Alfredo Locarini, Nautilus – Navigation in Space; Christian Monaldi, Nautilus – Navigation in Space; Virginia Angelini, Nautilus – Navigation in Space; Mattia Chiappalone, Nautilus – Navigation in Space; M. Cavallucci, Nautilus – Navigation in Space; Dario Modenini, University of Bologna; Giacomo Curzi, University of Bologna; Stefano Palmiotto, University of Bologna; Daniele Pecorella, University of Bologna; Stefano Cicalò, Space Dynamics Services s.r.l.; Francesca Guerra, Space Dynamics Services s.r.l.; Alessia Bertolucci, Space Dynamics Services s.r.l.; Davide Bracali Cioci, Space Dynamics Services S.R.L.; Enrico Paolini, University of Bologna; Lia Francesconi, University of Bologna; Gianni Pasolini, University of Bologna; Enrico Testi, University of Bologna; Albino Carbognani, Istituto Nazionale di Astrofisica – OAS; Orr Cohen, European Space Agency (ESA); Louis Walpot, European Space Agency (ESA)
https://doi.org/10.64861/QXVO8686
Keywords: Space Situational Awareness, Conjunction Assessment, Collision Avoidance, Onboard Autonomy, Distributed Traffic Management, Late Maneuver Strategy, Orbit Determination, Probability of Collision, Optical Tracking, Embedded Systems
Abstract:
The increasing density of active spacecraft and debris objects is placing growing pressure on current ground-centric conjunction assessment workflows. Although automated screening tools have improved responsiveness, decision-making remains largely ground-based, introducing latency, scalability constraints and coordination challenges in high-density orbital environments. Distributed and onboard autonomy is therefore emerging as a potential architectural evolution for space situational awareness (SSA) and space domain awareness (SDA) operations._x000D_
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This paper describes an autonomous onboard collision risk assessment and avoidance technology named OB-ASTRA. The project is funded under the ESA ARTES 4.0 Advanced Technology programme and is currently progressing toward TRL 5. OB-ASTRA is designed to execute conjunction detection, collision probability computation, maneuver planning, and coordination functions directly onboard the spacecraft, while remaining interoperable with existing SSA infrastructures, thanks to a ground companion application and a web server._x000D_
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OB-ASTRA is implemented as a dedicated satellite subsystem providing navigation services and autonomous collision avoidance to the host spacecraft. Navigation relies on a state-of-the-art GNSS receiver for precise orbit determination and a star tracker for attitude knowledge; these sensors also supply the measurements required for conjunction assessment and refinement. To support constellation operations, the unit integrates a LoRa transceiver for inter-satellite link, allowing exchange of GNSS-derived ephemerides between spacecraft involved in a conjunction and thereby improving risk assessment fidelity._x000D_
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The onboard processing architecture is organized into five main layers: (1) orbit determination, (2) conjunction screening, (3) collision probability estimation, (4) maneuver generation and optimization, and (5) coordination and supervision. An onboard ephemeris catalogue enables autonomous screening of potential conjunctions, while external Conjunction Data Messages (CDMs) may be ingested to enhance robustness and compatibility with established operational practices and existing space traffic management infrastructures._x000D_
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A central element of the operational concept is the progressive refinement strategy. As the Time of Closest Approach (TCA) decreases, state uncertainty is reduced through updated onboard orbit determination and, when feasible, refinement of the secondary object state. Refinement is enabled either through cooperative ephemeris exchange via a LoRa-based inter-satellite transceiver or through autonomous optical tracking of the secondary using an onboard camera unit integrated within the system. The combination of GNSS-based self-navigation, optical line-of-sight measurements, and inter-satellite data exchange supports late-stage improvement of conjunction geometry knowledge. This “late-commitment” approach enables just-in-time maneuver decisions, reducing overall Δv expenditure._x000D_
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The Concept of Operations (CONOPS) follows an action timeline centered on OB-ASTRA’s default basal function as an onboard orbit determination and periodic conjunction screening system. Collision assessment is triggered either by an onboard screening alert or by an external ground-issued warning (e.g., CDM request). Each alert undergoes a preliminary risk assessment to determine whether it qualifies as a High-Interest Event. If the conjunction is deemed unsafe, the system autonomously generates a maneuver draft consistent with mission objectives and operational constraints. This draft is completed ahead of a first decision deadline (e.g., ~2 days before TCA) to enable coordination. During the coordination phase, OB-ASTRA applies established “rules of the road” (e.g., precedence logic) and exchanges intent information with counterpart spacecraft or operators when applicable. A second deadline (e.g., ~12 hours before TCA) defines the point by which agreement must be reached; failing this, a predefined default maneuver is adopted to ensure safety, transparency and predictability, at the expense of optimality. In the final approach phase, the system performs just-in-time risk monitoring using targeted measurement updates to refine the collision risk index. The maneuver is executed only if risk remains above the threshold up to the maneuver execution time; otherwise, it is aborted. Post-maneuver operations include orbit restoration and dissemination of updated state and maneuver information._x000D_
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OB-ASTRA development has passed the Critical Design Review (CDR), held in March 2026, with the Test Readiness Review (TRR) planned for Q3 2026. In the next phase, the engineering model will be integrated with a high-fidelity sensors-in-the-loop testbed emulating representative conjunction scenarios. The testbed incorporates the three core sensors – GNSS receiver, optical camera, and inter-satellite link (ISL) transceiver – stimulated respectively by a GNSS signal emulator, an optical dynamic scene simulator, and an ISL signal emulator to reproduce realistic operational conditions. The validation campaign will assess conjunction detection effectiveness, collision probability convergence behavior, maneuver optimality, and the coherence between early screening and late-stage refinement decisions. Preliminary results from the engineering model test activities are expected following TRR and will be reported as available. The full paper will present the OB-ASTRA architecture, the engineering model and testbed implementations, the operational algorithms, and initial validation outcomes, with discussion of implications for future hybrid onboard-ground collision avoidance concepts._x000D_
Date of Conference: September 15-18, 2026
Track: SDA Systems & Instrumentation