Space Surveillance is a subset and contributor to space situational awareness (SSA), which in turn, is a subset and contributor to space domain awareness (SDA). To clarify the purpose and scope of this paper, these three terms are defined and their relationships illustrated.
SDA is the overall understanding of the space operational environment required to enable planning and execution of space operations involving satellites, supporting ground assets, plus the ground-to-space, space-to-ground and space-to-space communications links that connect satellites, users and operators. The space operational environment encompasses active satellites (their locations, capabilities and intentions), orbital debris, space weather, terrestrial weather, policies, politics, and intelligence. SDA activities include collecting raw observables, identifying physical states and parameters (such as orbit, attitude, size, shape), determining functional characteristics (such as active vs. passive, thrust capacity, payloads), inferring mission objectives (such as communications, weather), identifying behaviors, and predicting credible threats and hazards.
SSA is a subset of space domain awareness that focuses on the orbital segment. SSA is the requisite foundational, current, and predictive knowledge and characterization of space orbital objects including live satellites, dead satellites, and debris. It provides planners, users and operators with knowledge and characterization of objects in orbit to ensure safe, stable, and sustainable space activities.
Developing and maintaining SSA requires three basic activities: data collection (i.e., space surveillance), data analysis (fusing and interpreting collected and historical data; then turning all that into useful information), and data dissemination (getting useful information to the users and operators).
The definitions of SDA, SSA, and space-based surveillance have evolved differently according to culture and country. SDA language was introduced by the United States military in 2019 to emphasize that the space domain involves more than just objects in space. Authoritative literature and media from the Chinese military continues to use SSA terminology even as their growing infrastructure and organization clearly indicate that they, too, appreciate the bigger scope of SDA. At the same time, Chinese researchers almost always describe their work on tracking and identifying space objects as SSA, though the U.S. community would call that “space surveillance” and not “space situational awareness.”
This paper focuses on the collections piece of SSA. Using the “collect, analyze, disseminate” framework described earlier, space surveillance is the “collect” piece, which entails observing (or more generally, collecting upon) orbiting man-made objects often enough and with sufficient accuracy to enable the “analyze” piece to identify, characterize, and predict future locations of all those objects. That surveillance, or collecting, is currently done with three basic types of sensors: passive optical, active radar, and passive RF (listening to radio frequency signals that are either emitted or reflected by man-made objects). The vast majority of those sensors today are located on the ground. They can, however, be deployed on satellites themselves, enabling what is commonly called “space-based space surveillance” or, in Chinese literature, “space-based SSA”.
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