### Advese Impacts of RFI to SAR System
#### 1.2.3 Image Interpretation Process  
The ultimate goal of SAR imaging is to provide a physical understanding of the illuminated area. The existence of RFI would lead to the amplitude and phase distortion of the imaging data. Intuitively, strong interference would generate haze-like image artifacts or bright lines superimposed on the illuminated area, which lead to inaccurate spatial and radiometric measurements. What is worse, the resulting phase distortion would de-correlate the data, producing inaccurate post-products such as polarimetric descriptor [5], coherence [9], and retrieved biological or physical parameters [10].  
Figure 3: The effects of RFI on image formation process. (**a**) Variation of the estimated azimuth Doppler rate with range blocks in the case with and without interference affection. (Courtesy of [PERSON] et al. [8]) (**b**) Sectional drawings of the point target response after matched filtering using biased parameters under different interference to signal ratio (ISR). (Courtesy of [PERSON] et al. [5]).  
Figure 2: A particular example: the range spectrum of the synthetic aperture radar (SAR) echoes (**a**) without RFI and (**b**) with RFI.  
Consequently, the RFI hinders the subsequent image interpretation process like target detection, classification, etc.  
Figure 4 presents an example of National Aeronautics and Space Administration (NASA) Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) Hawaii dataset [5]. Obvious RFI stripes are observed and the patterns of the artifacts are very unique, which are not a duplicate of nearby scatterers or areas with strong reflectivity. The patterns are not comparable with the general radiometric artifacts such as the ambiguities, saturation effects, or processing effects. The patterns in the image are bright stripes with curvature, which could be a result of unmatched filtering of the interference. The interference in this dataset may originate from the nearby airport surveillance radar. A category of land cover classification scheme is based on the physical scattering characteristics. The presence of RFI would derive biased estimate of decomposition parameters, and subsequently lead to wrong classification results when using these incorrect parameters. Figure 4b,c shows the entropy and anisotropy parameters obtained by Cloud-Pottier polarimetric decomposition. The regions in dashed lines highlights the anomalies introduced by interference artifacts, where in Figure 4b is mainly the sea surface and in Figure 4c consists of the land, sea surface, and airport runway. Obvious differences are easily observed compared with the nearby regions outside the regions in dashed lines.  
Take a further insight, four areas inside the airport runway corresponding to various RFI conditions are selected for illustration, as marked in Figure 4a. They are referred to as No RFI Area 1, Low RFI Area 2, Moderate RFI Area 3, and Strong RFI Area 4, respectively. These areas possess the same scattering mechanisms and should have similar reflectivity. The co-polarized signatures are computed as shown in Figure 4d. These four distributed areas should have the same scattering mechanism with the one under ideal occasion, i.e., low ISR condition. However, with the raise of RFI energy level, the distortion to the shape and intensity of the co-polarized signatures become more severe. Therefore, the distortion of the image amplitude and phase caused by RFI would lead to erroneous  
Figure 4: Illustration of RFI’s resulting amplitude and phase distortion on real measured NASA UAVSAR data. (**a**) Pauli-coded image. (**b**) Entropy parameters and (**c**) anisotropy obtained by Cloud–Pottier polarimetric decomposition. The regions in dashed lines highlights the anomalies introduced by interference artifacts. (**d**) Evaluation of the distortion to the polarization signatures under various RFI conditions. (Courtesy of [PERSON] et al. [5]).  
polarization signatures. The erroneous polarization signatures would further bias the interpretation of scattering mechanisms.