This paper presents a methodology to design a spaceborne dual-beam along-track synthetic aperture radar interferometer to retrieve ocean surface velocity vectors. All related aspects and necessary tradeoffs are identified and discussed or reviewed, respectively. This includes a review of the measurement principle and the relation between baseline and sensitivity, the relation between wind and radar backscatter, a discussion of the observation geometry, including the antenna concept, polarization diversity, and all main error contributions. The design methodology consists of a sensitivity-based derivation of explicit instrument requirements from scientific requirements. In turn, this derivation is based on a statistical model for the interferometric phase error. This allows a quantitative, well-grounded instrument design offering an additional degree of freedom to the approach, which we call ''noise-equivalent-sigma-zero requirement space.'' Crucial tradeoffs for the system design, such as the resolution, the number of independent looks, the minimum wind speed, and the coherence and ambiguities, are pointed out and discussed. Finally, this paper concludes with a single platform system concept operating in Ku-band, which provides the measurement quality needed to achieve a surface velocity estimation accuracy of [5] cm/s, [200]-km swath coverage, for [$4x4$] km² L2-product resolution and winds starting at [3] m/s.

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.

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.

The capabilities of the current concept for a post-Sentinel-1 High ResolutionWide Swath (HRWS)C-band SAR system as discussed. Product-level performance analysis are given for two example applications (large scale deformation and ship detection), showing the clear improvement with respect to Sentinel-1 and exposing some of the trade-offs associated to the choice of operating mode.

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.

The architecture of framework to simulate Digital Beamforming reflector-based SAR systems is presented. The mai purpose of this tool is to allow the development and evaluation of DBF processing and calibration techniques unde realistic conditions. One of the main challenges is to model the antenna patterns, including their polarimetric characteristic and topography dependencies.

Ambiguities are one of the limiting factors of SAR product quality and are thus important subject to every mission performance analysis. Since target NRCS is highly heterogeneous, the resulting ambiguities affect different areas at very different levels. Therefore using global average ambiguity levels for product performance assessment is of limited use. In this paper we present a statistical analysis of signal to ambiguity levels taking into account the spatial variability of the NRCS by exploiting global backscattering maps.

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.

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.

This paper presents a spaceborne SAR mission concept in Ku-band for the measurement of the two-dimensional ocean surface current. The measurement concept is based on dual-beam along-track interferometry, which requires two highly squinted antennas each looking simultaneously in fore and aft direction. A single side-looking geometry and a ScanSAR imaging mode is chosen to complete the observational concept in order to achieve the requirements on revisit-time and coverage. The applied hybrid polarization is intended to increase the information level about different ocean scattering mechanisms. Furthermore a methodology of instrument requirement derivation is briefly discussed, ensuring the ocean current measurement accuracy requirements. Finally the instrument concept, including digital beamforming, and the SAR performance are presented.

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.

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.

This paper presents two reflectarray antenna configurations used as receive and transmit antennas for a ku-band Synthetic Aperture Radar system (SAR). The receive antenna is constituted by nine equal single offset reflectarray modules. The transmit antenna is based in two dual offset parabolic reflector antenna with a rectangular reflectarray as main reflector. Both receive and transmit antennas are able to work in two different modes using orthogonal polarizations, the narrow swath mode and wide swath mode.

In this paper the flexibility in modifying the resulting antenna characteristics in terms of location of phase centres and aperture widths, independent of the antenna type used by applying linear combinations of digital beams is discussed. Especially a parabolic reflector can be considered to have the same properties as a Direct Radiating Array (DRA) when used for SAR applications like along-track interferometry.

Instrument calibration has ever been essential to synthetic aperture radar. This paper reviews the calibration functionality of current state-of-the-art spaceborne SAR and then proceeds to suggest calibration strategies for future SAR systems. These systems will incorporate multi-channel digital beamforming capabilities which offer new opportunities but also challenges for digital calibration. At the same time, the increased complexity of instrument calibration can not be extrapolated to future systems. This requires a reconsideration of the calibration strategy for spaceborne SAR. The paper is seen as a step in this direction.

MIMO SAR stands for systems utilizing multiple-receive and multiple-transmit channels. The trade-space for SAR includes the instrument and antenna parameters taking into consideration the techniques, i.e. the operation modes of the radar. The purpose of exploring this trade-space is to improve and optimize the performance of the system, described through the performance parameters. The aim of this paper is to give examples showing that MIMO SAR offers new and unexpected ways of trading the system and performance parameters versus each other. Adequately exploring the trade-space yields a high flexibility and allows to simultaneously improve several performance parameters; conventional single channel SAR systems do not have the same flexibility. The content of this paper is considered novel in the sense that it deals with highly advanced techniques for SAR.

This paper describes the performance analysis of a Ka-band Synthetic Aperture Radar (SAR) system working in joint mode, which is able to provide an adequate Cross-Track Interferometric (XTI) and Ground Moving Target Indication (GMTI) performance. The common major problem of joint mode is the low Pulse Repetition Frequency (PRF). To overcome this problem a long receive antenna is considered, which allows a reduction of the PRF, resulting in an increase of the swath widths. The receive antenna is constituted by nine equal reflectarray modules and the transmit antenna consists of two reflectarray modules, both antennas are able to work in narrow and wide swath modes.