The reactive collision avoidance (RCA) algorithm allows a spacecraft to find a fuel-optimal trajectory for avoiding an arbitrary number of colliding spacecraft in real time while accounting for acceleration limits. In addition to spacecraft, the technology can be used for vehicles that can accelerate in any direction, such as helicopters and submersibles.
In contrast to existing, passive algorithms that simultaneously design trajectories for a cluster of vehicles working to achieve a common goal, RCA is implemented onboard spacecraft only when an imminent collision is detected, and then plans a collision avoidance maneuver for only that host vehicle, thus preventing a collision in an off-nominal situation for which passive algorithms cannot. An example scenario for such a situation might be when a spacecraft in the cluster is approaching another one, but enters safe mode and begins to drift. Functionally, the RCA detects colliding spacecraft, plans an evasion trajectory by solving the Evasion Trajectory Problem (ETP), and then recovers after the collision is avoided. A direct optimization approach was used to develop the algorithm so it can run in real time.
In this innovation, a parameterized class of avoidance trajectories is specified, and then the optimal trajectory is found by searching over the parameters. The class of trajectories is selected as “bang-off-bang” as motivated by optimal control theory. That is, an avoiding spacecraft first applies full acceleration in a constant direction, then coasts, and finally applies full acceleration to stop.
The parameter optimization problem can be solved offline and stored as a look-up table of values. Using a lookup table allows the algorithm to run in real time. Given a colliding spacecraft, the properties of the collision geometry serve as indices of the look-up table that gives the optimal trajectory. For multiple colliding spacecraft, the set of trajectories that avoid all spacecraft is rapidly searched on-line.
The optimal avoidance trajectory is implemented as a receding-horizon model predictive control law. Therefore, at each time step, the optimal avoidance trajectory is found and the first time step of its acceleration is applied. At the next time step of the control computer, the problem is re-solved and the new first time step is again applied. This continual updating allows the RCA algorithm to adapt to a colliding spacecraft that is making erratic course changes.
This work was done by Daniel Scharf, Behçet Açikmese, Scott Ploen, and Fred Hadaegh of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Information Sciences category.
The software used in this innovation is available for commercial licensing. Please contact Daniel Broderick of the California Institute of Technology at
This Brief includes a Technical Support Package (TSP).
Reactive Collision Avoidance Algorithm
(reference NPO-44771) is currently available for download from the TSP library.
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Overview
The document discusses NASA's Reactive Collision Avoidance (RCA) Algorithm, developed to address the challenges of multi-spacecraft missions where spacecraft operate in close proximity, sometimes as close as 1-5 meters. Such missions, including the Terrestrial Planet Finder (TPF-I) and planetary sample return missions, necessitate effective collision avoidance strategies due to the potential for close encounters between spacecraft.
Prior to the RCA algorithm, there was no general method for determining minimum-fuel trajectories to avoid collisions while considering realistic constraints, such as limited thrust capabilities and the need for real-time trajectory calculations. The RCA algorithm is notable for its ability to handle any number of colliding spacecraft, making it a versatile solution for various mission scenarios.
The algorithm employs a direct optimization approach, where a parameterized class of avoidance trajectories is defined. These trajectories follow a "bang-off-bang" control strategy, which involves the spacecraft first applying full acceleration in a constant direction, then coasting, and finally applying full acceleration to stop. This method is grounded in optimal control theory.
To ensure real-time performance, the algorithm utilizes a look-up table of pre-computed optimal trajectories based on collision geometry. When a potential collision is detected, the properties of the collision serve as indices to quickly retrieve the optimal avoidance trajectory from the table. For scenarios involving multiple colliding spacecraft, the algorithm efficiently searches for trajectories that can avoid all threats.
The implementation of the optimal avoidance trajectory is managed through a receding-horizon model predictive control law. This means that at each time step, the algorithm recalculates the optimal trajectory and applies the first step of acceleration, continuously updating the trajectory in response to any erratic course changes by the colliding spacecraft.
The RCA algorithm is groundbreaking as it is the first of its kind to provide a real-time, fuel-optimal solution for collision avoidance among multiple spacecraft, while also accounting for acceleration limits. It can determine whether a feasible trajectory exists under the algorithm's assumptions, although it acknowledges that certain situations may render collision avoidance impossible due to limited acceleration capabilities.
This document serves as a technical support package for the RCA algorithm, highlighting its significance in enhancing the safety and efficiency of future space missions.
