# Performance and Requirements of GEO SAR Systems in the Presence of Radio Frequency Interferences
## 1 Introduction  
L band, C band and X band spaceborne microwave systems play important roles in lots of remote sensing applications, such as the surface deformation measurements of landslides and earthquakes and vegetation mapping of forests. Nevertheless, the available microwave frequencies allocated for the spaceborne systems, called the active primary allocation frequencies, are given by the International Telecommunications Union's (ITU's) Earth Exploration-Satellite Service (EESS) [1]. Active primary allocations within the microwave frequency extension from L band to X band are provided in Table 1[1]. Beyond the primary allocation frequencies in different radio frequency bands, there are protected frequency bands or void frequency bands. Taking a classical protected frequency band in L band as an example, the frequency interval 1400-1427 MHz is protected, within which unwanted emissions of active service operations are forbidden and even the emissions from the nearby bands should be guaranteed to be lower than an expected value [2]. However, as more and more microwave devices are implemented, the primary allocation frequencies are becoming increasingly crowded. Different systems, which utilize the same or adjacent microwave frequency bands, will interfere with each other. Since only the active primary allocation frequencies can be used by the spaceborne microwave systems, the systems cannot avoid being impacted by the Radio Frequency Interference (RFI). Based on L band Aquarius scatterometers and the SMOS radiometer, strong RFI emissions, which might come from the terrestrial sources (e.g., military radars), have been detected [2; 3; 4]. Besides those detected by the radiometers, in 2016, [PERSON] et al. discussed the crosstalk RFI in the interferometric Global Navigation Satellite System Reflectometry (iGNSS-R), when the Delay-Doppler Map of the desired satellite is overlapped by other satellites [5]. Other detailed discussions about the classification of RFI sources, impacts analysis and mitigation methods in the global navigation satellite system (GNSS) system case were given in [6]. RFI is a common and serious problem in Synthetic Aperture Radar (SAR) as well. RFI decreases the SAR image quality by generating the noise-like artefacts, which is shown in Figure 1. RFI can also cause the saturation of the SAR echo and even produce spurious targets in the images [7]. Lots of RFI has been detected by SAR systems of different working bands. With respect to the L band SAR, the impact of RFI has been observed in ALOS-1 PALSAR data, which showed that RFI could not only cause haze-like image artefacts, linear pattern or blurs in the focused SAR image but reduce the capability of obtaining polarimetric signatures and the phase accuracies of SAR images [8]. Likewise, for C band systems, the RFI deriving from the Radio Local Area Networks (RLAN) could degrade the signals of the C band meteorological radars when the RFI power was comparable to that of the signal obtaining the nominal Signal-to-Noise Ratio (SNR) [9; 10]. RFI signals could be observed affecting a few range lines in the C band Sentinel-1A SAR image [11]. Many low power RFI sources with the whole bandwidth and a few very high-power temporal RFIs are detected by Sentinel-1A SAR image as well [12]. Significantly, Radarsat-2 and Sentinel-1A have mutual interferences because they share the same working band and have the almost same equatorial crossing time and multiple repeat periods of each other. Although RFI signals in X band are not as obvious as those in C band, increased RFI mitigation capability is needed due to the wide designed bandwidth, as based on the analysis of the C- and X- band Windsat microwaves radiometer data [13; 14]. Moreover, a small RFI modulated with a pulse wave has been observed in the X band TerraSAR-X system, which if notched out, will give rise to sidelobes in the SAR image [15]. Therefore, RFI is an important factor needed to be considered, including the future launched Geosynchronous orbit SAR (GEO SAR) system.