Fore and Aft Channel Reconstruction in the Dual Receive Antenna Mode

Kurzfassung

The technological progress in remote sensing applications based on spaceborne synthetic aperture radar (SAR) developed flexible radar systems to satisfy the user needs for global earth observation and reconnaissance. Recent satellite SAR instruments such as the German satellite TerraSAR-X, the Canadian satellite Radarsat-2 and the Italian satellite constellation COSMO Sky-Med are featuring multiple channels separated in along-track direction in order to not only produce high resolution images of the earth, but also detect movements. Example applications for ground moving target indication (GMTI) are detection of road traffic, ship surveillance or oceanography to name a few. Furthermore, multiple channels separated in along-track direction allows for receiving different polarizations at the same time (in the so-called Quad-Pol mode). Applications thereof are in the field of polarimetry such as soil moisture or biomass estimation. With TerraSAR-X two receive channels separated in along-track direction can either be realized by switching between the fore and aft antenna halves on a pulse to pulse basis, or with the dual receive antenna (DRA) mode. In the DRA mode the signals of the fore and aft antenna halves are combined by a hybrid coupler generating the sum and difference channel data. While the sum channel data are sampled by the main receiver unit, the difference channel data are sampled by the redundant receiver unit. Generating the sum and difference channel data instead of the fore and aft channel data is a low-cost and efficient method to implement a two-channel mode that requires little change to the frontend hardware at the cost of a more complex calibration procedure. Fore and aft channel reconstruction in the processing is necessary in the along-track interferometric mode (ATI mode) if GMTI algorithms such as the displaced phase center antenna technique (DPCA) or along-track interferometry (ATI) shall be applied, and for the Quad-Pol mode in order to separate the horizontally and vertically polarized received signal components, because one polarization is received by the fore, the other polarization by the aft channel. Although this technical note focuses on the compensation of the transmit and receive hardware impact and the fore and aft channel reconstruction of the ATI mode, it is expected that the same channel reconstruction procedure is valid for the Quad-Pol mode as well.