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This paper introduces and analyses 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 class of highly performant SAR systems employing novel and highly flexible radar imaging modes. Examples are adaptive high-...
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... in one and the same data take. This dilemma motivated further research towards the development of new radar techniques for spacebome high- resolution wide-swath SAR imaging. A promising candidate for such a new radar imaging technique is digital beamforming on receive where the receiving antenna is split into multiple sub-apertures (cf. Fig. 1). In contrast to analog beamforming, the received signals from each sub- aperture element are separately amplified, down-converted, and digitized. This enables an a posteriori combination of the re- corded sub-aperture signals to form multiple beams with adap- tive shapes. The additional information about the direction of the scattered ...
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... behavior of the scatterers and their surroundings. By this, it becomes possible to overcome the fundamental limitations of conventional SAR systems [4]- [10]. A prominent example for the recent developments is the high- resolution wide-swath (HRWS) SAR system which combines a small transmit antenna with a large receiver array as illustrated in Fig. 1 [6] [8]. The small transmit antenna illuminates a wide swath on the ground and the large receiver array compensates the Tx gain loss by a real time digital beamforming process called scanning on receive (SCORE). Multiple azimuth chan- nels allow furthermore for the acquisition of additional samples along the synthetic aperture by ...
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This paper reviews advanced radar architectures that employ multiple transmit and multiple receive antennas to improve the performance of future 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 channels. 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 challenge, a new class of short-term shift-orthogonal waveforms is introduced. The short-term shift orthogonality avoids mutual interferences from the radar echoes of closely spaced scatterers, while interferences from more distant scatterers are suppressed by digital beamforming on receive in elevation. Further insights can be gained by considering the data acquisition of a side-looking imaging radar in a three-dimensional information cube. It becomes evident that the suggested waveforms fill different subspaces that can be individually accessed by a multi-channel receiver. For completeness, the new class of short-term shift-orthogonal waveforms is also compared to a recently proposed pair of OFDM waveforms. It is shown that both sets of waveforms require essentially the same principle of range time to elevation angle conversion via a multi-channel receiver in order to be applicable for MIMO-SAR imaging without interference.
The trend in the conception of future spaceborne radar remote sensing is clearly 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 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. The paper compares the system architecture and SAR performance of a planar and a reflector antenna SAR.
