## 4 On-Orbit Thermal Analysis of S-STEP
### Results of Thermal Analysis  
Figure 10 shows the representative thermal analysis results for Case 1--the orbital temperature profiles of components newly developed in this program when the left-looking SAR imaging operation was executed in the hot environment. This is the worst hot condition for the S-STEP system among the analysis cases listed in Table 5. Temporal temperature variations occurred due to the changes in incidence angle of satellite with respect to the sun and earth in accordance with the orbital revolution of satellite, as shown in Figure 9. The thermal design to dissipate the heat of electronic units through SAR antenna controlled their temperatures well, especially high-power electronic units, such as PCDU, IAU, and battery, combined with sidewall radiators and thermal paths implemented by the graphite sheet. The key payload components of TRM assemblies were operated under a moderate temperature condition, ranging from 29.6 to 42.2 \({}^{\circ}\)C, despite their total heat dissipation of 1460 W (365 W per a single TRM assembly) during SAR imaging operation. Figure 11a,b, respectively, show the predicted maximum and minimum temperatures of all onboard components in Case 1 (left-looking) and Case 2 (right-looking) when the SAR imaging operation is executed in the hot environment. The graph also shows the operating and survival temperature limits of each component. In both cases, the temperatures of all components were maintained within their allowable limits for the operating conditions specified in Table 2. In particular, the high-power components of IAU and PCDU were operated with sufficient temperature margins of more than 6.7 \({}^{\circ}\)C by effective heat dissipation via the SAR antenna and sidewall radiators. The battery temperature was maintained above 25.3 \({}^{\circ}\)C, and the heater was not operated during the entire orbit. These analysis results indicated that the proposed thermal design strategies were effective for stable satellite operation and achieving the required SAR imaging duration of 60 s, regardless of SAR left- and right-looking modes, even in the worst hot environment. Figure 10: Predicted on-orbit temperature profiles of components newly developed in S-STEP program (Cases 1) (SAR left-looking mode in hot environment). Figure 12 shows Case 3 analysis results--the orbital temperature profiles of representative onboard components when left-looking SAR imaging operation is executed in the cold environment. The predicted maximum and minimum temperatures of the entire onboard components in Case 3 (left-looking) and Case 4 (right-looking) when the SAR imaging operation was executed in the cold environment are shown in Figure 13a,b, respectively. These figures show that all components were within the temperature ranges of operating conditions. The temperatures of TRMs were moderately maintained within the range of \(-5.6\) to \(0\)\({}^{\circ}\)C in both the left- and right-looking modes of SAR antenna. The  
Figure 11: Predicted maximum and minimum temperatures of S-STEP components in hot environment (Cases 1 and 2): (a) SAR left-looking mode and (b) SAR right-looking mode. battery temperature was maintained above 14 \({}^{\circ}\)C by heater control, and the duty was 36%. Figure 14 shows the thermal analysis results in Case 5 (safe-hold mode, cold environment). All non-operating components kept the temperature within the survival limits. The heater duty was 39% in this case. These on-orbit thermal analysis results showed that the proposed S-STEP system would be stably operated throughout the mission duration under the expected on-orbit thermal environment. Figure 12: Predicted on-orbit temperature profiles of components newly developed in S-STEP program (Cases 4) (SAR right-looking mode in cold environment). Figure 13: _Cont._  
\(\Delta\)T\({}_{\rm s}\) occurred during the left-looking SAR imaging in the hot case. Table 8 summarizes \(\Delta\)T\({}_{\rm s}\) of each TRM in the left- and right-looking modes in the hot and cold environments (Cases 1-4).