# Ten Years of TerraSAR-X Operations
## 8 Performance Monitoring
### Data Take Verification  
The Data Take Verification functionality provides an immediate feedback on data take quality and overall performance of the SAR system after commanding, execution, and SAR screening of the raw data of a TerraSAR-X or TanDEM-X acquisition. It combines information from data take ordering, commanding, as well as the data reception and processing chain, and it therefore provides additional value on top of the regular SAR data screening.  
Three main tasks are in the scope of Data Take Verification. The first one is the evaluation of the SAR screening results of an acquisition, where the raw data statistics of in-phase and quadrature channels are checked against predefined quality limits. These checks can identify instrument calibration issues like channel bias or imbalances, but also issues in the commanding like non-optimal receiver gain settings, which would result in insufficient raw data saturation or even overflows in the A/D converters. Another prominent quality parameter is the Doppler centroid that is estimated from the raw data, which is a sensitive parameter for the whole SAR system.  
Another check is the completeness of scene coverage w.r.t. the user-requested scene, which depends not only on precise instrument switch-on and switch-off, but also on the correct commanding of the radar echo window. As an example, Figure 15 shows a control plot for an acquisition in Stripmap mode taken over Morocco. The red polygon indicates the scene, as requested by a user, whereas the green polygon shows the boundaries of the recorded and processed scene.  
Figure 15: Example for scene coverage verification after data take acquisition, visualized in Google Earth. The red polygon indicates the requested scene, whereas the green polygon shows the acquired scene. In this case, a considerable part of the data take is not available, as the downlink was affected by severe weather conditions.  
In the third step of the verification of a data take, or data take pair in the case of a TanDEM-X acquisition, the correct on-board execution of each commanded data take is verified, down to the level of single pulses, via the correlation of the data take command parameters, as generated by the Request-to-Command Converter (R2 CC), with the range lines actually being recorded and downlinked by the radar instruments. This check has the potential to identify issues in the complex interleaved commanding of imaging pulses with other instrument-specific pulse types, like warm-up sequences, calibration pulses, and interleaved inter-satellite synchronization in the case of TanDEM-X. It does also provide valuable information in the analysis of issues regarding missing or corrupted range lines in the SAR data.  
Throughout the mission, data take verification provided valuable routine monitoring. Due to the reliability of the whole SAR system, major issues were detected in rare cases only. Yet, the system was used extensively during the commissioning phases of the satellites, where it allowed for verifying the SAR system performance and to derive and fine-tune initial calibration parameters.