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This work addresses the topic of advanced processing methods for multistatic SAR constellations with large along-track baselines. The main focus is put on the development of novel azimuth reconstruction approaches that accommodate both range-time and range-frequency dependent terms in the Doppler domain as well as in the time domain. Additionally, the influence of the available technology on the accuracy of the reconstruction are investigated, and then a realistic system concept is identified. Diese Arbeit beschäftigt sich mit fortschrittlichen Prozessierungsmethoden für multistatische SAR-Konstellationen mit großem Sender-Empfänger-Abstand entlang der Flugrichtung. Der Schwerpunkt liegt auf der Entwicklung neuartiger Rekonstruktionsansätze, die sowohl im Dopplerbereich als auch im Zeitbereich Terme mit Entfernungszeit- und Entfernungsfrequenzabhängkeit berücksichtigen. Außerdem wird in dieser Arbeit der Einfluss vorhandener Technologie auf die Exaktheit der Rekonstruktion untersucht und ein realistisches Systemkonzept ausgearbeitet.

This article analyzes the process of image synthesis for a formation flying synthetic aperture radar (FF-SAR), which is a multistatic synthetic aperture radar (SAR) based on a cluster of receiving-only satellites flying in a close formation, in the framework of the array theory. Indeed, the imaging properties of different close receivers, when analyzed as isolated items, are very similar and form the so-called common array. Moreover, the relative positions among the receivers implicitly define a physical array, referred to as spatial diversity array. FF-SAR imaging can be verified as a result of the spatial diversity array weighting the common array. Hence, different approaches to beamforming can be applied to the spatial diversity array to provide the FF-SAR with distinctive capabilities, such as coherent resolution enhancement and high-resolution wide-swath imaging. Simulation examples are discussed which confirm that array theory is a powerful tool to quickly and easily characterize FF-SAR imaging performance.

Conventional single-platform spaceborne synthetic aperture radar has a conflict between wide imaging swath width and fine-cross-range resolution. Displaced phase center antennas is one method to overcome this limitation. For spectrum recovery of nonuniform sampling, a detailed analysis on the relationship between coefficient matrix singularity and the number of effective phase centers (EPCs) passed by the platform during one pulse repetition time (PRT) is given. When the distance passed by the platform during one PRT is multiple of the distance between two EPCs plus a small value, the coefficient matrix is singularity. An explicit expression of the coefficient matrix is also derived. Ministry of Education (MOE) This work was supported in part by the Ministry of Education, Singapore, under Grant MOE2016-T2-1-070, and in part by the National Natural Science Foundation of China under Grant 61471386 and Grant 61631019.

In the context of state-of-the-art and next-generation spaceborne Synthetic Aperture Radar (SAR) imaging systems, such as the German Tandem-L mission proposal, a short revisit time coupled with high resolution presents itself as a requirement for the observation of numerous Earth dynamic processes in various scientific and environmental applications. This poses contradicting and demanding requirements on system design for whose realization digital beamforming techniques play a crucial role, in order to enable next-generation systems to significantly outperform current state-of-the-art systems and fulfill the increasingly stringent needs of applications. Multichannel SAR systems with digital beamforming in azimuth have been proposed as an option for coping with these challenging requirements in order to achieve high-resolution wide-swath (HRWS) imaging capabilities. These systems tend however to require a very long antenna to image wide swaths. As an alternative, the use of digital beamforming in elevation, in conjunction with a varying pulse repetition interval (PRI) for the SAR system radar pulses (Staggered SAR), has been proposed. The best azimuth resolution in this case is nevertheless limited by the antenna dimensions. The thesis focuses on the development of new SAR techniques and corresponding processing methodologies which allow the combination of the advantages of the multichannel system architectures in azimuth with those of staggered SAR. This introduces new and potentially highly flexible modes of operation, enabling HRWS imaging with a compact antenna and contributing to the new developments in the field of digital beamforming techniques. The thesis includes the proposal of novel algorithms for the resampling of multichannel staggered SAR data and examples of high-performance HRWS systems designed to make use of the technique. In addition, a proof-of-concept with experimental data from a ground-based radar system and a discussion of the possible effects of errors for the implementation of this new class of spaceborne imaging systems are presented.

Synthetic aperture radar (SAR) is an active microwave remote sensing technique that can provide high-resolution images independent of sunlight illumination. Moreover, because clouds, fog, and precipitation do not have a signifcant effect on microwaves, all-weather imaging is possible. Because of the coherent reception of the radar echoes, an effective long “synthetic” antenna can be generated by signal processing and exploited to achieve high resolution in the fight (or azimuth) direction. These peculiarities make SAR attractive for the systematic observation of dynamic processes on Earth's surface [1].

Several applications of synthetic aperture radar (SAR) require or at least benefit from full polarimetry. In the past, however, full polarimetry has been often discarded in the design of spaceborne SAR missions, as it implied a significant reduction of the swath width, already constrained by the requirement on azimuth resolution. Innovative SAR techniques, based on digital beamforming on receive, allow nowadays high-resolution wide-swath (HRWS) imaging through multiple elevation beams, which simultaneously map multiple sub-swaths. Furthermore, a continuous variation of the pulse repetition interval, i.e., staggered SAR, can be introduced to get rid of the “blind ranges” between the multiple sub-swaths, due to the fact that the radar cannot receive, while it is transmitting. This paper considers how the introduction of full polarimetry in HRWS imaging makes the design of the SAR system more challenging, but represents a viable solution for frequent and seamless high-resolution fully-polarimetric imaging of the Earth’s surface.

Although high-resolution wide-swath (HRWS) SAR imaging and ground moving target indication (GMTI) systems from the system architecture point of view look quite similar (both system types employ multiple receiving (RX) channels arranged along azimuth direction), they generally need to be operated in different modes, in particular with significantly different PRFs. HRWS systems require a comparatively low PRF, reflecting the overall length of the full antenna. The low PRF enables wide-swath imaging, and the multiple RX channels are used for signal spectrum reconstruction. In contrast, GMTI systems require a high PRF, reflecting the short length of the individual RX channel. The multiple RX channels are then used for clutter suppression and moving target parameter estimation. The high PRF additionally shall ensure that aliasing of the Doppler shifted moving target signal energy is avoided. It is known that the Doppler shift is proportional to the target’s velocity and inversely proportional to the radar wavelength. Thus, especially at Ka-band with small wavelengths in the millimeter range, the Doppler shift and, hence, the required PRF for avoiding aliasing may be very high. In general the high PRF required for GMTI prevents wide-swath SAR imaging as well as wide-swath GMTI. The resolution of this conflict is the issue addressed in this contribution. In the frame of a contract extension of the ESA-funded TRP Activity “Study of Multi-Channel Ka-Band SARs for Moving Target Indication”, novel low PRF system operation modes and processing approaches which allow for simultaneous GMTI and HRWS SAR imaging have been developed and investigated in detail. In particular, a special matched reconstruction filter bank (MRFB), which is the heart of all proposed processing approaches, has been developed. With the MRFB low PRF target signals acquired with a conventional HRWS system can be reconstructed and refocused. The application of the MRFB also provides accurate estimates of the target’s velocity. Furthermore, potential extensions of conventional HRWS systems have been proposed for additionally enabling effective clutter suppression without the need of increasing the PRF. In the final paper the results of the contract extension of the TRP Activity will be presented. The MRFB principle, the proposed and investigated HRWS-GMTI system concepts and the corresponding processing approaches will be introduced and explained in detail. The HRWS performance and the GMTI performance obtained from Monte-Carlo simulations will be presented and discussed. Also potential drawbacks of the proposed HRWS-GMTI modes will be addressed.

Synthetic Aperture Radar (SAR) is a well-proven imaging technique for remote sensing of the Earth. However, conventional SAR systems are not capable of fulfilling the increasing demands for improved spatial resolution and wider swath coverage. To overcome these inherent limitations, several innovative techniques have been suggested which employ multiple receive-apertures to gather additional information along the synthetic aperture. These digital beamforming (DBF) on receive techniques are reviewed with particular emphasis on the multi-aperture signal processing in azimuth and a multi-aperture reconstruction algorithm is presented that allows for the unambiguous recovery of the Doppler spectrum. The impact of Doppler aliasing is investigated and an analytic expression for the residual azimuth ambiguities is derived. Further, the influence of the processing on the signal-to-noise ratio (SNR) is analyzed, resulting in a pulse repetition frequency (PRF) dependent factor describing the SNR scaling of the multi-aperture beamforming network. The focus is then turned to a complete high-resolution wide-swath SAR system design example which demonstrates the intricate connection between multi-aperture azimuth processing and the system architecture. In this regard, alternative processing approaches are compared with the multi-aperture reconstruction algorithm. In a next step, optimization strategies are discussed as pattern tapering, prebeamshaping-on-receive, and modified processing algorithms. In this context, the analytic expressions for both the residual ambiguities and the SNR scaling factor are generalized to cascaded beamforming networks. The suggested techniques can moreover be extended in many ways. Examples discussed are a combination with ScanSAR burst mode operation and the transfer to multistatic sparse array configurations.

Innovative multi-channel synthetic aperture radar (SAR) concepts enable high-resolution wide-swath imaging, but the antenna length typically restricts the achievable swath width. This limitation can be overcome by a novel technique which is based on a single azimuth channel but operates the system with a continuously varied pulse repetition frequency (PRF) by this allowing in principle for arbitrary wide swaths. This paper introduces the basic principles and discusses design constraints for such a PRF variation. Further, a systematic performance analysis of an L-band reflector antenna system is carried out with focus on the sensitivity versus different input parameters.

Conventional Synthetic Aperture Radar (SAR) systems with a single transmit and receive antenna are not capable of simultaneously imaging a wide swath with high spatial resolution. New techniques employing multiple receive apertures to overcome this system-inherent limitation are examined in this work, with particular emphasis on multiple channels in azimuth. The work introduces an innovative processing algorithm that is suited to cope with the multi-channel data. Comprehensive analysis of this novel “multi-channel reconstruction algorithm” covers the theoretical investigation of its impact on system performance as well as its verification with measured and simulated data. The simulation results are given in the context of a complete high-resolution wide-swath SAR system design which highlights the intricate relationship between system architecture and multi-channel processing. Subsequently, different innovative optimization strategies to improve the system performance and increase the operational flexibility are demonstrated and theoretical analysis is extended to these approaches. Furthermore, multi-channel systems operating in burst modes such as TOPS (Terrain Observation with Progressive Scan) and ScanSAR are investigated as a solution to new imaging modes enabling for compact antenna dimensions an unprecedented combination of an ultra-wide swath with reasonable geometric resolution in azimuth.

Staggered SAR is an innovative synthetic aperture radar (SAR) concept, based on the continuous variation of the pulse repetition interval (PRI). This concept allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple apertures.