidance from NASA’s Earth Science Division, a team has been formed to perform an architecture study – the SDC Study – associated with the Surface Deformation and Change (SDC) Targeted Observable. Surface deformation measurements are critical for studies of ice sheet stability and their contributions to sea level rise, permafrost thaw, surface change and strain rates related to earthquakes, volcanoes, landslides, and changes in groundwater levels and corresponding subsidence or uplift. The Decadal Survey report recognizes this criticality for its Earth Surface and Interior objectives, and also for a number of Hydrology and Climatology objectives, identifying twenty-three surface deformation-related objectives throughout the report.

SDC observational requirements for many of these 23 SDC-related objectives cover a range of performance parameters such as spatial resolution, deformation precision and temporal frequency. In reaching its final recommendation on a cost-effective strategy for the SDC observable, the Decadal Survey Committee presumed that the measurement implementation will involve Synthetic Aperture Radar (SAR) and Interferometric SAR (InSAR) technologies. The Decadal Survey references the NASA-ISRO SAR (NISAR) Mission design performance of 12-day repeat interferometry, and calls for shorter repeat cycle (sub-weekly to daily), potentially at the expense of spatial resolution if necessary to stay within the recommended development cost.

The SDC Study has three main objectives: 1) Identify and characterize a diverse set of observing architectures, including innovative observing systems that have the potential for disrupting the norm for SDC observations; 2) Assess the ability of each of the architectures to meet SDC objectives, including cost effectiveness; 3) Perform sufficient in-depth design study of one selected architecture to enable initiation of a Phase A concept study. To accomplish these objectives, the study team is engaging US national expertise in Earth Science research, applications, technology, mission formulation and implementation. The team comprises NASA centers with relevant expertise and is engaging the international community, government, academia, and industry.

In the future, SDC observational needs may be met with a heterogeneous constellation of satellites, some in the public and some in the private sector. The SDC architecture study will examine the research and applications benefits of the data sets derived from these existing and planned systems, which may be complicated by different data access modalities for various satellites, ranging from free and open data to commercial but restricted data sets. In this study, we will work with other agencies (space agencies and data sponsors), and commercial providers to understand and quantitatively assess the ways in which the variety of data can be applied to scientific research and other applications. The study team will develop a simulation tool to quantitatively assess the performance of the existing and planned SAR constellations, which will be considered as an observing system. This will allow research and applications community members to gain a more quantitative understanding of the critical gaps in our observations from the government Programs of Record and the commercial sector that NASA must fill to meet the SDC science and application objectives.

While the Decadal Report Surface Deformation and Change Targeted Observable is focused on surface geodesy (i.e. change in position of the surface), NASA has directed that the scope of the study include some architectures that intrinsically support research- and applications-grade measurements of observables such as soil moisture, vegetation structure, disturbance, agricultural monitoring, wetlands processes, coastal processes, ocean processes, sea ice hazards monitoring (e.g. icebergs and polar sea-lane variability). This implies that there should be a good balance between architectures that optimize for phase-based geodetic performance and those that also support amplitude-based radiometry.

The SDC Study commenced in October 2018 and is planned to run for 5 years. In the initial 2 years, the community needs will be collected and parsed, technology and partnerships assessed, and a set of architectures for study identified. In the following 1-2 years, these architectures will be evaluated against the needs, and a down-selection will occur. The remaining time will be spent in more detailed studies of the down-selected architectures, with a final down-selection and report at the end in preparation for mission implementation. In this paper, we will describe the flow of the study and how potential partners can become involved.

Within the context of potential future InSAR missions, we describe (1) the desired characteristics of the earth deformation observables as outlined in the recent NASA earth science decadal survey, (2) additional desirable capabilities of a future observing system that could provide a quantum leap in geodetic quality, and (3) different imaging scenarios that warrant further definition and formulation. The range of capabilities to consider involve repeat time, spatial resolution, line-of-sight diversity, and ability to suppress confounding signals that limit the quality of the geodetic measurement. We discuss a range of solutions but emphasize the potential of large, coordinated constellations. Each satellite within such constellations may have a reduced capability relative to a single flagship satellite, but the entire observation system's measurement capability exceeds what is attainable from current or imminent missions.

Remote sensing data products best serve users if the data are readily accessible and easy to understand. Designing user-friendly, or analysis ready data (ARD), will lead to the much wider use of these products by enabling non-remote-sensing specialists to apply them. A prime illustrator of this is InSAR data, a technically challenging application that has traditionally required not only detailed familiarity with the mechanics of the technique but also the need for significant metadata and software suites in order to use the observations. Over the past several years understanding of the method has matured and now permits generation of products in common coordinate systems with the imaging geometry and surface terrain phase corrections already applied. Under this user-friendly framework, many potential users can readily produce InSAR deformation maps and time series. Here we present a user guide to the analysis of InSAR data by non-specialists with examples of data from several operating and recent radar satellites. Interferogram formation requires only that one cross multiply arbitrary image pairs, and sets of these from each satellite may be combined over an area of interest using SBAS or similar methods. By implementing the vexing co-registration and elevation/geometry phase corrections within the data processing system for each radar mission, the user is relieved of the need for InSAR-specific expertise. This opens up practical InSAR usage to a very wide spectrum of users, who otherwise would be unable to take advantage of InSAR methods. The enabling technology is to reduce today’s standard data products to geocoded, phase compensated, and common coordinate system single look complex images. Our proposed approach implements the analysis-ready data recommendations of the Visibility and Access working group of the International Coordination for Spaceborne Synthetic Aperture Radar Data Acquisition, Processing and Analysis workshop held at Caltech in May, 2018. We advocate for all data producers to adopt common standards to achieve this goal.