"It's a milestone paper and the culmination of 11 years of effort by my lab, which was founded with this goal of surpassing the current state of the art and practice in distributed autonomy in space," said Simone D'Amico, associate professor of aeronautics and astronautics and senior author of the study. "Starling is the first demonstration ever made of an autonomous swarm of satellites."

Named the Starling Formation-Flying Optical Experiment, or StarFOX, the test successfully guided four small satellites in tandem, utilizing only visual information from onboard cameras to determine their orbits. The team presented their initial findings from the StarFOX test at the Small Satellite Conference in Logan, Utah, a gathering of experts in swarm satellite technology.

D'Amico highlighted the longstanding challenge: "Our team has been advocating for distributed space systems since the lab's inception. Now it has become mainstream. NASA, the Department of Defense, the U.S. Space Force - all have understood the value of multiple assets in coordination to accomplish objectives which would otherwise be impossible or very difficult to achieve by a single spacecraft," he said. "Advantages include improved accuracy, coverage, flexibility, robustness, and potentially new objectives not yet imagined."

Navigating the swarm robustly presents significant technological hurdles. Existing systems depend on the Global Navigation Satellite System (GNSS), necessitating frequent communication with ground-based systems. Beyond Earth's orbit, the Deep Space Network offers support but is relatively slow and not easily scalable for future missions. Additionally, neither system aids satellites in avoiding "non-cooperative objects" like space debris that could incapacitate a satellite.

D'Amico emphasized the need for a self-contained navigation system for the swarm, allowing for high autonomy and robustness. Such systems are further appealing due to the minimal technical requirements and reduced costs of today's miniaturized cameras and other hardware. The cameras employed in the StarFOX test are proven, cost-effective 2D cameras known as star-trackers, commonly found on satellites today.

"At its core, angles-only navigation requires no additional hardware even when used on small and inexpensive spacecraft," D'Amico said. "And exchanging visual information between swarm members provides a new distributed optical navigation capability."

StarFOX integrates visual measurements from single cameras mounted on each satellite in the swarm. Much like ancient mariners using a sextant to navigate the seas, the known stars in the background serve as a reference to determine bearing angles to the swarming satellites. These angles are processed onboard using precise physics-based force models to estimate the satellites' positions and velocities relative to the orbited planet - Earth in this instance, though the system is adaptable to the moon, Mars, or other planetary bodies.

Employing the Space Rendezvous Lab's angles-only Absolute and Relative Trajectory Measurement System (ARTMS), StarFOX integrates three innovative space robotics algorithms. An Image Processing algorithm detects and tracks multiple targets in images, computing target-bearing angles - the directions in which objects, including space debris, are moving relative to each other. The Batch Orbit Determination algorithm then estimates each satellite's general orbit from these angles. Finally, the Sequential Orbit Determination algorithm refines swarm trajectories by processing new images over time, potentially supporting autonomous guidance, control, and collision avoidance algorithms onboard.

Data is exchanged over an inter-satellite communication link, forming a wireless network. This collective information enables the calculation of precise absolute and relative positions and velocities without relying on GNSS. Under challenging conditions, using just a single observer satellite, StarFOX achieved a relative position accuracy within 0.5% of the satellites' distance. With multiple observers, error rates decreased to just 0.1%.

The promising results from the Starling test have led NASA to extend the project, now termed StarFOX+, through 2025. This extension aims to further explore these capabilities and pave the way for future advancements in space situational awareness and positioning technologies.

Research Report:Starling Formation-Flying Optical Experiment: Initial Operations and Flight Results

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