Space situational awareness (SSA) is the knowledge and characterization of space objects and their operational environment. This is a critical component of Space Domain Awareness (SDA), which is the ability of decisionmakers to understand, as completely as necessary, their current and predicted operational environments.

SSA is the practice of tracking objects in space, identifying them, establishing their orbits, understanding the environment they’re operating in, and predicting their future positions and threats to their operations. The data is used to predict conjunctions between objects and warn space operators of potentially dangerous close approaches to enable collision avoidance maneuvers. Occasionally, there may be a need to predict and react to debris from fragmentation events, meteor storms, or other natural events that might affect operations. SSA is foundational to all space safety and space traffic management (STM) activities. Making use of SSA to accomplish a real-time goal is usually considered STM.

The Aerospace Corporation has been deeply involved in all aspects of SSA as it has been practiced by the U.S. government, including the acquisition and operation of the Space Surveillance Network’s (SSN’s) radars and telescopes. We have been instrumental in developing the processes and algorithms of orbit determination and the maintenance of the catalog of space objects. We are experts in the practical details of turning observations from sensors into knowledge of orbits. We have developed many of the algorithms used for orbit propagation, conjunction prediction, and collision probability estimation that are used operationally by the U.S. Space Force. To educate the community, we have held conferences and workshops to acquaint satellite operators with threats posed by meteor storms and radio frequency interference. We have worked with satellite operators to investigate the spacecraft charging environment and have patented innovative techniques for measuring the charge on spacecraft. Aerospace has ongoing projects that use CubeSats to collect information on the low Earth orbit (LEO) environment.

The Space Safety Institute’s goal is to examine the space environment holistically, to understand both the debris that can’t be tracked and how our current actions will affect the future environment. The risks to space operators stem from far more than just the objects we can see and track—the focus of most SSA efforts. For decades, under Aerospace’s Center for Orbital and Reentry Debris Studies (CORDS), we have been developing models of the untrackable debris environment to quantify unseen risks now and in the future. The Aerospace Debris Environment Projection Tool (ADEPT) models the future environment under multiple scenarios and provides insight into the effects of various behaviors and policies on future debris generation, collision frequencies, and satellite operating conditions. Results from ADEPT help craft policies and norms of behavior that achieve desired outcomes.

Aerospace and Space Safety Institute technical staff connect the analysis of the present with practical assessments of the future. Some examples of this include:

• We directly managed and provided public access to the unclassified space object catalog derived from SSN data as part of pilot commercial and foreign entity support (CFE) from 2003 through 2006. Aerospace created space-track.org and handed over a functioning system to a contractor for continuing operations.
• We have deep experience with current catalog maintenance and sensor processing. We help manage the SSN Metrics Improvement Project Team (SSN MIPT), and we developed a process for catalog covariance corrections. We lead multiple efforts in managing new observations for difficult-to-track objects and new observations, and we have developed techniques aimed at tracking rendezvous and proximity operations and minimizing observation cross-tagging. We have deep expertise in the algorithms of operational orbit propagators.
• We have created prototype tools for daily SSA analysis. These include daily prediction of all-on-all close approaches, automatic detection of maneuvers, a space weather dashboard, and tools for SSA big data mining.

Independent Assessments

As operator of a federally funded research and development center, Aerospace provides independent technical assessments of systems and capabilities. As an illustration, Aerospace, working cooperatively with the Naval Research Laboratory, provided an independent assessment of state-of-the-art uncorrelated track (UCT) processing tools for the Air Force Research Laboratory. These tools enable individual observations of satellites to be identified as a single object. This capability is critical to maintaining a catalog of space objects. The assessment covered a wide range of orbital altitudes and techniques, including radar and telescope observations. The evaluations included testing against real-world cases. The UCT tools are critical not only for maintaining space object catalogs but also for the efficient tracking of debris after fragmentation events.

Standards and Best Practices

The U.S. Space Force maintains a set of standards for astrodynamic algorithms to process SSN data. Aerospace recently rewrote the government’s SGP4 orbit model to improve accuracy and extend its range of applicability. Aerospace worked with the U.S. government to define changes to the two-line element (TLE), used to define satellite orbits in the U.S. Air Force SSN catalog, to accommodate this new model. SGP4 is the most widely used propagator worldwide, and TLEs are the most commonly used format for orbit data. The new propagator will be included as a new astrodynamics standard.

Research and Development

As new types of space operations are implemented, they can create new challenges for maintaining accurate information on space object orbits. Aerospace conducts research in a variety of areas to find practical solutions to these new challenges.

There has been an increase in the number of space launches with tens or even more than 100 payloads. Uniquely identifying and determining orbits for these satellites rapidly is difficult, especially shortly after deployment. Aerospace modified the government’s breakup processor, used to identify the orbits of individual fragments after a breakup event, to function effectively with satellite deployments. This tool is designed to quickly separate and catalog the satellites from multi-payload launches. 

The accuracy of orbit propagation is critical to determining the utility of tracking data for safety tasks, such as collision avoidance. At the request of the government, Aerospace extensively modified the orbit models inside the SGP4 propagator. These modifications provide one to two orders of magnitude improvement in accuracy as well as extending the model’s applicability to cislunar space.

We believe that active identification of satellites and their locations will be critical for effective STM. For several years Aerospace has been researching independent GPS-based active transponders and has been drastically reducing their required size, weight, and power. In addition, we are actively working with the Combined Space Operations Center (CSpOC) to incorporate this type of data into the space surveillance catalog.

Infrastructure, Tools, and Data

As the characteristics of space operations rapidly evolve, there is increasing need to support those operations with new techniques and capabilities. Customers have frequently turned to Aerospace to develop these new capabilities. These tools are accessible through the Space Safety Institute.

When a fragmentation event occurs in space, hundreds or thousands of trackable fragments can be generated as well as tens of thousands (or more) untrackable but mission-ending fragments. Even for the trackable-sized fragments, it can take days to weeks to catalog them. During that time, they are an invisible hazard to satellites. Aerospace developed the tools needed to model the generation of debris from satellite and upper-stage fragmentation events and the associated risk to satellites. This capability is regularly called on by the government when on-orbit breakups occur.

As space activity increases, it becomes more important to have an accurate understanding of the orbits of all the satellites. One way to improve tracking capabilities is to integrate data from more sensors, including those of our allies. Incorporating external data comes with a number of challenges with no guarantee that more data will result in improved tracking. Aerospace has supported government exercises examining satellite tracking data from foreign sensors to determine the effects and benefits of incorporating that data into U.S. space object catalog maintenance.

Policy and Strategy

Space situational awareness depends on the collection and interpretation of data. There are a number of issues associated with collecting data from the range of available sources including government, commercial and international sensors. Standard methods and formats of data transfer must be developed and agree upon. The Space Safety Institute, utilizing Aerospace’s experience, is helping develop the common understanding and language needed for effective space situational awareness.

References

• Paper: GPS Transponders for Space Traffic Management (Apr 2020)
• Paper: Space Traffic Management: The Challenge of Large Constellations, Orbital Debris, and the Rapid Changes in Space Operations (Sep 2020)

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