On top of the nominal right-looking mode, by performing a roll manoeuvre of the satellite, data can also be acquired in left-looking geometry. Beyond the 3 basic modes, operation in the following experimental modes is planned:
Due to this multitude of operation modes based on an active phased antenna array with hundreds of transmit/receiver modules (TRM), we obtain a large number of different antenna beams (~ 10000 for TerraSAR-X). Furthermore, the demand on the absolute radiometric accuracy with only a couple of tenths of dB means that the complete TerraSAR-X system has to be adjusted with the accuracy of laboratory equipment.
Thus, a conventional calibration approach of SAR systems like ERS or X-SAR, involving internal calibration of a single transmit/receive chain (via special calibration loops), the determination of a single antenna beam pattern and the absolute calibration in one or two operational modes, is not feasible. Hence, new, more efficient and affordable methods have been developed. The two most important innovations are:
Taking these aspects into account, the main efforts have been concentrated on developing the calibration concept for TerraSAR-X since 2003. As part of the TerraSAR-X Ground Segment the concept has been realized according to the design depicted in Figure 2.
The TerraSAR-X Calibration System provides all necessary sub-systems to perform the different calibration tasks. The major functions are:
With regard to these functions, new software tools have been implemented and existing algorithms were updated (Web page ‘Algorithms and Tools’). Further calibration targets are being developed and purchased.
Currently the detailed calibration activities required during the commissioning phase of TerraSAR-X are being planned. Starting from the calibration strategy, different constraints have to be considered here, e.g. the coverage on the Earth’s surface or the required number of point target measurements assumed in the radiometric accuracy budget. One example is shown in Figure 3. In order to obtain as many as possible passes over deployed calibration targets, the test sites will be set up in the cross-over points of ascending and descending orbits. Mainly for logistic reasons we aim to select these test sites near Oberpfaffenhofen, as shown in Figure 4.
The calibration strategy of TerraSAR-X has been adapted to TerraSAR-L (Web page ‘TerraSAR-L’) and to Sentinel-1 (Web page ‘Sentinel-1’).