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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 beam-forming 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.

One of the benefits of synthetic aperture radar (SAR) systems utilizing digital beamforming is the ability to increase the receive power. The relevant SAR technique is known as SCan-On-REceive (SCORE), which steers the receive antenna beam such that it follows the radar pulse echo traversing the ground. This allows the use of a narrow receive beam in elevation, and therefore, the height of the receive antenna can be increased, resulting in a higher gain, which explains the higher receive power. Although advantageous, this technique has some pitfalls, which impose an upper limit on the antenna size and constrain the selection of SAR operation parameters. These limitations (which are often neglected in the system conception) are caused by the pulse extent on the ground and the way it is modulated by the receive antenna pattern. This letter addresses and quantifies the effects caused by the transmit pulse length (here denoted as pulse extension loss) through a rigorous analysis, with the purpose of introducing an important SAR performance figure. Closed expressions are derived for the simplified case of a uniform linear antenna array.

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This paper reviews advanced multi-channel SAR system concepts for the imaging of wide swaths with high resolution. Several novel system architectures employing both direct radiating arrays and reflector antennas fed by a digital array are introduced and compared to each other with regard to their imaging performance. In addition, innovative SAR imaging modes are proposed which enable the mapping of ultra-wide swaths with high azimuth resolution. The new techniques and technologies have the potential to enhance the imaging performance of future SAR systems by one order of magnitude if compared to state of the art SAR sensors like TerraSAR-X, ALOS, Radarsat-2 or Sentinel-1.