The Starling Formation-Flying Optical Experiment (StarFOX) is a groundbreaking project designed to advance autonomous navigation for small spacecraft. Developed by NASA in collaboration with Stanford University, StarFOX leverages angles-only navigation techniques to enhance the autonomy and operational efficiency of satellite swarms.
Study: Starling Formation-Flying Optical Experiment: Initial Operations and Flight Results. Image Credit: Alones/Shutterstock.com
Background
As satellite missions become more complex, there is an increasing need for advanced space navigation systems. Traditional methods often rely on Global Positioning System (GPS) signals, which may be unavailable in deep space or during certain mission phases.
The StarFOX experiment seeks to overcome these limitations with a vision-based navigation system that uses angles derived from images captured by the spacecraft's cameras. This approach enhances spacecraft autonomy and reduces reliance on external signals, making it a promising solution for future satellite constellations and formations.
Central to the StarFOX initiative is the Absolute and Relative Trajectory Measurement System (ARTMS). ARTMS employs advanced algorithms for image processing and orbit determination, allowing spacecraft to autonomously track and navigate relative to one another.
The system generates time-tagged bearing angle measurements for each target, enabling precise identification and tracking of space objects. Additionally, Multi-Hypothesis Tracking (MHT) techniques enhance the system’s capability to detect new targets and maintain accurate navigation in dynamic environments.
The Current Study
The StarFOX experiment utilized a swarm of small satellites equipped with the ARTMS. It was carried out in multiple phases: formation acquisition, in-train formation, and passive safety ellipse formation. Ground control initially provided target relative orbit estimates, which were then refined onboard using the Image Processing (IMP) module and State Observer Dynamics (SOD). The IMP module was crucial for processing images to generate inertial bearing angles for both known and potential targets.
The experiment aimed to evaluate ARTMS performance under diverse operational conditions. The spacecraft relied on a combination of onboard sensors, such as cameras and star trackers, to collect navigation data. However, the integration and testing phases encountered challenges, notably due to the lack of realistic image measurements. This issue led to software crashes during initial tests, underscoring the need for accurate sensor inputs and precise time synchronization for effective navigation.
To overcome these challenges, the team implemented in-flight software updates that allowed user-specified time offsets for star tracker data. This flexibility was vital for maintaining navigation accuracy and ensuring the experiment's success. Additionally, the team enhanced the system's robustness by incorporating adaptable software execution paths and contingency measures to handle varying operational conditions.
Results and Discussion
The initial flight results of the StarFOX experiment demonstrated the ARTMS’s effectiveness in meeting its performance goals. The system successfully tracked known targets and detected new ones, validating the angles-only navigation approach.
Data from the mission indicated that the executed maneuvers improved angles-only observability, helping the filter better distinguish target ranges. However, the small delta-v maneuvers performed did not significantly reduce state uncertainties, suggesting that further optimization will be needed for future missions.
A key takeaway from the StarFOX experiment was the critical importance of realistic pre-flight simulations and validation processes. The team found that generating realistic space images was crucial for effective software validation.
Despite efforts to account for non-ideal measurement conditions, in-flight challenges exceeded expectations. Issues such as the dimness of target spacecraft, unexpected sun intrusion into the field of view, and overly optimistic signal-to-noise ratios highlighted the need for software improvements to handle varying optical conditions.
The results also emphasized the importance of accurate time synchronization in autonomous systems. The discrepancy between star tracker time-tags and the onboard clock underscored the necessity for precise timing mechanisms. Exploring the ability to estimate differential clock offsets using inter-satellite angles emerged as a promising avenue for future missions.
Conclusion
In conclusion, the StarFOX experiment marks a significant advancement in autonomous navigation for small spacecraft. By effectively implementing angles-only navigation techniques through the ARTMS, the project has proven the feasibility of vision-based systems for managing satellite swarms.
The initial flight results show that the system can successfully track and navigate relative to multiple targets, setting the stage for future missions that demand greater autonomy and reduced dependence on external signals. The success of StarFOX not only underscores the potential of angles-only navigation but also opens the door to further innovations in autonomous spacecraft technology.
Journal Reference
Kruger J., Hwang Soon S., et al. (2024). Starling Formation-Flying Optical Experiment: Initial Operations and Flight Results. arXiv 2406.06748. https://arxiv.org/abs/2406.06748