## 1 Introduction  
The spaceborne synthetic aperture radar (SAR) has been successfully used to systematically and continuously monitor the dynamic evolution of the geosphere, biosphere, hydrosphere, and cryosphere of the Earth's surface in scientific, civil, commercial, and military applications. This is because the SAR can provide high-resolution images for remote sensing applications regardless of sunlight illumination and weather conditions [1; 2; 3; 4]. In military applications, the maximum possible number of images of a particular area of interest (AoI) are required to be captured in a short time period according to the established mission plan to improve the surveillance and reconnaissance mission performances at national borders and military facilities [5; 6]. In addition, the observer-orient-decide-act (OODA) loop model is predominantly used to establish a command and control (C2) system for rapid response and operation against arbitrary threats, especially time-critical or time-sensitive targets, such as the transporter-erector-launcher (TEL) [7]. The modified or extended OODA loop comprises inner and outer loops [8], wherein the inner loop cycle should be minimized for timely information acquisition. Accordingly, a microsatellite SAR system with a fast-revisit interval has been recently introduced and widely reviewed. Based on the \"Faster, Better, and Cheaper (FBC)\" strategy [9] promoted by NASA since 1992, numerous new space companies have accelerated their innovation under the umbrella of the \"New Space Paradigm\" (NSP). As a representative example of these innovations, the spaceborne SAR is actively undergoing industrial innovation owing to its aforementioned advantages. The emergence of the NSP has revolutionized the development philosophy and methodology of the global space industry [10]. NSP, which is also called Space 4.0 or Space Renaissance, is a collective reference expression of a methodology that boldly breaks through the existing government-led \"Old Space\" development philosophy. Instead, it leads to the rapid development of new space systems by adopting agile approaches and exploiting the latest commercial-off-the-shelf (COTS) technologies. The introduction of concepts such as \"Smaller\" and \"Lighter\" to the FBC development strategy, especially in the field of small satellites, has accelerated the development and implementation of several innovative space development programs under the NSP. A key area of application of the NSP is in the small satellite constellation for providing services such as surveillance and reconnaissance or communications over a large area, including the world. In addition, the shortening of the development period and reduction in the cost through large-scale mass production using standardized satellite platforms, innovative manufacturing methods such as additive 3D printing techniques, and automated test processes yield small satellite constellations for diverse challenging missions. Such missions include SAR imaging with a short revisiting period, near-real-time or real-time remote sensing, global internet services, and low-latency high-speed communications. Thus, owing to its several advantages, small SAR satellite constellations can complement or replace existing medium/large SAR satellite missions such as Sentinel-1, TerraSAR-X [11; 12], and KOMPSAT-5 [13]. A graphical comparison of the SAR image resolutions in terms of the mass of the microsatellites being developed recently is shown in Figure 1[14]. As indicated in the figure, these microsatellite SAR constellations, such as ICEYE and Capella SAR, can provide cost-effective and practical SAR images with a resolution comparable to those of medium/large SAR satellites such as KOMPSAT-5. Developed by ICEYE in Finland and launched in 2018, ICEYE X2 (92 kg) is known as the world's first commercial SAR in the microsatellite class based on its proof-of-concept model, ICEYE X1 [15; 16].