## 5 Results and Discussion
### Radiometry  
Other than the comparatively trivial constant gain offset of Section 4.5, the calibration process affects the radiometric accuracy of the processed SAR data through the pointing estimate \(\Delta\vec{A}_{e}\). The impact of the antenna pointing correction is illustrated by the C-band examples of Figure 8 (One of the many responses shown in this Figure was also used as a basis for Figure 6 above.) In the polar plots of the first two rows, each reflector response is represented as a line in a coordinate system similar to (37): azimuth indices are mapped to a propagation direction. This direction is parameterized by squint and off-nadir angles in the antenna reference frame. The trajectories are non-linear and differ from acquisition to acquisition due to variations in the platform attitude angles. The plots in the third row, meanwhile, are based on the analysis of reference targets in focused SAR imagery. In these plots, each reference target is represented by a single data point. Lines connect all targets imaged within a single channel. The polar plots in the left column clearly show a systematic RCS variation of \(\pm\)1.5 dB along azimuth, which is largely compensated for by the pointing correction introduced. By contrast, the evaluation in the third row is based on focused SAR data after azimuth compression. As such, it is therefore insensitive to large, systematic variations along the range history of targets. In this light, the estimated standard deviation of 0.3 dB in the bottom left plot is seen to be a gross misrepresentation. Instead, errors on the order of 1 dB are to be expected as the squint angle varies. Such variation is to be expected when data are acquired under different conditions in the field. The plots in the right column show that the pointing estimate effectively reduces the systematic errors along azimuth. Indeed, it even reduces the RCS mismatch measured in the focused SAR data, as shown in the last row. The polar plots also show, however, that residual RCS errors, which cannot be attributed to antenna pointing, remain. The residual errors and their visualization are helpful in characterizing these types of errors and identifying potential causes. In this case, residual errors are possibly linked to an issue with the antenna pattern measurement. Figure 9 illustrates the impact on antenna pointing calibration for data acquired by the DBFSAR sensor. The acquisitions were carried out over the Kaufbeuren calibration site during the _DATLAS_ test flights of 2018. The results pertain to one of the channels of the six-phase center ATI/GMTI configuration of DBFSAR. They were acquired at X-band in VV polarization and with a bandwidth of 800 MHz. The sensor, in this particular configuration, uses a single transmit antenna with a narrow beam of 2\({}^{\circ}\) and six separate, wider receive antennas. The pointing calibration yields separate corrections for the transmit and receive antennas. As the Figure caption indicates, the transmit pointing correction is significant: it is comparable to the antenna beam width. The large correction is consistent with the large initial RCS mismatch observed. It is most likely due to a misalignment of the antenna during the on-ground measurement of the antenna pattern. Figure 10 shows the impact of this pointing estimate on focused imagery. These acquisitions took place over the city of Landsberg am Lech, Germany. Although there are no reference targets available for this site, the calibration quality is clearly improved when the estimated pointing correction is taken into account during the antenna pattern correction in SAR processing. Figure 8: A comparison of RCS analysis results before and after C-band antenna pointing calibration. The first two rows visualize the residual RCS errors \(\delta\text{RCS}^{s}_{pq}(az)\) within the range history of reference targets in the co-polar channels of four independent calibration data acquisitions.