## 2 S-STEP System Overview  
The S-STEP program has been started to develop an 80 kg class lightweight small SAR satellite for high-resolution SAR imagery for environmental investigation, surface mapping, and disaster monitoring. The mission operation concept of the S-STEP system is shown in Figure 1[22]. The S-STEP system is operated at LEO and provides a 1 m resolution stripmap image, 4 m resolution ScanSAR image, and 1 m resolution VideoSAR for 10 s duration to capture the earth's targets of interest. The SAR payload of S-STEP is an aperture-coupled cavity-backed microstrip patch array antenna, which can achieve high radio frequency (RF) gain. A total of 192 TRMs were applied to implement electronic beam steering capability in the elevation direction of the antenna. The SAR image data are transferred to an integrated avionics unit (IAU) for storage. The stored data are transmitted to the ground station through a 1 Gbps X-band downlink module. The expected mission lifetime of the S-STEP system is 3 years. The S-STEP program ultimately aims to implement a satellite constellation for near real-time SAR observation. Therefore, the S-band inter-satellite link (ISL) system is considered for immediate communication link between the satellites. Table 1 lists the basic system specifications of S-STEP [22]. Figure 2 shows the mechanical configurations of the S-STEP satellite. The primary feature of its mechanical design is the bus-payload integrated flat panel-shaped satellite structure. This mechanical configuration is advantageous to ensuring the dimensional stability of the satellite, especially the SAR antenna, in extreme on-orbit thermal environments. One more essential feature of this satellite configuration is the additional 0 g compensation device for on-ground testing of SAR antennas, as it is a nondeplovable-type antenna. All payload and bus electronic units are mounted on the top aluminum-skin honeycomb composite panel as the configuration shows in Figures 2c and 3. Four reaction wheels (RWs) in a pyramidal configuration with an optimal tilt angle are mounted at the center of the satellite to minimize the moment of inertia and their power consumption. The X-band antenna for payload data downlink and two S-band antennas for telecommand/telenetry (TM/TC) communication are installed on the side walls in \(-x\), \(+y\), and \(-y\) sides of the satellite. The GPS and ISL antennas are also installed on various locations of the side walls. To operate the satellite, 340 W of electrical power is produced at beginning of life (BoL) and stores the power in the battery with 648 Wh capacity. Among the on-board instruments of satellite, the four TRM assemblies require the largest amount of power consumption. In the SAR imaging operation, the stored battery power is mainly used to operate these TRM assemblies. The satellite is integrated on a VFOD introduced in a previous study [22], which has the dual functions of the orbital insertion of satellite and the entire spacecraft vibration isolation. The main function of VFOD is an on-orbit satellite separation combined with a side-mounting ride-share platform. The additional function of launch vibration isolation \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Specification** \\ \hline Mission lifetime & 3 years \\ Satellite mass & 80.3 kg \\ Satellite volume & 1970 mm \(\times\) 1060 mm \(\times\) 200 mm \\ Generation & 340 W (BoL) \\ Power & Storage & 648 Wh \\ Inter-satellite link & RF (S-band) \\ TM/TC/Image downlink & S-band/X-band \\ Attitude pointing accuracy & 0.085\({}^{\circ}\) (3\(\sigma\)) \\  & - & 1 m (Stripmap) \\ Resolution (@ SAR look angle: 25\({}^{\circ}\)) & - & 4 m (ScanSAR) \\  & - & 1 m (VideoSAR) \\  & - & 5 km \(\times\) 420 km (Stripmap) \\ Swath (elevation \(\times\) azimuth) & - & 15 km \(\times\) 420 km (ScanSAR) \\  & - & 5 km \(\times\) 5 km (VideoSAR) \\  & - & 60 s (Stripmap) \\ Image acquisition time (per single pass) & - & 10 s (VideoSAR) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: System specifications for S-STEP [22].