## 2 Geometry and Signal Model  
### Geometry Model  
For the SAR system, most RFI sources are related to electronic equipment on the surface of the earth [16]. Therefore, this paper establishes a simulation model of the normal working spaceborne SAR being simultaneously interfered with by multiple RFI sources on the ground. In the process of data acquisition, the SAR satellite moves in an elliptical orbit around the earth, and the antenna points to the imaging area on the side of the flight direction. Multiple RFI sources on the ground may exist in or outside the imaging area, which may make RFI signals enter the receiver from the main lobe of the antenna, or from the side lobes. Figure 1a shows the geometric model between the SAR system and multiple RFI sources when acquiring real echo signals and being interfered with by RFI signals in the east-north-up (ENU) coordinate system. It should be noted that, unless indicated, all geographic coordinates in this article are in the ENU coordinate system. The light yellow area on the ground shows the main beam coverage area of the SAR system antenna at the azimuth time of \(t_{a}\). As shown in this figure, there are some RFI sources on the ground that continuously transmit interference signals to the SAR system, and when the azimuth time is \(t_{a}\), the distance between the \(k\)-th interference source and the SAR system can be expressed as \(R^{k}(t_{a})\). During the data acquisition process, with the relative movement between the SAR system and RFI sources, the distance between them changes with the azimuth time. Figure 1b shows that the arrangement of the multi-channel array of the SAR system in Figure 1a. As can be seen from Figure 1b, (2M-1) channels are arranged at equal distance on the \(Y\)-axis and the \(Z\)-axis of the antenna coordinate system, and there are M channels on each axis and the distance between adjacent channels is d. Among them, the channel located at the origin of the coordinate belongs to the \(Y\)-axis and \(Z\)-axis arrays at the same time, and it  
Figure 1: The geometry model of signal acquisition of synthetic aperture radar (SAR) and the channel array arrangement: (**a**) The geometric relationship between the SAR system and multiple radio frequency interference (RFI)sources when the azimuth time is \(t_{a}\); (**b**) The arrangement of the multi-channel array used by the SAR system in (**a**) and the definition of the elevation angle \(\theta^{k}\) and the azimuth angle \(\varphi^{k}\). is used to transmit and receive signals, while other channels only receive signals. Thus, at each azimuth time, the SAR system transmits signals through channel 1, and all channels receive echo signals. Moreover, we define the elevation angle \(\theta^{k}\) and the azimuth angle \(\varphi^{k}\) of the \(k\)-th RFI signal as shown in Figure 1b. The purpose of DOA estimation is to estimate the value of these two angles. The reason for choosing the L-shaped multi-channel array is as follows. First of all, different from the study of single interference source localization [17], in order to estimate the two-dimensional DOA of the interference signal at each azimuth time and apply it to multiple RFI sources geolocation, deploying multiple receiving channels as a non-linear array on the SAR system is necessary. Furthermore, for the SAR system, the accurate estimation of the geographic coordinates of the RFI source must be achieved based on multiple azimuth time DOA estimation results. Therefore, this method requires the system to have a good estimation accuracy of the direction of the RFI signal at each azimuth time. In addition, if the calculation complexity required for DOA estimation at each azimuth time is too high, the calculation burden of the system will be very heavy.