Advanced Digital Beamforming Architectures and Operation Modes for and Enhanced SIGNAL Mission Concept

Kurzfassung

Recently, a team of scientist lead by the German Aerospace Center (DLR) have proposed a Ka-band single-pass interferometric mission concept named SIGNAL (SAR for Ice, Glacier aNd globAL Dynamics). SIGNAL main goal is to estimate accurately and repeatedly topography and topographic changes variations associated with mass change or other dynamic effects on glaciers, ice caps and polar ice sheets. To achieve the required elevation accuracy, in the order of a few decimeter, a formation flying constellation of two compact satellites was proposed as the only way to obtain the desired cross-track baseline. One of the attractive features of Ka-band is the possibility to design relatively compact systems. However, traditional fixed beam designs Ka-band immediately fail to provide either or both the desired sensitivity (due to the small receive antenna) or the required swath. Thus, some form of digital beamforming is a must. In addition, short antennas lead to high azimuth resolution, but also to a narrow unambiguous swath. In previous work this swath limitation was partially solved by proposing a dual-swath solution using beamforming in elevation only, with a relatively large gap between swathes. This approach improves the global mission coverage, but also adds complexity to the processing and geophysical interpretation. An alternative to achieve wider continuous swathes is to use multiple azimuth phase centers. By introducing multiple antennas spaced in the along-track direction, the system immediately gains the capability to acquire along-track interferometric data. Thus, if properly designed, the resulting SAR can be operated in a wide-swath mode, and in a narrower swath ATI mode. In the particular case of SIGNAL, this means that the two-satellite system could be switched between a wide-swath cross-track interferometric mode, and an also wide swath ATI mode, where in this second case the wide swath would be achieved by combining the narrower ATI mode swathed of each of the two satellites. With this, the SIGNAL would gain the capability to accurately measure ocean currents, which would clearly extend the potential scientific user base. The paper will present the proposed enhanced SIGNAL architecture, and describe possible operating modes. Besides the aforementioned wide-swath and ATI-modes, the paper will also discuss ping-pong multi-baseline acquisitions modes exploiting a novel sub-pulses technique in order to keep a low PRF. For all these modes, the SAR performance will be described. Also the final interferometric performance, in terms of relative height errors or of ocean surface velocity errors will be discussed.