## 1 Introduction
Typically, the reduction of such antenna interactions involves methods to mitigate the coupling between array elements or to improve the cross-polarization level. The last one plays a key role in the performance of SAR systems that makes use of the polarization diversity to take advantage of the retrieved information of the scattered signals. Some previous research has been made and different techniques have been proposed in order to reduce the cross-polarization level. An extended approach is splitting a planar array into subarrays of \(2\times 2\) elements according to a rotation pattern based on the image theory arrangement, which also minimizes the side lobe level [18]. This method has been widely used in a broad range of applications [19; 20; 21]. A further analysis extends this approach by considering the inter-element coupling [22]. Some other techniques improve the cross-polarization level by rotating the antenna elements in reflectarray applications [23], or combining the 180\({}^{\circ}\) rotation of the antenna structure along with a 180\({}^{\circ}\) alternate feeding based on the overlapping of different multipolar moments [24]. Further rotation topologies such as the sequential rotation is typically used for arrays of circular polarized antennas, as it leads to an improvement of the axial ratio bandwidth or the polarization purity [25; 26; 27; 28; 29]. Generally, the majority of cross-polarization suppression techniques start from the decomposition of planar arrays into subarrays, and a very few applications consider linear arrays. In [30], a 90\({}^{\circ}\) sequential rotation technique is implemented in a circularly polarized linear array of circular patches. Later, this analysis is extended by considering the scanning properties of linear phased arrays [31]. However, this approach is simplified as the radiation effect of higher order modes is neglected and no real geometrical 90\({}^{\circ}\) sequential rotation is applied. To the best of the authors' knowledge, no similar 90\({}^{\circ}\) sequential rotation approach, as presented in this work, was applied before, not only by performing the presented rotation topology in linear array applications, but also by considering identical fed dual polarized antenna elements. In this manuscript, a novel approach to perform a 90\({}^{\circ}\) sequential rotation of the same identical dual polarized cavity backed aperture coupled patch antenna elements of a linear array, for airborne SAR applications, is presented. This technique improves the cross-polarization level within the main antenna beam of up to 14 dB in comparison with the standard array arrangement, providing cross-polarization suppression values above 40 dB. This work is structured as follows. First of all, the scope of the antenna system according to the required airborne SAR sensor requirements is presented. Later, the design of a cavity backed dual-polarized antenna element is introduced, whose compaction allows for achieving a high degree of integration and thus to maximize the number of the antenna elements. This design will be validated by means of measurements of a manufactured prototype. Later, three different configurations of a linear array composed of five identical antenna elements are constructed and measured. Each configuration that corresponds to a different rotation pattern of the array elements is analyzed when applying a uniform phase and amplitude distribution, and beamforming case with triangular amplitude tapering. Finally, these results are examined and the improvement of the cross-polarization level within the antenna main beam when performing a 90 deg sequential rotation is discussed.