## 2 The Evolution of Space Situational Awareness Policies  
Research on space situational awareness originated in the 1980s and gained significant attention in the 1990s [5]. In August 1998, the U.S. government underscored the critical significance of space situational awareness in its official document [6]. In 2018, space situational awareness was officially recognized as one of the four primary U.S. space missions. The related document explicitly emphasized that a comprehensive understanding of the space environment is foundational to the execution of U.S. space commands and operations in the space domain [7]. In 2019, the United States formally introduced the term Space Domain Awareness (SDA) to supplant space situational awareness. This signifies the official acknowledgment of space as an operational domain, alongside land, sea, and air [8]. The documents titled \"Space Capstone Publication, Spacepower\" published in 2020 and \"Space Doctrine Notes, Operations\" published in 2022 by the U.S. government underscored that the Space Domain Awareness mission entails understanding all factors, intentions, capabilities, and current conditions that may impact U.S. space activities. These documents particularly highlighted the importance of space situational awareness in anticipating potential future scenarios in the space domain [9; 10]. Important milestones in the development of the U.S. space situational awareness policies are shown in Figure 3.  
In addition to the United States, European countries have embarked on establishing an independent system to detect, assess, and forecast space threats since the last century. The development of a European space situational awareness system was initially led by European spacefaring countries, notably France [11]. In 2008, the Council of the European Union adopted resolutions to advance European space policies, emphasizing the need for Europe to enhance its monitoring capabilities for space facilities and debris to strengthen its global standing in space endeavors [12]. Since 2009, the European Space Agency (ESA) has spearheaded the Space Situational Awareness Program, categorizing it among the six European major space programs alongside the Galileo Global Satellite Navigation System [13]. The primary objectives of the European space situational awareness system include monitoring and tracking space objects, observing and forecasting space environments, and detecting space targets in low Earth orbits (LEOs). These objectives aim to achieve space collision avoidance, re-entry trajectory analysis, and orbit evolution of space targets to ensure the safety of European space facilities [14; 15; 16]. In 2010, the European Council formally acknowledged the importance of space situational awareness in both military and civilian areas in its seventh Space Council resolution [17]. Subsequently, the European Council reiterated the significance of space situational awareness in a series of related meetings in 2011 [18; 19]. In 2014, the European Parliament adopted Resolution No. 541 concerning the establishment of the Space Surveillance and Tracking (SST) System, systematically outlining concepts, necessity, mission goals, and series of resolutions related to SST [20]. In 2015, the EU established the SST Alliance to coordinate SST-related affairs. In 2016, the European  
Figure 3: Milestones of the U.S. SSA policies. Space Strategy was promulgated, emphasizing Europe's commitment to strengthening existing space target monitoring and tracking capabilities and evolving them into a more robust space situational awareness system in the future [21]. In 2018, the EU Commission submitted the European SST system development report for the years 2014-2017 to the European Parliament.