## 1 Introduction
The TMS mainly consisted of rollout-type flexible deployable radiator, heat pipes integrated with the satellite structure, and phase change material with high heat storage capacity. They applied TMS to the 12U-class CubeSat and verified its effectiveness through the numerical thermal analysis. [PERSON] et al. [18] proposed functional thermal control systems for 100 W-class high-power 3U CubeSat. They used a heat storage panel with pitch type CFRP, micro loop heat pipe, and flexible re-deployable radiator. The effectiveness of adopting those thermal hardwares was numerically and experimentally verified. In all, a considerable number of previous studies reported thermal designs for small satellites [16; 17; 18; 19; 20; 21]. For small SAR satellites, the light weight and compactness of onboard instruments are essential factors due to the limited mass and volume of satellite platforms. For a small satellite constellation mission, these factors would be far more essential because they are directly related to the available number of satellites launched at once. Currently, the adoption of the abovementioned passive thermal hardware might be the most suitable solution for the thermal control of small SAR satellites; however, the thermal design of small SAR satellites needs to be optimized for minimizing system mass and power budget allocated for a thermal control subsystem. In addition, it is necessary to minimize the heater's power consumption to achieve the compactness of the satellite by solar panel and battery size reduction. An 80 kg class high-resolution earth observation small satellite with X-band active phase array SAR, named the Small SAR Technology Experimental Project (S-STEP) [22], is under development. For developing an S-STEP system in accordance with a smaller, lighter, faster, better, and cheaper NewSpace paradigm, several mechanical design strategies are being adopted for the satellite [22]. The design strategies involve a bus-payload integrated flat panel-type satellite platform, a multifunctional TRM structure, and a vibration-free orbital deployer (VFOD). The flat panel-type satellite structure does not require flexible deployable structures, and it could be beneficial in terms of the dimensional stability of SAR antenna in extreme on-orbit thermal environments. The TRM assembly of S-STEP functions as a structural stiffener for the satellite structure and provides thermal dissipation through a radiator integrated with TRM. Thus, it can minimize the application of additional stiffener for the structural reinforcement of the satellite. These strategies make it possible to minimize the system mass budget while ensuring the structural safety of vibration-sensitive onboard instruments under launch loads. In particular, the structural design load reduction achieved by VFOD contributes to minimizing the development period and cost by optimizing the verification test process, i.e., streamlining and skipping the test phase or a step at a subsystem level can be positively considered. These advantages make it attractive to be used for implementing the NewSpace small SAR satellite. The effectiveness of these mechanical design strategies has been investigated by [PERSON] et al. (2020). However, this study did not perform the numerical or experimental verification of those thermal designs. To date, no other previous studies addressed the thermal design strategy for this form of small SAR satellite. In particular, no previous study qualitatively verified the thermal design to achieve the designated SAR imaging duration in both left- and right-looking modes of the small SAR satellite, although it is one of the most important system performance factors. This study proposes a new thermal design strategy for assuring the functionality and thermal stability of the S-STEP system in an on-orbit thermal environment.