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.

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.

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.

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.

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.

This paper presents a novel concept of a spaceborne Digital Beamforming (DBF) Synthetic Aperture Radar (SAR) based on a multi-beam reflectarray antenna. The proposed solution offers a number of advantages compared to more conventional DBF SAR systems using planar arrays or reflector antennas. The paper focuses on the spaceborne DBF SAR and discusses its architecture, system parameters and an imaging scenario. A reflectarray antenna design to be used in a combination with the considered imaging platform is presented in the paper. The reflectarray model is used to estimate DBF SAR performance characteristics discussed in the final part of the paper.

A transmit scheme that uses several timed and directed sub-pulses to cover one common swath is proposed. Timing and ambiguity suppression is presented. The advantages of the proposed scheme are an increased sensitivity compared to one transmit pulse per swath and a simpler activation scheme of the receivers in combination with lower data compression effort on-board a satellite.

The term MIMO SAR is used for Synthetic Aperture Radar (SAR) systems utilizingmultiple-receive andmultiple-transmit channels (Multiple-Input Multiple Output: MIMO). The trade-space for such SAR systems 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 show examples of how 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 multiple performance parameters; a feature not available in conventional single channel SAR systems. The content of this paper is considered novel in the sense that it deals with highly advance SAR techniques.

The paper presents the instrument concept and performance of a Ka-band single-pass interferemetric mission proposed for measuring topography and topographic changes. A formation flying constellation of two compact synthetic aperture radars (SAR) satellites equipped with innovative digital beamforming hardware and advanced operation modes is described.

This paper presents a novel concept of a spaceborne Digital Beamforming (DBF) Synthetic Aperture Radar (SAR) based on a multi-beam reflectarray antenna. The proposed solution offers a number of advantages compared to more conventional DBF SAR systems using planar arrays or reflector antennas. The paper focuses on the spaceborne DBF SAR architecture using a reflectarray antenna designed for the imaging platform. DBF SAR system parameters, an imaging scenario and a reflectarray antenna model are described in detail. The results of a performance analysis of the SAR system are presented and discussed in the paper.

In the general effort of understanding climate change, one of the key challenges is understanding the feedback loops between the global climate system and the Cryosphere. Current observations qualitatively show that the Cryosphere reacts very sensitively to climate change. However, improved observation capabilities are needed in order to better quantify some of the observed processes. In particular, in order to quantify the complex dynamics of ice mass balances, a detailed and accurate knowledge of the time varying ice surface topography is needed.

TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model of the Earth surface by means of single-pass X-band synthetic aperture radar (SAR) interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in nonnominal modes have been conducted with the main purpose of validating the performance of both space and ground segments in very demanding scenarios. In particular, this letter reports about the first bistatic acquisition and the first single-pass interferometric (mono-/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focusing on the analysis of the experimental results. Even in the absence of essential synchronization and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

This paper presents the strategy for the development of TOPMEX-9, an innovative concept for Earth observation based on synthetic aperture radar (SAR) and nanosatellite clusters. The concept is intended as a, as a start up project for future collaboration between the Microwaves and Radar Institute (HR) of the German Aerospace Center (DLR), the Mexican Space Agency (AEM) and the Mexican Talent Network (RDTM). The idea is based on a nanosatellite formation flying around a microsatellite using a distributed constellation in multistatic SAR mode. This is an analogy with the sun providing illumination to passive optical receivers or cameras. The microsatellite acts as a speaker (Tx) while the nanosatellites around behave as listeners (Rx). Multistatic SAR mode allows the separation of radar payloads, thus decreasing volume, weight, power and consequently the mission costs. It allows permits retrieval of multi-angle measurements, thus obtaining more information about the illuminated scene than the monostatic SAR systems. The design of each TOPMEX-9 nanosatellite is based on the CubeSat standard and includes a single receiver reflector antenna in H or V polarisation in the Ka-band (32.6-37.0 GHz). The SAR system distributed architecture (i.e. radar, TM/TC, tracking and intercommunication) has the advantage of maximizing the energy of the radar antenna, thus having a better signal-to-noise ratio. TOPMEX-9 is predicted to a great impact in future low-cost Earth observation missions. This mission is focused on applications in oceanography such as ocean wave spectra and sea surface roughness measurements, coastal area monitoring, wind speed estimation and atmosphere studies. Other applications are ice roughness in cryosphere research and ship detection. The limited lifetime of a nanosatellite is compensated by the fact that new radar cluster configurations can be launched based on lessons learned, contributions in the acceleration of technology development and proving innovative data acquisition schemes.

This paper reviews radar architectures that employ multiple transmit and multiple receive channels to improve the performance of synthetic aperture radar (SAR) systems. These advanced architectures have been dubbed multiple-input multiple-output SAR (MIMO-SAR) in analogy to MIMO communication systems. Considerable confusion arose, however, with regard to the selection of suitable waveforms for the simultaneous transmission via multiple antennas. In this paper, it is shown that the mere use of orthogonal waveforms is insufficient for the desired performance improvement in view of most SAR applications. As a solution to this fundamental MIMO-SAR problem we had previously suggested to exploit the special data acquisition geometry of a side-looking imaging radar equipped with multiple receiver channels in addition to appropriately designed waveforms transmitted by multiple antennas. Here, we extend this approach to a more general set of radar waveforms with special correlation properties that satisfy a short-term shift-orthogonality condition. We show that the echoes from simultaneously transmitted pulses can be separated if the short-term shift orthogonality is combined with digital beamforming on receive in elevation. This enables the implementation of a fully functional MIMO-SAR without correlation noise leakage for extended scattering scenarios.