## 3 SAR Payload Design
### System Architecture Design
The three anomaly states correspond to the init, stand-by, and stabilized states, respectively. Moreover, an internal calibration was performed to maintain the stable performance and operating states of the SAR payload. In particular, this internal calibration mode was implemented to calibrate the amplitude and phase of the active components of the TRMs in the active phased-array antenna. Unlike external calibration, the calibration state for the internal calibration should be conducted before and after every image acquisition. Based on the above-mentioned iterative design optimization process, a brief overview of the key parameters of the S-STEP SAR payload is presented in Table 2. The simulation and analysis of the SAR system design were performed using the SAR system design tool, as described in Section 6. The dimensions of the designed antenna were 1970 \(\times\) 1060 mm. The essential concepts of the innovative S-STEP satellite design include the bus-payload integrated flat-panel-type structure and highly modularized SAR antenna with a tile structure. In addition, an efficient unit design with simplified and integrated functions, a fatigue-life prediction methodology for electronic printed circuit boards (PCB), a dedicated VFOD, and a unique thermal control based on passive thermal control hardware are also applied. The hardware and software of the SAR payload were designed to achieve the given system performance requirements. As depicted in Figure 8, the lightweight microsatellite SAR payload comprises four units: SAU, RFU, DCU, and PSU. As shown in Figure 8a, each EFE assembly of the SAU has two heat radiators for effective heat radiation of heat from each channel of TRMs as mentioned previously. In case of the lightweight S-STEP microsatellite, the data link subsystem (DLS) was integrated into one of the platform units, IAU. The SAR antenna is one of the essential design drivers for SAR satellites, because the antenna gain, cross polarization, and sidelobes directly impact the SAR image quality, and thus influence the final imaging performance. Therefore, a lightweight, high-gain antenna with a low footprint is vital for implementing a small X-band SAR microsatellite. These requirements involve the use of deployable antennas. Figure 7: Mode transition diagram of the S-STEP SAR payload. Although several promising developments are ongoing for deployable inflatable antennas, the difficulty of gas pressure control still poses serious problems for surface accuracy; therefore, this antenna is considered unsuitable for SAR microsatellites. Recently, reflect-array antennas have been mounted on the ISARA and MarCO CubeSat missions [34] that can potentially act as inexpensive, lightweight, and stowage-efficient SAR antennas but are affected by leakage issues, as demonstrated in space. Although the deployable mesh reflector antenna is ultralight and can attain high antenna gain, the deployment mechanism is extremely complicated, and the beam steering is limited [35]. Deployable active phased-array antennas are a common selection for SAR antennas, but these are generally heavy, expensive, and exhibit complex design [28]. In the S-STEP, an active phased-array antenna was applied as a single panel and integrated into the mechanical structure of the microsatellite SAR without performing the deployment mechanism to reduce the weight and maximize the stowage volume efficiency. Overall, various SAR imaging modes such as ScanSAR can be implemented through beam steering in the microsatellite SAR using an active phased-array antenna. This enabled the operation of complex and flexible imaging scenarios. In particular, the use of an active phased-array-based antenna may be advantageous for electronic counter-countermeasures (ECCM), especially in tactical military applications [36]. However, the active antenna poses a disadvantage: the weight of the overall system increases because its development cost can be higher than that of the passive antenna, and the cooling system can become complicated owing to severe heat generation.