## 4 SAR Payload Unit Design
### SAR Antenna Unit
The external configuration and an exploded view of the EFE are presented in Figure 12a,b, respectively. As depicted in Figure 12c, the block diagram of the single-channel TRM using a 15 W X-band GaN HPA MMIC (UMS CHA8610-99F) is fabricated using the pseudomorphic high electron mobility transistor (pHEMT) process [40]. Specifically, the EFE contained six 4-channel TRMs and a control assembly. The transmission path of the TRM comprised an amplifier to drive the HPA and a GaN HPA for the high-power amplification of the transmission signal. In contrast, the receiver path contained a limiter (TriQuint TGL2201) to prevent damage to the low-noise amplifier (LNA) and a GaAs LNA MMIC (UMS CHA2110-98F) for low-noise amplification of the received signal. Additionally, the common path included a core-chip component to control the amplitude and phase of the transmit/receive signal and a circulator component connecting to the  
Figure 11: (**a**) Configuration of an element antenna, (**b**) sectional structure, and (**c**) SIW cavity of the radiating board, and (**d**) configuration of a \(1\times 16\) subarray of the S-STEP SAR antenna. antenna. Moreover, the core chip controlled the amplitude and phase of the Tx and Rx signals. The 6-bit X-band core-chip GaAs MMIC (Ommic CGY2170 YHV/C1) is a common MMIC containing a 6-bit attenuator, 6-bit phase shifter, and switches. In recent T/R module research studies, the scholars used multilayer substrates composed of low-temperature co-fired ceramic (LTCC). In addition, hermetic sealing is used in aerospace applications to protect T/R modules against environmental factors. These aspects ensure the compactness and robustness of the T/R module; however, they require a complex and costly fabrication process. In the S-STEP SAR payload, the TRM was designed with a low-cost heterojunction (ceramic and FR4) multilayer substrate instead of an LTCC substrate. Furthermore, the TRM was sealed using a laser welding process. Eventually, the designed heterojunction multilayer substrate for the TRM comprised three FR4 substrates (185 HR) for digital signals and two ceramic substrates (RO6035 HTC and RO4450F) for RF signals. The control assembly separately configured from the TRM performed power supply voltage conversion, power supply control, amplitude and phase control for beam steering, and distribution and combination of transmit/receive signals, respectively. In particular, a single TRM contained four channels for size and weight reduction and was manufactured using materials applicable in space, such as for outgassing and satellites, adhering to ECSS-Q-ST-70-02C. The T/R module housing material was AL6061, which is lightweight, inexpensive, and exhibits suitable heat conduction characteristics. However, this material has a different  
Figure 12: (**a**) Configuration of the electronic front-end (EFE) assembly, (**b**) exploded view of the EFE, and (**c**) architecture of the transmit/receive module (TRM) of the S-STEP SAR antenna unit. thermal expansion coefficient as compared to the GaAs HPA MMIC. Accordingly, GaAs MMIC chips were attached using H20E epoxy at 120 \({}^{\circ}\)C. The thermal analysis results of the SAR antenna, including the TRM, were detailed in a prior study on the S-STEP [26].