# Thermal Optimization Design for a Small Flat-Panel Synthetic Aperture Radar Satellite
## 1 Introduction  
With the rapid advancement of satellite and rocket manufacturing technology, particularly the emergence and application of reusable rockets, the average launch cost of a single satellite has seen a significant reduction, ushering the global aerospace industry into a new era [1, 2, 3]. A satellite constellation is a system in which multiple satellites work together as a whole, also known as a distributed satellite system (DSS). In DSS, two or more satellites are deployed in the same or multiple orbits to complete specific space tasks through mutual cooperation. Compared to a single satellite, DSS significantly enhances the satellite's global coverage efficiency, also referred to as temporal resolution [4]. Small- and medium-sized satellites have become the primary components of DSSs, owing to their advantages in terms of lower cost, reduced weight, and shorter development time. LEO is frequently employed as the operational orbit for DSSs, featuring an orbital altitude not exceeding 2000 km. This implies reduced launch costs and offers satellites with high bandwidth and low communication latency. Currently, the scarcity of LEO orbits is intensifying competition amongglobal companies for constellation deployment, leading to a production-centric approach in the design of constellation satellites [5; 6]. SAR is an active earth observation system that emits continuous radio pulses towards the target scene, receives and records the echoes of each pulse, and achieves high-resolution microwave imaging. SAR can be mounted on various platforms, including airborne, spaceborne, and missile-borne platforms, and it finds widespread applications in numerous fields. SAR satellites are capable of imaging at night or in areas obscured by clouds, demonstrating all-weather and all-day imaging capabilities that are unachievable by optical remote sensing satellites [7]. Currently, typical small SAR satellite constellations include ICEYE, Capella, StriX, and so on [8; 9; 10], which almost achieve hourly revisits for a single target. In addition, to provide users with higher-precision data, medium and large SAR satellites are continuously being upgraded, such as Sentinel-1, FIA, CSG, RadarSat, and others [11; 12; 13; 14]. The commonly used SAR satellite antennas include slotted waveguide antennas and microstrip phased array antennas, both of which are of a plate antenna structure. Additionally, there are deployable mesh reflector antennas and deployable paraboloid antennas [15]. For plate-type SAR antennas, a reasonable thermal design is necessary to ensure the temperature stability of high-power electronics and supporting structures. SAR satellites often employ methods such as multi-layer insulation (MLI), thermal control coatings, TIM, heat pipes, and heat conduction devices for their thermal design [16; 17]. MLI is a vacuum thermal insulation component made of multiple reflective screens and spacers interlaced. The MLI forms a high thermal resistance to radiant heat flux through the layers of reflective screens, and, theoretically, its thermal conductivity can reach \(10^{-5}\) W/(m\(\cdot\)K) to achieve good thermal insulation. And TIM is a kind of gap filling material, which is used to increase the heat transfer between two contact surfaces. So far, numerous studies have proposed thermal control methods for SAR satellites, all emphasizing heat dissipation and temperature uniformity in the transmitter/receiver module (T/RM) [18; 19]. In terms of thermal design, the on-orbit working time of the payloads on optical remote sensing satellites and SAR satellites is quite similar. Specifically, each orbital cycle only includes short imaging tasks, during which the heat consumption of related electronics is high. Therefore, the excellent thermal design concepts of small optical remote sensing satellites can serve as a reference.