## 1 Introduction
It is the most outstanding representative microsatellite under the NSP that captures SAR images using stripmap, spotlight, and ScanSAR modes. These are based on the X-band active phased-array SAR antenna, which is a 3.2 \(\times\) 0.4 m microstrip patch-array antenna with 320 transmit/receive modules (TRM). The Capella SAR, developed by Capella Space in the United States, is a 107 kg microsatellite SAR (Sequoia) that was successfully launched in 2020 [17; 18; 19; 20]. In particular, this satellite features an X-band SAR payload that acquires spotlight-mode SAR images with a ground resolution of 0.5 m and uses a deployable tension-truss mesh reflector antenna with a 3.6 m diameter, similar to the AstroMesh [21] of the L-band SMAP SAR satellite or Oxford Space Systems [22], to minimize transmission power. This feature enables an outstanding continuous SAR image acquisition period of at least 9 min. The effective area of the Capella SAR antenna is 8 m\({}^{2}\) compared with the 1.3 m\({}^{2}\) antenna of ICEYE. Therefore, the Capella SAR antenna is more advantageous for obtaining higher SAR image quality, such as noise equivalent sigma zero (NESZ), than the ICEYE antenna. StriX-\(\alpha\) is an X-band SAR microsatellite constellation containing 25 satellites, planned by Synspective in Japan, based on the development results of a prototype called MicroXSAR [23]. It is a 130 kg microsatellite that acquires SAR images with a ground resolution of 1-3 m. Specifically, Strix-\(\alpha\) features a deployable passive phased-array antenna with 4.9 \(\times\) 0.7 m honeycomb panels of a slotted waveguide array containing solar panels mounted on its rear. Furthermore, QPS-SAR is an X-band SAR microsatellite constellation developed by iQPS in Japan, and two launches have been completed in 2019 and 2021, respectively [24]. Similar to the Capella SAR, a total of 36 launches are planned for the QPS-SAR. These satellites use a deployable wrapped-rib parabolic mesh reflector antenna. This satellite weighs 100 kg with a 3.6 m diameter antenna that provides a high resolution of 1 m in spotlight mode and a standard resolution of 1.8 m in stripmap mode. The S-STEP system has adopted various challenging strategies to actively pursue a new space approach that is faster, cheaper, smaller, and lighter than the old space approach. The fundamental strategies are a flat-panel-type satellite structure integrated with both a bus and payload, a multifunctional mechanical structure of TRM, and a dedicated vibration-free orbital deployer (VFOD) [25; 26]. As well as these strategies, the key design goal in terms of an SAR payload is to develop a lightweight SAR payload that can be applied to an 80 kg class microsatellite. In particular, the S-STEP mission aims to develop a multimode SAR payload system that can be mounted on a microsatellite. This payload is designed to provide various SAR image qualities in multiple modes, e.g., striripap and ScanSAR modes, by utilizing antenna beam scheduling. Moreover, by integrating an SAR antenna with the satellite structure, the satellite mass can be reduced to achieve an 80 kg class microsatellite. Furthermore, a microstrip patch antenna is applied for lightweight miniaturization and cost reduction. An aperture-coupled structure that can produce a high antenna gain by increasing the antenna efficiency was applied to the array radiating elements, and a substrate integrated waveguide (SIW) cavity-backed microstrip patch antenna was applied instead of the existing metal cavity-back technique to enhance its manufacturability and antenna gain performance. Essentially, the S-STEP SAR payload can provide a high resolution of 1 m in the striripap mode, for which the resolution usually corresponds to that in the spotlight mode of medium or large SAR satellites. The spotlight mode SAR images exhibit limited coverage; however, the stripmap mode can acquire SAR images with continuous long coverage, which is advantageous for land-based security applications.