# Performance and Requirements of GEO SAR Systems in the Presence of Radio Frequency Interferences
## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Band** & **Frequency Range (MHz)** & **Services in Band** \\ \hline L & 1215–1300 & Radio Navigation Satellite System (RNSS) and radiolocation \\ C & 5250–5570 & Aeronautical RNSS and radiolocation \\ X & 9300–9900 & Radiolocation and radio navigation \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Active EESS primary allocations in L, C and X bands. Figure 1: RFI in ENVISAT image before removal (**left**) and after removal (**right**) (data are courtesy of European Space Agency (ESA)). To improve the revisit and the coverage performance of the spaceborne SAR system, GEO SAR has been proposed [16]. Unlike a totally geostationary communication satellite, GEO SAR satellite runs in a geosynchronous orbit with non-zero inclination and eccentricity. Thus, a GEO SAR can obtain the relative movement with the targets on the ground, which ensures sufficient Doppler bandwidth after long time integration for realizing two-dimensional SAR imaging. It can achieve a revisit of the target region of one day and a wide beam footprint of more than hundreds of kilometres [17]. According to the inclination, the designed GEO SAR systems can be classified into the inclined orbit GEO SAR system (large inclination) and the quasi-geostationary system (small or zero inclination, small eccentricity but not zero) [18]. To satisfy the SNR requirement, inclined orbit GEO SAR system needs a higher power and a larger antenna size [19]. It achieves the designed azimuth resolution by a relative shorter integration time and it is sensitive to ionosphere and cluster disturbances [20; 21; 22; 23; 24; 25; 26; 27; 28]. In contrast, the geostationary system has lower requirements of the power and the antenna size [20]. Moreover, it has better revisit and observation performance for the target [29]. Nevertheless, it needs an integration time of hours. Thus, atmosphere distortion is a serious problem in this design [30]. GEO SAR has a longer integration time and a larger coverage compared with a Low Earth Orbit SAR (LEO SAR). Thus, it has an increased probability to be impacted by the RFI and the related impact will be more severe as well. Shown in Figure 2, the high orbit of GEO SAR gives rise to a large swath of about 400 km, while the swath of a LEO SAR is only around tens of kilometres in across track. Resultantly, more RFI signals will come into the beam of GEO SAR, compared with that in a LEO SAR case. Although the RFI impact depends on its time-frequency characteristics, such as the bandwidth and the transmitting duration, it is general noise-like, which can finally decrease the imaging quality of GEO SAR images by reducing the Signal-to-Interference-plus-Noise Ratio (SINR). Therefore, we need to consider and evaluate the RFI impact in the GEO SAR system design. The paper is organized as follows. The RFI impact on the SINR and the required average transmitted power to compensate RFI impacts in the GEO SAR case are discussed in Section 2. In Section 3, focusing on the bistatic scattering RFI case, the RFI impact on SINR and the required transmitted powers in the specular scattering and non-specular scattering cases are studied. Moreover, under the assumption of the point-like target case, the Doppler filtering effect in the LEO SAR bistatic scattering RFI case is specially analysed. In Section 4, in the presence of the RFI, simulations are conducted to analyse the GEO SAR system design requirements in detail. Finally, Section 5 concludes this paper. In addition, some important real RFI measurement results in different frequencies are given in the Appendix A.