## 3 SAR Payload Design
### System Architecture Design  
The system architecture of the SAR payload was designed by applying the system parameters derived from the design process of the S-STEP SAR payload, as depicted in Figure 5. According to the original definition proposed by INCOSE, model-based system engineering (MBSE) is the formalized modeling procedure to support system requirements, design, analysis, verification, and validation activities initiating in the conceptual design phase. These should continue throughout the development and later life-cycle phases [31]. The architecture analysis and design integrated approach (ARCADIA) with its open-source model editor Capella [32; 33] was selected for the present study. The adoption of digital modeling-based engineering, instead of document-oriented engineering, reduces inconsistencies and errors and facilitates the system analyses for the rapid and effective development of small satellites, such as the S-STEP, according to the NSP. The functional and logical architectural diagrams of the S-STEP SAR payload using Capella are presented in Figure 6a,b, wherein the primary functional chains of the S-STEP SAR payload are depicted in Figure 6a based on the ARCADIA approach, e.g., Tx, Rx, and frequency synthesis. The physical architecture of the S-STEP SAR payload comprising the SAR antenna unit (SAU), radio frequency unit (RFU), digital control unit (DCU), and power supply unit (PSU) is illustrated in Figure 6c. The architectural block diagrams of the SAR system are shown in Figure 6c. The SAR system comprises a DCU for waveform signal generation and digital receiving processing; an RFU for up- and down-conversion of the RF signal; a SAU for beamforming, beam steering, high-power amplification, and radiation; and a PSU for power supply and regulation. A relatively high-transmission duty cycle of 30% can produce a higher-than-average Tx power. Based on the standpoint of the Tx amplifier, augmenting the peak power is more challenging than increasing its duty cycle under appropriate thermal management. As indicated in Table 2, the system loss of 2.6 dB comprises the transmission loss inside the satellite from the X-band TRM to the antenna input through blind-mate bullet connectors. Additionally, ohmic loss, dielectric loss, and reflection loss inside the antenna are included in the antenna gain. The system noise figure corresponds to the cascaded noise figure from the antenna input to the low-noise amplifier (LNA) via the filter, circulator, and limiter and includes the noise figure of the LNA if the antenna is facing the Earth at 290 K noise temperature [23]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Center frequency & 9.65 GHz \\ Polarization & single (VV) \\ Antenna size & 1970 \(\times\) 1060 mm \\ Number of panels & 1 \\ Array columns (azimuth) & 4 \\ Array rows (elevation) & 48 \\ Antenna gain & 41 dBi \\ Antenna beamwidth & 0.8\({}^{\circ}\) in azimuth \\  & 1.6\({}^{\circ}\) in elevation \\ Transmitted power & 1.92 kW max \\ Average power consumption & 2.8 kW max \\ Transmitted bandwidth & 400 MHz max \\ Pulse width & 150 μs max \\ Transmit duty cycle & 30\% max \\ Noise figure & 4 dB \\ Pulse repetition frequency & 2–10 kHz \\ Quantization & 10 bits \\ Date rate & 1.5 Gbps \\ System loss & 2.6 dB \\ Payload mass & 28.9 kg max \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Key system parameters of the S-STEP SAR payload. Figure 6: System architectural block diagrams of the S-STEP SAR payload: (**a**) functional architecture, (**b**) logical architecture, and (**c**) physical architecture. The mode transition diagram of the S-STEP SAR payload is presented in Figure 7, which includes three operating modes--supporting, imaging, and anomaly modes--and ten operating states--off, reset, init, stand-by, stabilize states for the support mode, calibration and mission states for the imaging mode, and init anomaly, stand-by anomaly, and stabilize anomaly states for the anomaly mode.