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.

This paper introduces the concept of a fractionated MirrorSAR which is based on a set of mutually separated transmitter and receiver satellites. As opposed to previously published bi- and multistatic SAR systems, the receiver satellites are considerably simplified, as their main functionality is reduced to a kind of microwave mirror (or space transponder) which routes the radar echoes towards the transmitter. The forwarded radar signals are then coherently demodulated within the transmitter by using the same oscillator that had been used for radar pulse generation. This avoids the necessity of a bidirectional phase synchronization link as currently employed in TanDEM-X. Since the needs for fully equipped radar receivers, on-board memory and downlink are also overcome, the weight and costs of the receiver satellites can be significantly reduced. This allows for a scaling of their number without cost explosion, thereby paving the way for novel applications like multi-baseline SAR interferometry and single-pass tomography. Several additional opportunities make the MirrorSAR concept even more attractive. First, the separation of the transmitter and receiver front-ends reduces not only RF losses by avoiding switches and circulators, but it may also lower the peak power in the transmitter satellite by employing a frequency-modulated continuous wave (FMCW) illumination. This simplifies the design of the high-power amplifier and increases its efficiency. Second, the opportunity for continuous radar data collection enables new modes for the imaging of ultra-wide swaths with very high resolution, thereby overcoming an inherent limitation of conventional monostatic SAR systems. Third, the joint availability of all receiver signals in a centralized node offers new opportunities for efficient data compression, as the multistatic radar signals from close satellite formations are characterized by a high degree of mutual redundancy. Fourth, the use of a sufficiently separated transmitter satellite can avoid the risk for mutual illumination, which challenges the design and operation of fully-active multistatic SAR systems. Further advantages arise from the scalability and reconfigurability, which support new redundancy concepts and pave at the same time the way to new modes like MIMO-SAR tomography.

SESAME (SEntinel-1 SAR companion Multistatic Explorer) is a passive SAR satellite mission proposed for the ESA Earth Explorer Program. SESAME comprises two receive-only C-band SAR satellites flying in close formation to build a single-pass SAR interferometer (SP-InSAR) using the active signal of the European Sentinel-1 satellite. The SESAME mission addresses applications in geoscience and climate research that require repeat measurements of high precision elevation data over land surfaces including ice covered areas and forests, exploiting the SP-InSAR and multistatic observation geometry of the satellite formation. The objectives, the measurement approach and geo-biophysical products of the mission are described.

This paper presents the correlating synthetic aperture radar (CoSAR) technique, a novel radar imaging concept to observe statistical properties of fast decorrelating surfaces. A CoSAR system consists of two radars with a relative motion in the along-track (cross-range) dimension. The spatial autocorrelation function of the scattered signal can be estimated by combining quasi-simultaneously received radar echoes. By virtue of the Van Cittert-Zernike theorem, estimates of this autocorrelation function for different relative positions can be processed by generating images of several properties of the scene, including the normalized radar cross section, Doppler velocities, and surface topography. Aside from the geometric performance, a central aspect of this paper is a theoretical derivation of the radiometric performance of CoSAR. The radiometric quality is proportional to the number of independent samples available for the estimation of the spatial correlation, and to the ratio between the CoSAR azimuth resolution and the real-aperture resolution. A CoSAR mission concept is provided where two geosynchronous radar satellites fly at opposing sides of a quasi-circular trajectory. Such a mission could provide bidaily images of the ocean backscatter, mean Doppler, and surface topography at resolutions on the order of 500 m over wide areas.

Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.

SAR instruments with multiple transmit/receive channels allow for a variety of operation modes. The trade-space includes the instrument and antenna parameters designed to enable various operation modes. These are utilized to achieve a required SAR performance derived from the mission and user requirements. Finding a solution in this multi-dimensional trade space is not trivial and often involves the compromise of parameters. This paper addresses the topic of SAR instrument design tailored to a set of operation modes. It explains and compares the SAR techniques and gives the performance.

Correlating SAR (CoSAR) has been recently proposed as a geosynchronous remote sensing mission capable of delivering continental coverage of ocean surfaces with a high repeat cycle. The main specific measurements provided by CoSAR are estimates of the normalised radar cross section (NRCS), Doppler shifts, and surface topography with higher resolution than microwave radiometers and larger coverage than state-of-the-art LEO SAR constellations. This paper analyses the specific geometrical characteristics of CoSAR surveys and discusses the challenges of efficient CoSAR image formation approaches. The space-variance of CoSAR surveys is expected to be small, hence CoSAR image formation can benefit from the available knowledge in efficient bistatic and multistatic SAR image formation approaches.

This paper proposes the wrapped staring spotlight (WSS) SAR imaging mode, which is a new method to extend the azimuth steering capability for phased array SAR to achieve either an improved azimuth geometric or radiometric resolution. It investigates the utility of steering directions with main lobe gains that are smaller than that of the grating lobes and exposes how these directions can be exploited. Furthermore, two methods are proposed to reduce the speckle and the image noise at once, i.e., the Look-Normalized Pattern Correction and the Ω-weighting. Based on two example TerraSAR-X WSS acquisitions, the image performance of extended and point targets is discussed.

This paper reviews advanced SAR system architectures and modes for high-resolution ultra-wide-swath SAR imaging. The comparison includes both direct radiating array antennas and reflector-based system configurations operating in either a single-Transmit multiple-receive (SIMO) or a multiple-Transmit multiple-receive (MIMO) mode.

SAR instruments with multiple transmit/receive channels allow for a variety of operation modes. The trade-space includes the instrument and antenna parameters designed to enable various techniques (operation modes). These are utilized to achieve a required SAR performance derived from the mission and user requirements. Finding a solution in this multi-dimensional trade space of inter-dependent parameters is not trivial and often involves the compromise of parameters. This paper addresses the topic of SAR instrument design tailored to a set of operation modes. It explains the SAR techniques and reports the performance highlighting possible compromises between techniques and system efficiency.

Correlating SAR (CoSAR) is an innovative remote sensing concept which delivers large-scale coverage of ocean surfaces with a high repeat cycle. The main specific measurements provided by CoSAR are estimates of the normalised radar cross section (NRCS), Doppler shifts, and surface topography with higher resolution than microwave radiometers and larger coverage than state-of-The-Art LEO SAR constellations. This paper analyses the specific geometrical characteristics of CoSAR surveys, keeping an eye in the future development of efficient CoSAR image formation approaches. Far from being foreign to SAR, the paper will show that CoSAR image formation can benefit from the available knowledge in efficient bistatic and multistatic SAR image formation approaches.

The paper proposes wrapped staring spotlight SAR, a method to extend the azimuth steering capability for phased array SAR systems. Based on two TerraSAR-X (TSX) wrapped staring spotlight example acquisitions, the image performance of point and extended and targets is discussed. In the provided examples, the steering is extended until the gain of the grating lobes becomes much stronger than the gain into steering direction. By this, it becomes possible to improve the azimuth resolution by a factor of two when compared to that of the starring spotlight mode implemented in the operational TSX processing chain.

This paper discusses various aspects of Synthetic Aperture Radar (SAR) missions in Medium Earth Orbits (MEO). It covers the design of suitable orbits and their corresponding coverage, with emphasis on repeat ones. Furthermore, it analyses the changes in SAR performance as altitude increases, while addressing the potentials and limitations of high orbits. Throughout the paper one interesting orbit, repeating its ground track every 3 days and providing near-global coverage, is studied.

The paper reports about a basic analysis of a distributed SAR system realized by several small satellites. The potential for Pulse Repetition Frequency (PRF) reduction and the corresponding swath width extension is discussed. It is derived how to maintain a gap-free phase center (PC) sequence while increasing the inter-satellite distance. The paper presents different kinds of configurations. The one that solves the problem combines alternating transmit (tx) and dispersed receive (rx) satellites. An example configuration suitable for ship monitoring is established. It serves to illustrate the problem statement and its solving.

One of the most ubiquitous, yet treacherous relations in radar is the equation describing the ratio of signal-to-noise powers, known as the radar equation. It is straight forward to derivation the radar equation for a point scatterer, as commonly done on an undergraduate courses level when radar is introduced. Typically, the point scatterer based approach is extended to derive the radar equation for distributed scatterers as applicable for imaging synthetic aperture radar. There are, however, several hidden pitfalls in the radar equation when applied to imaging radars. The most common one, is, that it appears to include a gain, i.e. an increase in the signaltonoise ratio, associated to image focusing (also know as range and azimuth compression). A conclusion which is incorrect and misleading. As it turns out, understanding the radar equation requires a profound SAR and signal processing understanding. This paper attempts a rigorous approach interpreting the signal and noise power of a multi-channel digital beam-forming SAR system.

SAOCOM-CS denotes a satellite formation based on SAOCOM, Argentina's L-band SAR satellite, and a small passive companion satellite (CS) of the European Space Agency (ESA). The realization of this mission is currently under evaluation by ESA. A possible acquisition geometry of particular interest for its scientific and innovative value is a large baseline bistatic (LBB) configuration. This paper discusses the impact of the LBB geometry on the SAR imaging performance computation and presents the expected performance results in a reference scenario based on a stripmap, full polarization operational mode.

ALOS-Next/Tandem-L is a proposal for a highly innovative L-band SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unparalleled quality and resolution. It is based on a collaboration between DLR and JAXA which started with a pre-phase A study in 2013 and is currently undergoing a phase A study. Thanks to the novel imaging techniques and the vast recording capacity with up to 8 Tbytes/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere. By this, the new L-band SAR mission will make an essential contribution for a better understanding of the Earth system and its dynamics. ALOS-Next/Tandem-L will, moreover, open new opportunities for risk analysis, disaster management and environmental monitoring by employing especially designed acquisition modes and techniques in combination with a reconfigurable tandem satellite configuration and an L-band SAR instrument with advanced digital beamforming techniques.

The signal-to-noise ratio of a radar system is described through the well-known radar equation, which can be adapted to apply to imaging Synthetic Aperture Radar (SAR). Extension to multi-channel SAR systems utilizing digital beam-forming turns out to be treacherous; the pivotal question being, if and how to include the gain of the multiple antenna apertures. This paper puts an effort into answering this question in a simple and comprehensible, yet mathematically rigorous way.

A method to generate several phase centres on a single parabolic reflector antenna fed by a digital beamforming array is presented. Among other applications, along-track SAR interferometry at higher frequencies, as for example, in the Ka-band, is considered. Flexibility in size, position and amount of phase centres is similar as for a setup using a direct radiating array.