Digital beamforming: A paradigm shift for spaceborne SAR

Radar Conference, …, 2009

The trend in the conception of future spaceborne radar remote sensing is towards the use of digital beam-forming techniques. These systems will comprise multiple digital channels, where the analog-to-digital converter is moved closer to the antenna. This dispenses the need for analog beam steering and by this the use of transmit/receive modules for phase and amplitude control. Digital beam-forming will enable Synthetic Aperture Radar (SAR) which overcomes the coverage and resolution limitations applicable to state-of-the-art systems. Moreover, new antenna architectures, such as reflectors, already implemented in communication satellites, are being reconsidered for SAR applications. This paper introduces a new digital beam-forming radar concept based on the combination of a reflector with a digital feed array. For a system example the SAR performance is estimated. Finally a novel digital signal processing approach, exploiting the signal properties of the transmitted waveform, is presented.

Synthetic Aperture Radar (SAR) is an indispensable source of information in earth observation since SAR is the only spaceborne sensor that has highresolution, all-weather and day-and-night imaging capability. Within the next 10 years more than 20 spaceborne SAR systems from different nations will be launched. Increased demand for larger swath width and higher geometric resolution poses contracting requirements on the system design. This paper gives an overview of the latest developments on the digital beamforming technique that allows overcoming the fundamental limitations of conventional SAR systems. Finally, this paper summarizes the potential of bi-and multi-static SAR systems and gives an outlook on the TanDEM-X mission.

2006

Multi-aperture synthetic aperture radar (SAR) systems in combination with an appropriate coherent processing of the individual aperture signals enable high resolution wide swath (HRWS) SAR imaging [1]-[8]. An innovative reconstruction algorithm for such a digital beamforming on receive was presented in [9]-[12] that allows for HRWS even in case of a non-uniformly sampled data array in azimuth. This paper will compare this algorithm to different azimuth processing strategies regarding their performance in dependency of the overall sampling. Further, optimization strategies are discussed to maximize the system's performance by pattern tapering on transmit and "Pre-Beamshaping on Receive" networks that allow for pattern tapering on receive and adaptively adjust the virtual sample positions.

International Journal of …, 2009

The trend in the conception of future spaceborne radar remote sensing is clearly toward the use of digital beamforming techniques. These systems will comprise multiple digital channels, where the analog-to-digital converter is moved closer to the antenna. This dispenses the need for analog beam steering and by this the used of transmit/receive modules for phase and amplitude control. Digital beam-forming will enable Synthetic Aperture Radar (SAR) which overcomes the coverage and resolution limitations applicable to state-of-the-art systems. On the other hand, new antenna architectures, such as reflectors, already implemented in communication satellites, are being considered for SAR applications. An open question is the benefit of combining digital beamforming techniques with reflector antennas. The paper answers this question by comparing the system architecture and digital beam-forming requirements of a planar and a reflector antenna SAR. Further elaboration yields the resulting SAR performance of both systems. This paper considers multiple novel aspects of digital beam-forming SAR system design, which jointly flow into the presented system performance.

2007

This paper introduces and analyses the innovative paradigm of multidimensional waveform encoding for spaceborne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new class of highly performant SAR systems employing novel and highly flexible radar imaging modes. Examples are adaptive high-resolution wide-swath SAR imaging with compact antennas, enhanced parameter estimation sensitivity for applications like along-track interferometry and moving object indication, and the implementation of hybrid SAR imaging modes that are well suited to satisfy the hitherto incompatible user requirements for frequent monitoring and detailed mapping. Implementation specific issues will be discussed and examples demonstrate the potential of the new technique for different remote sensing applications.

2005

Synthetic Aperture Radar (SAR) is a well- proved instrument for remote sensing. Multi-aperture SAR- systems allow for digital beamforming on receive that will enable a wide swath SAR with high azimuth resolution. In classic multi-aperture systems, stringent requirements are imposed on sensor velocity and PRF to ensure uniform sampling of the synthetic aperture. This paper shows that an unambiguous reconstruction of the SAR signal is also possible in case of such a non-optimum PRF. For this, an innovative reconstruction algorithm is derived, which en- ables a recovery of the unambiguous Doppler spectrum also in case of a non-uniform sampling of the synthetic aperture. Further, the influence of perturbations on the reconstruc- tion is presented and general system aspects are discussed.

2006

Multi-aperture synthetic aperture radar (SAR) systems in combination with an appropriate coherent processing of the individual aperture signals enable high resolution wide swath (HRWS) SAR imaging [1]-[8]. An innovative reconstruction algorithm for such a digital beamforming on receive was presented in [9]-[12] that allows for HRWS even in case of a non-uniformly sampled data array in azimuth. This paper will compare this algorithm to different azimuth processing strategies regarding their performance in dependency of the overall sampling. Further, the algorithm is characterized theoretically regarding aliasing and residual ambiguities. In this context, optimization strategies are discussed to maximize the system's performance by pattern tapering on transmit and "Pre-Beamshaping on Receive" networks that adaptively adjust the virtual sample positions.

2015 IEEE Radar Conference (RadarCon), 2015

Since its invention 60 years ago the synthetic aperture radar (SAR) principle has been continuously pushed further in terms of information density. Future SAR systems are required to collect data in different frequency bands and polarizations with finer spatio-temporal resolution and higher quality. These requirements drive technology developments with a trend towards digital radars. This type of radar is highly flexible and configurable compared to its analogous counterpart and allows a relatively easy implementation of digital beamforming (DBF) as well as multiple input multiple output (MIMO) techniques. The demand of highly sensitive SAR sensors enforces antenna concepts with large aperture. A promising candidate are large deployable mesh reflector antennas fed by an array of feed elements. Each feed element, or a subset of feed elements, defines a digital channel to be further processed on board the spacecraft or on ground. These innovative class of high-information content SAR sensors are the result of a carefully optimized design in both, the hardware and the signal processing domain. Here, an advanced SAR sensor concept based on array-fed reflector antennas shall be presented. Space, time and frequency adaptive beamforming techniques which complement the antenna design are introduced and demonstrated by means of numerical simulations.

IEEE Transactions on Aerospace and Electronic Systems, 2012

Kontaktlose Informationsgewinnung mittels elektromagnetischer Wellen spielt für den wissenschaftlichen und industriellen Fortschritt eine immer wichtigere Rolle. Insbesondere das Prinzip des Radars mit synthetische Apertur (SAR), das zu den bildgebenden Verfahren zählt, gehört heute zum Standardrepertoire ziviler und militärischer Erdbeobachtung. Zukünftige SAR-Systeme, deren Leistungsfähigkeit signifikant von der verwendeten Hardware sowie der Datenverarbeitung abhängt, sollen immer größere Gebiete in immer kürzeren zeitlichen Abständen erfassen können. Die vorliegende Arbeit beschäftigt sich mit der Sensorik, dessen beide Hauptbestandteile die Antennentechnik sowie die nachfolgende analoge und digitale Signalverarbeitung umfasst. Der Fokus liegt hierbei auf digitalen Strahlformungstechniken, die es erlauben, den SAR-Sensor besonders effizient zu betreiben. Hierzu werden raum-zeit-adaptive Algorithmen abgeleitet, die geeignet sind, das Systemrauschen des Sensors zu minimieren sowie räumliche Interferenzen zu unterdrücken. Diese Algorithmen werden anhand eines satelliten-getragenen SAR-Systems simuliert, bei dem das innovative Konzept einer großen entfaltbaren Reflektorantenne in Kombination mit einem digitalen Speisearray zum Einsatz kommt. Sinn dieses Radartyps ist es, das elektromagnetische Signal möglichst kurz nach dem Empfänger zu digitalisieren, um eine maximale Flexibilität des SAR-Systems zu erreichen und gleichzeitig Kosten für teure analoge Elektronik zu sparen. Der zweite Schwerpunkt der Arbeit behandelt Optimierungsaspekte, wobei unter anderem eine neue Methode aufgezeigt wird, die es erlaubt, eine inhärente Problematik solcher Reflektor-Speisearray-Konstellationen abzumildern. Da bei Ausfall eines oder mehrerer Elemente des Speisearrays ein blinder Bereich im Aufnahmefeld entsteht, wird zur Lösung ein defokussiertes Reflektorantennen-Konzept vorgeschlagen, das den Vorzug planarer Arrayantennen, nämlich der Erhaltung der Funktionsfähigkeit bei Ausfall eines Antennenelements, mit dem Vorteil entfaltbarer Reflektorantennen, nämlich der Bereitstellung großer Aperturen, kombiniert.

IEEE Transactions on Geoscience and Remote Sensing, 2008

This paper introduces the innovative concept of multidimensional waveform encoding for spaceborne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new generation of SAR systems with improved performance and flexible imaging capabilities. Examples are high-resolution wide-swath radar imaging with compact antennas, enhanced sensitivity for applications like alongtrack interferometry and moving object indication, and the implementation of hybrid SAR imaging modes that are well suited to satisfy hitherto incompatible user requirements. Implementationspecific issues are discussed and performance examples demonstrate the potential of the new technique for different remote sensing applications.