## 6 Conclusions  
Several new SAR missions are proposed [14] or are currently being implemented [2; 15] which are based on large deployable reflector antennas. The huge size of these antenna systems make them prone to mechanical deformation, which also leads to variations of the electrical antenna pointing along the orbit. To keep the calibration performance on the same level as for current SAR system, this problem has been addressed in this paper.  
The performance of a novel method for antenna pointing estimation in elevation for SAR systems has been presented. It is based on two coherent SAR datasets to extract the antenna pattern difference while mitigating the effect of the underlying scene. Based on simulations, experimental results and the derived mathematical expressions in Section 3, it can be deduced that the proposed technique for elevation pointing estimation based on difference scene types is very promising:  
1. Scenes of homogeneous distributed targets can be used for the novel and for the conventional method.
2. Scenes with high variability and many point-like targets like urban or mountainous region can exclusively analyzed and used by the proposed novel technique.
3. Scenes of low backscatter like deserts, calm water bodies and melting ice suffer from low SNR can only be used with long integration intervals along azimuth.  
Apart from scenes listed in point 3, most land areas on Earth become available for elevation pointing estimation and, hence, also allow the tracking of the antenna pointing along the satellite's orbit, which is an important feature of the proposed technique. Hence, the applicability of the novel technique for pointing estimation was extended from homogeneous distributed targets to nearly any area on Earth with sufficient SNR. Due to the coherent nature of this technique, the phase can additionally be used for antenna pointing estimation in contrast to the conventional amplitude only method.  
The novel technique has been described along with the traditional method. Performance simulations have shown the influence of SNR and scene length on the pointing estimation uncertainty. Finally, by means of TerraSAR-X datasets, the applicability of the novel method has been proven. It has been shown that the novel technique is working under realistic conditions although additional range ambiguities due to doubling the PRF in the experimental setup became apparent. Nevertheless, for the two exemplary scenes shown, the antenna pointing has been estimated down to a few millidegrees which is in the same order of magnitude as the conventional method and is applicable to nearly all areas around the globe. For future SAR missions with digital beam forming the additional range ambiguities can be avoided by synthesizing a supplementary (notch) beam pattern using digital signal processing instead of doubling the PRF and, hence, avoiding the appearance of additional ambiguities.  
[PERSON] developed the algorithms, set up the experiments and analyzed the data. [PERSON], [PERSON] and [PERSON] helped improving the data analysis strategy. U.S. performed the commanding of the TerraSAR-X satellite for the experimental acquisitions and [PERSON] processed these data to focused SAR images.  
This research received no external funding.  
The authors declare no conflict of interest.