## 2 Mission and Satellite Design
### Overview  
The satellites for the S-STEP mission involve 80 kg class X-band SAR microsatellite constellations based on an active phased-array antenna. In particular, there are six major mission requirements for the S-STEP, and the design lifetime of the mission is at least three years with a total mass of 80 kg. The mission involves two distinct SAR modes: a stripmap mode and 3-beam ScanSAR mode as showing Figure 2. Accordingly, the high-resolution mode acquires the SAR raw data with a 5 km swath with a ground resolution greater than \(1\times 1\) m (single look) using the stripmap technique. Comparatively, the wide-swath mode acquires the SAR raw data with a 15 km swath with a ground resolution at greater than 4 \(\times\) 4 m (single look) using the ScanSAR technique. The continuous SAR image acquisition period is greater than 60 s for both imaging modes. Moreover, the raw data are transmitted to the ground segment in near-real-time via the X-band data link of the platform.  
The key factors that influence the micro-SAR satellite performance are the average/maximum revisit cycle, response period, and coverage. Although more performance factors can be considered, the S-STEP mission derives the optimal satellite orbit and system parameters based on these key factors.  
The design of the constellation is vital for satisfying the major mission requirements, such as the lifetime, revisit time, and SAR image quality. The constellation orbit significantly impacts the mission performance because multiple satellites perform a single mission. Additionally, the constellation of the S-STEP mission is designed by adopting the Walker-Delta technique that considers the number of launches, number of satellites, probability of collision, and the revisit time to the area of interest based on the mission concept and requirements.  
The frozen orbit and repeat ground track (RGT) orbit are considered during the design of the satellite constellation orbits to enhance the mission plan efficiency and usability of the SAR images. In this study, the RGT orbit was designed with an altitude range of 510 \(\pm\) 10 km and repeated every 14 d to facilitate mission planning.  
To minimize the revisit time for the region of interest up to \(\sim\)30 min, the orbit incidence angle was selected according to the longitude of the region of interest and subsequently optimized. This was because the revisit time of the satellite for a certain area depends on the orbit inclination. Therefore, the final constellation and orbit of the S-STEP mission were selected using multidisciplinary design optimization with FreeFlyer, commercial application software for satellite mission analysis, design, and operations based on the essential performance factors. The constellation of the microsatellite with a Walker-Delta pattern is operated on four orbit planes, and eight satellites are equally spaced on each plane to operate a total of 32 satellites. The mission orbit is nominally circular, non-sun-synchronous, and inclined. More specifically, the nominal altitude, orbits per day, repeat period, and inclination of the mission orbit are 510 km, 15 2/11, 13.75 d, and 44.2\({}^{\circ}\), respectively [27]. A comparison of various satellite constellations based on their individual performance indices is illustrated in Figure 3[28].  
Figure 3: Comparison of the SAR performance indices as functions of satellite constellation masses.  
Figure 2: Operating concept of the S-STEP mission in the high-resolution and wide-swath modes.