## 3 Scattering Study  
The main activity carried out with GRECOSAR has been focused to derive vessel scattering maps for diverse observation conditions [25]. Three different geometries have been considered (see Fig. 4), namely: 1) a Spanish fishing vessel (SPA) 27 m long and 10 m wide, 2) an Icelandic fishing vessel (ICE) 70 m long and 12 m wide and 3) a common passenger ferry (FER) 200 m long and 30 m wide. They have been processed in the Inverse SAR (ISAR) imaging geometry of Fig. 5 for a signal bandwidth of 1 GHz and angular aperture \(\Delta\Omega\) of 5 degrees. This imaging geometry is a type of circular spotlight mode for which image resolutions close to centimeters can be reached. In order to make image interpretation easier, those phenomena degrading the quality of images have been discarded. Specifically, neither ship motions nor sea clutter have been considered as they can cause undesired blurring effects [26, 27]. The three models have been processed at L, S, C and X bands for seven bearings ranging from \(295^{o}\) to \(355^{o}\) in steps of \(10^{o}\). All the derived images are fully-polarimetric and have been analyzed with the Pauli, SDH [5] and Cameron [6] CTD theorems. In the paper, only some cases analyzed with the Pauli theorem are provided as the results obtained with the remaining images and CTD theorems are very similar. Particularly, Fig. 6 presents the scattering maps retrieved at X, C and L bands for an incidence angle of \(\phi=20^{o}\) and three different target orientations, \(\beta=\{295,315,335\}^{o}\). The images depict the normalized Pauli significance with a RGB-based color code (red for the first Pauli mechanism -trihedral, sphere, flat plane, \(\ldots\)-, green for the second -dihedral,\(\ldots\)- and blue for the third -anti-symmetric mechanisms-) over a transparent snapshot of the vessel under the sensor point of view. A local coordinate system is also included to support PePS location, which are highlighted with the black circles. With the previous scenario configuration, ISAR scattering maps become an important aid for improving the interpretation of SAR images. They allow to analyze the scattering properties within the different pixels making the isolation of small-scale details easier. As a result, the relation between ship geometry and the measured scattering may be properly defined. Figure 4: Snapshots of the SPA (a), ICE (b) and FER (c) models used in this work. Figure 5: ISAR imaging geometry of GRECOSAR. Figure 6: Scattering maps obtained for the SPA (a), ICE (b) and FER (c) vessels at L, C and X band with \(\beta\in\{295,315,335\}^{o}\) and \(\phi=20^{o}\). They have been analyzed with the Pauli CTD theorem for a dynamic range of 25 dB (red \(\to 1^{st}\) channel, green \(\to 2^{nd}\) channel, blue \(\to 3^{rd}\) channel). The lengths of the SPA, ICE and FER ships are 30, 70 and 200 meters. Circles highlight _PePS_ (stable dispersors) with which the feature sets of Table 1 are defined. The images of Fig. 6 show that each type of geometry presents a particular distribution of strong scatters, _PePS_ candidates, with a high RCS and a well-defined polarimetric pattern that remains stable within a specific range of views. They mainly correspond to dihedral interactions of cylindric structures (like masts, funnels,...) and trihedral behaviors due to corner geometries (like buttresses,...). Two important issues arise from these results [25]: 1) the mechanisms of the main scattering centers are present in all the CTD bases and this makes the polarimetric interpretation to be independent of the selected theorem [28]; and 2) such mechanisms are related to scatterers with large electrical dimensions (around 1 meter for the analyzed geometries), which may explain an apparent scattering stability along frequency and the high RCS values observed in simulated scenes [2]. This asseveration is however preliminary and should be corroborated with real images when available.