# Performance and Requirements of GEO SAR Systems in the Presence of Radio Frequency Interferences
## 4 Performance Evaluation and Discussion: Example of GEO SAR Design
### Impact of Bistatic Scattering RFI
#### 4.2.2 GNSS Bistatic Scattering RFI Case Analysis  
GNSS has been designed to work only in L band. Nevertheless, although Global Positioning System (GPS), Global Navigation Satellite System (GLONASS) and Galileo satellites are Medium Earth orbit (MEO) satellites, Beidou-2 satellites have different orbit types, including the MEO, geostationary orbit (GEOS) and inclined geosynchronous orbit (IGSO). Some important utilized system parameters of the main GNSS are given in Table 6[46; 47; 48; 49; 50]. Orbiton software is used to calculate the number of GNSS satellites simultaneously in the sky above Beijing. GPS satellite, Beidou-2 IGSO satellite and GEOS satellite are used as the examples of MEO satellite, IGSO satellite and GEOS satellite to calculate the occurrence probabilities of non-specular scattering case, the specular scattering case and the mainlobe to mainlobe specular scattering case in the GNSS bistatic scattering RFI case. The occurrence probabilities are given in Table 7. We obtain the occurrence probabilities by virtue of 10-day orbit simulation of GEO SAR and GNSS satellites. Because of the higher orbit and the wider beam width of GNSS, GNSS bistatic scattering RFI cases have very high occurrence probabilities. Probabilities for non-specular and specular scattering cases in GEOS and IGSO cases are 100%. The MEO satellite even has nearly 100% for non-specular and the specular scattering cases. Because of the very large beam width of the GNSS antenna, mainlobe to mainlobe specular scattering case can even occurrence. The average transmitted power requirements in GEO SAR in the presence of GNSS bistatic scattering RFI is shown in Table 7. The geometry parameters used in the simulation are marked in the caption. As the mainlobe to mainlobe specular scattering case is the worst case, we discuss this case. The GNSS satellites given in Table 6 are used as the input RFI sources. To simplify the analysis, different satellites are regarded to have the same illuminating geometry for GEO SAR. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Navigation System** & **GPS** & **GLONASS** & **Galileo** & **Beidou-2** \\ \hline \multirow{2}{*}{Transmit power (W)} & \multirow{2}{*}{240} & \multirow{2}{*}{135} & \multirow{2}{*}{265} & \multirow{2}{*}{130 (MEO)} \\  & & & & 185 (IGSO/GEO) \\ \hline \multirow{2}{*}{Satellite number above the scene} & \multirow{2}{*}{13} & \multirow{2}{*}{11} & \multirow{2}{*}{8} & \multirow{2}{*}{6 (GEO)} \\  & & & & 8 (IGSO) \\ \hline \multirow{2}{*}{Bandwidth (MHz)} & \multirow{2}{*}{2} & \multirow{2}{*}{10} & \multirow{2}{*}{4} & \multirow{2}{*}{20} \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Utilized system parameters of the main GNSS in the simulation. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Parameters**} & \multicolumn{2}{c}{**Value**} \\ \hline \multirow{5}{*}{**Probability (\%)**} & \multirow{2}{*}{**Non-specular scattering**} & **MEO** & 96.9 \\  & & **IGSO** & 100.0 \\  & & **GEO stationary** & 100.0 \\ \cline{2-4}  & & **MEO** & 96.9 \\  & & **GEO stationary** & 100.0 \\ \cline{2-4}  & & **GEO stationary** & 100.0 \\ \cline{2-4}  & & **MEO** & 9.0 \\ \cline{2-4}  & & **IGSO** & 22.5 \\ \cline{2-4}  & & **GEO stationary** & 48.4 \\ \hline \multirow{5}{*}{**Non-specular scattering**} & \multirow{2}{*}{0.7} \\  & & **Non-specular scattering**} & 0.7 \\ \cline{2-4}  & & **Specular scattering** & - \\ \cline{1-1} \cline{2-4}  & & **Mainlobe to mainlobe specular scattering** & \(2.2\times 10^{8}\) (\(A_{s}\approx 30\) km \(\times\) 30 km (maximum)) \\ \cline{1-1} \cline{2-4}  & & **specular scattering** & 0.8 (\(A_{s}=100\) m \(\times\) 100 m) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Occurrence probability of the GNSS bistatic scattering RFI case and the average transmitted power requirement in GEO SAR (HH polarization, Shrub scene, in the specular scattering case: \(\theta_{1}=\theta_{2}=30^{\circ}\), in the non-specular case: \(\theta_{1}=30^{\circ},\theta_{2}=45^{\circ}\)).