## 2 Interfering Mechanism and Major RFI Sources
### General Signal Model for Typical RFI Sources
The WBI could not be identified intuitively in time domain, but the interference to signal ratio could be maximized in frequency domain. Figure 9 shows a particular range spectrum of the radar echo with WBI. This data is acquired from a Chinese airborne SAR which operates at X-band. The bandwidth of the transmitted signal is almost 1 GHz. From Figure 9a, it is shown that the target echo is contaminated with multiple WBIs with different envelop and bandwidth. Their magnitudes are varying and the strongest one is much larger than that of the target echoes, which makes the target echoes nearly invisible in the range spectrum. The pulse is transformed onto the time-frequency representation using the short-time Fourier transform (STFT), as shown in Figure 9b. It is straightforward to observe the evolvement of the interference along with the frequency and time, which show that the WBI is highly non-stationary. The time-frequency representation characterizes the nonstationary property very well, in which the energies of WBIs are concentrated into a few frequency bins. The WBI signatures possess properties that are a combination of the CM WBI and SM WBI, which verifies the effectiveness of the model assumption. (3) Broadband Continuous Wave (CW) RFI  
This class depicts the signals that have relatively broad bandwidth due to modulation, and usually are continuous in time, with respect to the integration time. Common cases are broadband communication systems, or coded signals like global navigation satellite system. In this case, they have noise-like characteristics. For example, the digital video broadcasting-terrestrial (DVB-T) signal can be modeled as [32]:  
\[I_{DVB-T}=\text{Re}\Bigg{\{}\exp(j2\pi f_{c}n)\sum_{m=0}^{\infty}\sum_{l=1}^{ l_{\text{max}}}\sum_{k_{SC}=1}^{K_{\text{max}}}\text{s}\varphi(n)\Bigg{\}}, \tag{5}\]  
with:  
\[\varphi(n)=\Bigg{\{} \begin{array}{c}\exp\Big{(}\frac{2\pi k\big{(}n-T_{g}- Tl_{S}-l_{\text{max}}mT_{S}\big{)}}{T_{g}}\Big{)},\quad([l+l_{\text{max}}m])T_{S} \leq n\leq(l+l_{\text{max}}m+1)T_{S}\\ 0\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad \qquad\qquad\qquad\qquad\qquad\qquad\qquad\text{else}\end{array} \end{array} \tag{6}\]  
where \(f_{c}\), \(m\), \(l\), and \(k_{SC}\) denote center frequency, frame number, symbol number and subcarrier number, respectively. \(n\) denotes the time samples. \(k\) denotes the subcarrier number corresponding to \(f_{c}\). \(s\) denotes the information modulated on the subcarriers. The transmitted signal is organized in frames. \(T_{g}\) is the duration of the guard interval. Each frame has a timed duration of \(T_{S}\), and consists of  
Figure 9: Illustration of wide-band interference in (a) range spectrum and (b) its spectrogram. \(l_{\max}=68\) Orthogonal Frequency Division Multiplexing (OFDM) symbols. \(K_{\max}\) denotes the carrier number, and \(K_{\max}=6817\) for the 8K mode. Figure 10a shows a particular echo spectrum corrupted by broadband noise like RFI, and its spectrogram is shown in Figure 10b. The data is acquired by an L-band airborne SAR system with the central frequency of 1.3 GHz and bandwidth of 200 MHz. In Figure 10a, the interference is very strong and have large bandwidth that comparable with the transmitted signal. In the time-frequency domain, the interference overlaps with the target echoes at a large extent, which raise the difficulty of interference detection and separation. (4) Heterogeneous Environment  
For practical SAR system with wide illuminated swath, there will be multiple RFI sources observed simultaneously, either from the main beam or high-gain side lobes. The existence of various encountered RFI sources make it difficult to characterize the RFI problem and the associated solutions.