## 1 Introduction  
The so-called NewSpace paradigm refers to the recent commercialization of the space sector. The technology innovations of satellite and launch vehicles, as well as various relevant services, are now leading the rapid advances and growth of global space industries [1]. One of the technologies that plays an important role in the NewSpace era is the emergence of a small satellite constellation. The constellation of multiple satellites effectively reduces the system's temporal performances, such as revisit and response time, which enables frequent access to the target. The smaller, lighter, faster, better, and cheaper small satellite platform realized a large-scale constellation in orbit based on satellite mass production, which was nearly impossible with large-scale satellites [2]. The reduction of satellite launch cost, via reusable launch vehicles, small-scale launchers, and air-launch-to-orbit, have increased the satellite launch capability, thus contributing to the rapid orbital insertion of small satellites. These advantages make the small satellite platform attractive for implementing various low earth orbit (LEO) constellation missions, such as near real-time earth observation, global internet services, and communication networks, for commercial and military purposes [3, 4, 5]. The concept of a synthetic aperture radar (SAR), proposed in the 1960s, significantly changed the field of spaceborne remote sensing [6]. Its advantage compared with traditional optical imaging is the ability to obtain high-quality images, even when the target area is in nighttime or covered with obstacles, such as cloud and fog. Owing to this imaging capability, the demand for SAR imagery in space has rapidly increased in various sectors, including government, military, and commercial sectors [7]. The constellation of small satellites with SAR instruments is a solution to meet the increasing demands. To date, many small SAR satellites, such as ICEYE X2, Capella X-SAR, StriX, and QPS-SAR, have been developed and launched by industries globally. They are in-orbital operations to provide SAR images at a 1 m class resolution [8; 9; 10; 11]. To meet the various mission needs on spaceborne SAR imagery, the demand for an active phase array-type SAR antenna has been increasing for small satellite applications, as its electronic beam steering capability provides various SAR imaging modes, including stripmap, spotlight, and ScanSAR. Proper thermal control is crucial to guarantee the functionality and thermo-mechanical stability of an SAR satellite with many high-power instruments in extreme on-orbit thermal environments. Traditionally, most SAR satellites have adopted flexible deployable SAR antenna panel structures [12] to achieve the desired SAR imaging performance. Thermal designs for SAR satellites have been mostly based on passive thermal control approaches using suitable thermal coatings, paints, thermal fillers, thermal insulators, radiators, multilayer insulation (MLI), and sun-shield. In addition, heat pipes and doublers have been applied to the radiator for effective heat dissipation of some high-power instruments, such as transmit/receive modules (TRMs) and electronics. The heat loss through the radiator was compensated by a heater. To date, several studies [13; 14; 15] have proposed and investigated the thermal design for SAR satellites based on thermal control hardware. In addition to the small SAR satellites, [PERSON] et al. [16] proposed a quasi-all-passive thermal control system (TCS) design for the Luojia 1-01 satellite with the problems of low thermal inertia and poor thermal conductivity of the satellite structure. They adopted surface coatings, thermal interface materials, MLI, and active heater control to achieve the minimum system mass and power budgets. [PERSON] et al. [17] proposed the thermal management system (TMS), which is capable of rejecting 1 kW of waste heat produced in the satellite.