## 3 Typical Space-Based Situational Awareness Facilities  
The current global systems for detecting and alerting against threats to space assets are broadly categorized into ground-based and space-based surveillance systems based on their spatial distribution [34; 35; 36; 37]. Ground-based systems, being the initial development, formed the early space surveillance infrastructure for major spacefaring nations, leveraging their high power and capacity for large detection payloads. However, the Earth's curvature and susceptibility to atmospheric and cloud cover may limit ground-based systems, imposing significant constraints on their detection range and effective operational time. Additionally, their distance from space targets results in lower resolution for detecting targets on HEO. In contrast, space-based surveillance systems utilize detection payloads on satellites, mitigating issues related to atmosphere and clouds on Earth. They offer a broader field of view compared to ground-based systems through maneuverability. Moreover, these systems, equipped with satellite maneuvering and multi-satellite networking capabilities, can ensure continuous monitoring and close observation of specific space targets or regions. To achieve comprehensive, continuous, and high-precision space situational awareness on a global scale, nations worldwide have increasingly emphasized the development of space-based platforms in recent years [34]. In this section, typical detection payloads for space situational awareness systems shown in Table 1 are presented to provide a more intuitive and comprehensive understanding of current development status of this field. In the 1990s, the United States launched one of the earliest and most representative space-based situational awareness satellites, the Midcourse Space Experiment (MSX). It carried a suite of optical detection instruments, including an infrared imaging telescope, a space-based visible (SBV) camera, and the ultraviolet and visible imagers and spectrographic imagers (UVISI) [38]. The SBV camera is considered a groundbreaking space-based visible light detection instrument. The optical aperture of SBV is 15 cm, operating in the spectral range of 0.3-0.9 \(\upmu\)m. Its focal plane is composed of four low-noise visible light CCD chips with a field of view of \(1.4^{\circ}\times 1.4^{\circ}\) each, stitched together to form a total field of view of \(1.4^{\circ}\times 5.6^{\circ}\)[39]. This camera signifies the validation of space-based optical detection technology and provides crucial insights for the development of subsequent space detection systems. Building upon the success of the MSX satellite, the United States further strengthened its space situational awareness capabilities by establishing the space-based surveillance system (SBSS). The first satellite of the SBSS mission, launched in 2010, carried a TMA visible light camera with 30 cm aperture, \(3^{\circ}\times 3^{\circ}\) field of view, and 2.4 million pixels. Mounted on a gimbal, TMA demonstrated the capability to flexibly adjust its field of view under a fixed satellite posture. The detection range of TMA covered LEO, MEO, and HEO, with a positioning accuracy of 10 m for low-orbit targets and 500 m for high-orbit targets. According to the plan of the SBSS project, the system was originally intended to establish a constellation of space-based monitoring satellites operating in LEO, thereby enhancing positioning accuracy for targets in HEO to approximately 250 m [40]. To address the issue of too many false collision alerts caused by insufficient ground-based detection resolution, the Space-based Telescopes for the Actionable Refinement of Ephemeris (STARE) satellite was launched into a polar orbit to validate the capability of space-based surveillance systems to update the orbital parameters of on-orbit targets.