## 3 SAR Payload Design
### System Architecture Design
Therefore, a precise thermal design and analysis is essential for active phased-array antennas. In context, a passive antenna that performs beam steering through maneuvering and controlling the posture of the main satellite body is occasionally used in small satellites. Slotted waveguides and microstrip patch elements are primarily used as radiators for phased-array-type SAR antennas. The RF characteristics of the slotted waveguide-type antennas are highly suitable because distinct types of waveguides must be applied for each polarization, and the implementation of dual polarization is extremely challenging. Despite  
Figure 8: SAR payload units of S-STEP containing (**a**) integrated electronic front-end (I-EFE) assembly, (**b**) radio frequency unit, (**c**) digital control unit, and (**d**) power supply unit. being expensive, aluminum was selected owing to its small thermal deformation, and the carbon fiber reinforced polymers (CFRP) were applied for weight reduction. However, the microstrip patch element can be easily designed and manufactured. Additionally, it is inexpensive, lightweight, and exhibits a wider bandwidth than a slotted waveguide. In contrast, the construction of the beamforming network using a microstrip line causes relatively large losses. The losses can be minimized for performance maximization, and the antenna efficiency can be improved only through precise design/manufacturing and surface treatment. Alternatively, reflector-type antennas are deployed in a lightweight mesh or solid-type antenna. The system architecture of the TR module can be classified into four major categories, i.e., separate transmit and receive modules, shared phase-shifter module, common-leg module, and common-leg module with calibration path architectures [37; 38]. In general, the mechanical structure or configuration of the transmit/receive module can be categorized into tile (plank-type) or brick-type modules [39]. The brick-type module exhibits a planar structure with several devices mounted on a substrate, whereas the tile-type module displays a stacked structure with several stacked modules. Although the brick-type structure can be easily implemented as a flat structure, it poses the disadvantages of increased weight and size. Specifically, the tile type was implemented as an MMIC with stacked chips to enable miniaturization; however, the structure was complicated, and the development was challenging. The brick-type structure requires a wider space compared to the tile-type structure, and it was, therefore, advantageous for integrating a heat dissipation system and realizing a high transmission output. However, the tile-type structure is advantageous for compact, lightweight, scalable, and modular design owing to the development of system-on-chip technology. The S-STEP SAR payload adopted an active phased-array antenna based on a microstrip patch-array antenna, which is advantageous for miniaturization/lightweight, easy design/manufacturing, and brick-type TRM. Thus, a large space can be secured to install the SAR payload and body components on the rear of the antenna. All electronic units were installed inside the satellite structure, specifically on the rear of the SAR antenna, as depicted in Figure 9. The design results for each unit of the SAR payload are described as follows.