The effectiveness of swath and Doppler spectrum reconstruction algorithms in cross-track and along-track multistatic synthetic aperture radar (SAR) formations heavily relies on the precise relative positioning of each spacecraft. Ideally, a static array configuration is desired, where the satellites are evenly distributed in an Earth-fixed frame. However, maintaining this ideal configuration is not feasible without constant active flight control to counter the satellites’ relative dynamics. Alternatively, one can design natural trajectories that result in the desired formation geometry within an acceptable margin for a significant fraction of the orbital period. In this paper, we present a relative motion description in an Earth-fixed geometry for distributed SAR concepts. We derive a general expression for the transformation matrix from the Hill-Clohessy-Wiltshire (HCW) frame to the SAR-appropriate zero-Doppler (ZD) frame, and propose a general optimization method to fit the best natural non-drifting formation solution for any desired formation configuration. By applying the developed technique to optimize along-track and cross-track relative positioning requirements, we obtain conforming baselines over extended orbit stretches. We present SAR imaging examples for along-track distributed formations and formations with small cross-track baselines, using distributed platforms spanning a 72° latitude range. The demonstrated configurations achieve orbit duty cycles exceeding 20%, with baseline errors kept below 5% relative to the ideal array element positions. These results indicate the feasibility of these concepts using natural orbit solutions.

High Earth orbit Synthetic Aperture Radar (SAR) systems offer high temporal sampling and moderate spatial resolution on a global scale, potentially outperforming conventional Low Earth Orbit (LEO) systems in revisit times. However, this requires complex system architectures such as burst operation modes with multiple subswaths, large antennas, and digital beamforming. Similar temporal sampling and coverage enhancements can be realized with constellations of classical monostatic SAR instruments in LEO. This letter compares the complexity of such equivalent monostatic LEO-SAR constellations to complex high-altitude SAR systems and provides design numbers for two Medium Earth Orbit (MEO)-SAR mission examples and their LEO counterparts.

Over the last decades, repeat-pass SAR interferometry (InSAR) for deformation measurement and topographic mapping has revolutionized our understanding of many geophysical processes on Earth. A new mission concept, currently in development at the Jet Propulsion Laboratory (JPL) and Caltech, aims at using orbital repeat-pass InSAR for deformation and topography mapping of Saturn’s ice-covered and geologically active moon Enceladus. In this paper, we present an initial performance assessment of the system and the suggested SAR processing approach, along with simulated InSAR acquisitions using a DLR in-house End-to-End performance simulator.

The effectiveness of swath reconstruction algorithms in cross-track multistatic synthetic aperture radar (SAR) constellations heavily relies on the precise positioning of individual spacecrafts. Ideally, a static array configuration is desired, where the satellites are evenly distributed in a Local Vertical Local Horizontal (LVLH) frame, emulating a fixed antenna. However, achieving this ideal configuration is not feasible without the implementation of constant active control to counter the relative dynamics between the satellites. Alternatively, it is possible to design natural trajectories that yield the desired formation geometry for a significant portion of the orbital period.In this paper, we present a comprehensive analysis of the feasibility of a continuous control concept for building a static array in space. Additionally, we propose suitable formation strategies that eliminate the need for continuous control.

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.

We present in this paper a sensitivity analysis of swath reconstruction algorithms for multistatic SAR constellations, mainly focused on geometric aspects of the formation, such as the irregularity of the sampling and the tilt.

This paper presents our new SAR processing framework known as PRISM (Processor for Interferometric SAR Missions). This flexible approach allows the efficient and accurate processing of SAR data independent of the sensor and the acquisition mode. The two main PRISM components (the focusing and the interferometric chains) are described in this paper together with the philosophy of the software architecture. Experimental results are presented and discussed based on the impulse response function analysis of simulated data as well as the focusing and interferometric results using real TerraSAR-X data.

Hydroterra has been considered as one of three candidates for ESA's Earth Explorer 10 mission, and consists of a geosynchronous Synthetic Aperture Radar to measure key processes of the diurnal water cycle. These measurements would improve the prediction of intense rainfall, flooding and landslides. This concept has been the subject of Phase 0 studies, in which the mission requirements were consolidated and initial system design possibilities were investigated. This paper summarizes some of the elements of the mission concept contributing to the requirement consolidation process, namely the orbit design aspects, with focus on potential sub-sidereal interferometric capabilities, and the optimization performed in the bandwidth/integration time trade-space to define the operational scenarios and coverage zones, which are key inputs for the mission design.

This paper discusses multistatic SAR, highlighting the potential of constellations to deliver scalable performance, which allows for the simplification and reduction of the single units in terms of size and power. In particular, the paper illustrates the impact of technical challenges such as navigation control, clock synchronization, and data processing. The paper also includes the discussion of the main applications of multistatic SAR, including the measurement of single-pass interferometric and tomographic signatures as part of the calibration process. The paper is closed with examples of possible constellations offering comparable performance to SAR missions of the present and next generations.

Existing microwave remote sensing instruments used for Earth observation face a clear tradeoff between spatial resolution and revisit times at global scales. The typical imaging capabilities of current systems range from daily observations at kilometer-scale resolutions provided by scatterometers to meter-scale resolutions at lower temporal rates (more than ten days) typical of synthetic aperture radars (SARs). A natural way to fill the gap between these two extremes is to use medium-Earth-orbit SAR (MEO-SAR) systems. MEO satellites are deployed at altitudes above the region of low Earth orbits (LEOs), ending at around 2000 km and below the geosynchronous orbits (GEOs) near 35 786 km. MEO SAR shows a clear potential to provide advantages in terms of spatial coverage, downlink visibility, and global temporal revisit times, e.g., providing moderate resolution images (some tens of meters) at daily rates. This article discusses the design tradeoffs of MEO SAR, including sensitivity and orbit selection. The use of these higher orbits opens the door to global coverage in one- to two-day revisit or continental/oceanic coverage with multidaily observations, making MEO SAR very attractive for future scientific missions with specific interferometric and polarimetric capabilities.