# Ten Years of TerraSAR-X Operations
## 5 Radar Experiments with TerraSAR-X
### Prerequisite: Flexible Instrument Commanding  
The flexible instrument commanding in combination with a homogenous order chain for standard SAR data takes, as well as for experimental data takes, also called system data takes, enables the mixed operation for both types of data take within the same timeline, see Figure 3. The configuration of the SAR instrument does not have to be changed, since both, nominal and experimental data takes contain similar building blocks. This approach enables the development, test, and optimization of a new SAR mode with the final operational settings.  
The flexible data take commanding is provided by the combination of four main building blocks. One building block describes the pulse to be transmitted (see Figure 4 for a set of parameters describing the pulse). Up to eight different pulses can be used within one data take. Another building block controls the echo window w.r.t. position, length, receive gain. Transmi  
Figure 3: Data take generation chain for nominal and experimental data takes.  
can be changed from pulse to pulse. The third building block controls the antenna beam direction, pattern, and polarization. In general, the shortest change interval between two antenna settings is some milliseconds, but one selected set of antenna parameters can be changed from pulse to pulse. Up to 127 different antenna settings can be applied during one data take. A switching between H and V antenna polarization from pulse to pulse (toggling) is controlled by a parameter within the antenna properties. The controlling flag for polarization toggling can be set for transmit and receive independently. The fourth building block contains the control parameters for the structure of the data take, e.g., the number of pulses for which the radar is operating with the same radar settings. Sequences of radar settings can be repeated several times. Therefore, this repetition feature enables a convenient commanding of sub-swathes in ScanSAR data takes. The following chapters illustrate the wide range of useful applications utilizing the flexible instrument commanding.  
To build an along track interferometer with just one receiving chain, the ability of pulse to pulse switching between two antenna settings with different receive azimuth apertures is used. One pulse is received with a dedicated part of the antenna in flight direction (fore part); the next pulse is received with the part that is opposite to the flight direction (aft part). The echoes that were received by both parts are SAR processed separately and result in two SAR images with a baseline in flight direction. This mode is called Along Track Interferometer Switching (ATIS).  
The six-beam wide-ScanSAR mode (see Section 3.1) uses the ability to transmit dedicated pulses for each sub-swath, where the transmit bandwidth is optimized for constant radiometric performance over the full incidence range. With this, only the receiving noise of the transmitted bandwidth contributes to the image noise and the signal to noise ratio is increased.  
TOPSAR (see Section 5.3) data takes demonstrate the flexibility in commanding both, azimuth and elevation antenna steering, and the repetition of this structure. This mode reaches the limit of the current TerraSAR-X instrument design in using the maximum number of antenna settings, since each azimuth and elevation combination counts as one setting for the data take.  
Figure 4: Building blocks and structure of a data take.