## 2 Interfering Mechanism and Major RFI Sources
### Space-Borne RFI Sources  
Besides the terrestrial RFI sources, there are also few cases with broadcasting signals from other space-borne satellites, such as global navigation satellite systems (GNSSs) constellations, communication satellites or other active remote sensing system, which are regarded as space-borne RFI sources. Figure 7a shows the geometry of interfering mechanism from space-borne RFI source. One possible way is the direct jamming signal to the antenna side-lobe or back-lobe generated by other satellites. According to the analysis for NASA (Soil Moisture Active Passive) SMAP radar, this kind of interference power due to direct reception by back-lobes is non-negligible but tolerable [10].  
The other is the terrain scattered interference (TSI) reflected off of the earth. When different space-borne systems share nearby illuminated area, strong main lobe-to-main lobe coupling between the two systems may occur [27]. These kinds of reflections off of the earth surface are potentially large  
Figure 7: Illustration of terrain scattered interference. (**a**) Geometry of space-borne RFI interfering case. A case example in real measured C-band Sentinel-1 SAR data: (**b**) VV image, (**c**) VH image and (**d**) pseudo color coded image with VV-Red, VH-Green, and VV/VH-Blue.  
when the main beam coincides with a specular point for specular surfaces (lakes, wetlands, etc.). Long duration mutual interference that occurred between Sentinel-1 and the Canadian Radarsat-2 satellite, the Chinese GAOFEN-3 satellite has been reported and investigated [28]. This kind of terrain scattered interference would be more common for future geosynchronous synthetic aperture radar (GEO SAR) system with large imaging swath and frequent revisits, as well as for forthcoming SAR constellations with more frequent occurrences of orbit crossing [29].  
For this case, the received RFI signal could not be regarded as a single point-like source anymore, since it undergoes a remodulation by the illuminated area. The estimates of the received RFI power is derived in detail in [29]. Figure 7 shows a TSI example in real measured C-band Sentinel-1 SAR data acquired in Italy in August, 2015. This data is acquired in Terrain Observation by Progressive Scans (TOPS) mode, which consist of three sub-swaths. Each sub-swath is composed of many bursts. The VV, VH, pseudo color coded images are shown in Figure 7b-d, respectively. The horizontal and vertical direction are range and azimuth respectively.  
It is shown that many strong stripes with high intensity appeared in VH image, in which produce irregular texture patterns and bury the true echoes of illuminated area. It is caused by simultaneous SAR imaging of the same area from both the Sentinel-1 and Canadian Radarsat-2. The Sentinel-1 and Radarsat-2 both operate using the same central frequency of 5.405 GHz. However, in VV image, the interference pattern is not obvious from visual inspection, and the image amplitude appears not significantly deteriorated by interference effects. This indicates the interference pattern is highly related with the wave polarimetry. For different sub-swaths, the interference situations are also varying. The third sub-swath is less severe than the first and second sub-swaths. This means that RFI characteristics depend strongly on the geographical region of data acquisition. Unwanted interference causes distortion to the amplitude and phase of the echoes. This would definitely produce inaccurate post-products and pose hindrance to subsequent or image interpretation applications like target detection, classification, etc.