## 4 SAR Payload Unit Design  
### SAR Antenna Unit  
The SAU was an active phased-array antenna with a mass of 21 kg and dimensions of 1970 \(\times\) 1060 mm. It comprised 4 columns and 48 rows of phase centers, wherein each  
Figure 9: Internal layout of the S-STEP satellite units. active row contained a 16-element subarray. The antenna was a single polarization (VV) antenna, and the radiating subarrays were implemented using a honeycomb sandwich microstrip patch construction, which is lightweight, inexpensive, and increased the structural stiffness. In addition, the SAU comprised a radiating microstrip patch radiating board, electronic front-end (EFE), and feeding network. More specifically, the distances between the microstrip patch elements were designed as 0.9653 and 0.6723 in azimuth and elevation, respectively, considering the antenna gain, beam-steering interval, and grating lobe levels. We selected an aperture-coupled SIW cavity-backed microstrip patch-array antenna with active phased-array technology, considering the wide bandwidth with low reflection loss, inexpensiveness, lightweight, and simple fabrication for mass production. As displayed in Figure 10, the tile-based SAR antenna structure is advantageous for shortening the development period and reducing the costs by enabling qualification tests for each antenna tile that minimizes the testing for the entire antenna unit. In addition, if the requirements of the SAR antenna (e.g., antenna gain and transmit power) have to be altered for various missions, these can be immediately reflected by simply adjusting the number of antenna tiles. A single radiation element constituting a subarray including a test jig is presented in Figure 11a. A single radiating element with a total thickness of 4.9 mm contains an upper patch, substrate, feeding line, substrate, lower patch, substrate, and spacer, as depicted in Figure 11b. In particular, the antenna radiated electromagnetic waves through the feeding line and upper patch, and the rear radiation was reflected through the lower patch and its surrounding metal via a gain enhancement in the front direction. Additionally, the insertion of a honeycomb-based spacer between the upper patch and feeding line secured the wide bandwidth characteristics, ensured structural stability, and reduced the weight. As this type of antenna structure is a low-profile antenna based on a microstrip patch, it possesses suitable mechanical characteristics for microsatellites. Moreover, the RF-35 TC thermally conductive low-loss laminate was selected for the upper and lower substrates with a dielectric constant of 3.5 and thicknesses of 0.5 and 0.508 mm, respectively. The two substrates were separated by a 3.0 mm thick quartz fiber honeycomb core, wherein the bonding sheets for the honeycomb and substrates were 3M AF163-2 and isola 185 HR, respectively. According to the European Space Agency standard ECSS-Q-ST-70-02C, all these materials for the subarray are used in space (outgassing) and applied to satellites. As depicted in Figure 11c, the subarray is designed using 1 \(\times\) 16 patches, considering the grating lobe and beam-steering interval. The radiating board assembly was cured under pressure in an autoclave. In particular, the antenna structure was designed and optimized using the commercial electromagnetic (EM) simulation software CST Microwave Studio. The antenna was designed using a honeycomb-structured spacer for weight reduction and structural stiffness. Figure 10: Mechanical structure of the tile-based S-STEP SAR antenna unit (SAU). Although the existing TRM for satellites is developed using GaAs-based high-power amplifiers (HPAs), recent studies have applied GaN-based HPAs to TRM for satellites. In comparison to GaAs HPAs, GaN-based HPAs can operate at a higher supply voltage and exhibit a high-power density. Therefore, a high-power amplification can be achieved with a small area. Additionally, it has a low thermal resistance and exhibits appropriate thermal properties.