## 3 Experiments  
The San Francisco area in California (USA) was selected as the test site because this region has several typical terrain types, such as urban areas with different block directions, vegetated areas,  
Figure 1: The \(\Delta\alpha_{\text{B}}/\alpha_{\text{B}}\) scattering mechanism classification diagram, where the blue dash line indicates tilted boundaries. Zone 1: high entropy Multiple scattering; Zone 2: high entropy vegetation scattering; Zone 4: high entropy double-bounce scattering; Zone 5: medium entropy vegetation scattering; Zone 6: medium entropy dominant surface scattering; Zone 7: low entropy double-bounce scattering; Zone 8: low entropy dipole scattering; Zone 9: low entropy surface scattering. and sea surface. Both GF-3 polarimetric SAR data and Radarsat-2 polarimetric SAR data acquired over this region are used for analysis. Radarsat-2 data is used as reference here, because the GF-3 sensor and the Radarsat-2 sensor both operate at C-band and have similar system parameters. Pauli-basis images and Google Earth images are shown in Figure 2. The GF-3 polarimetric SAR data was acquired on 15 September 2017 on ascending passes with right looking direction. The incidence angle ranges from 19.86 degrees to 22.59 degrees. The pixel space is about 5.37 \(\times\) 2.25 m\({}^{2}\). The image shown in Figure 2a has 1288 \(\times\) 3250 pixels. The Radarsat-2 data was acquired on 9 April 2008 on ascending passes with right looking direction. The incidence angle ranges from 28.02 degrees to 29.82 degrees. The pixel space is about 4.73 \(\times\) 4.82 m\({}^{2}\). The image shown in Figure 2b has 1441 \(\times\) 1988 pixels. Both data sets were filtered by a 7 \(\times\) 7 sliding window. Two kinds of experiments are carried out. First, the \(H/alpha\) method [6] and the \(\Delta\alpha_{\rm B}/\alpha_{\rm B}\) method are used to analyze and evaluate the capability of GF-3 data for representing typical scattering mechanisms. Since the \(H/alpha\) method and the \(\Delta\alpha_{\rm B}/\alpha_{\rm B}\) method have no relation with the total backscattered energy, in the second part of the experiments, the iterative Wishart classifier [16] is applied for further assessment based on the initial classified results obtained by the \(H/alpha\) and the \(\Delta\alpha_{\rm B}/\alpha_{\rm B}\) diagrams. The outlined themes in Figure 2a,b is typical terrain types representing the urban, tilted urban, forest and ocean surface areas, which are used for quantitative assessment in the following experiments. In Figure 2, it is observed that there are two kinds of city blocks. The city block with buildings aligned along the azimuth direction is named as the urban area, and the city block with buildings/streets aligned approximately at 45 degrees off the azimuth direction is named as the tilted urban area. The tilted urban area often has specific orientation angles. We compared the Google Earth images of this area obtained in 2008 and 2017, and found that by visual inspection the terrain types in the selected areas were barely changed. The physical scattering mechanism classification results by the \(H/alpha\) and \(\Delta\alpha_{\rm B}/\alpha_{\rm B}\) methods are shown in Figure 3. It is observed that by using the GF-3 data, both methods tends to classify less pixels to the classes dominated by double-bounce scattering, i.e., the red pixels in Figure 3a,b are less than those in Figure 3c,d. Compared with results of the Radarsat-2 data, results of the GF-3 data by using the \(H/alpha\) classification plane show little difference between the urban area and the forest area. This implies that the \(H/alpha\) classification plane may not be suitable for GF-3 image classification.