In the future, instead of relying on large, costly individual satellites, teams of smaller satellites, known as a "swarm," will collaborate to enhance accuracy, agility, and autonomy. Researchers at Stanford University's Space Rendezvous Lab have recently achieved a significant milestone by completing the first in-orbit test of a prototype system that navigates a swarm of satellites using only visual information shared through a wireless network.

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

" The test, known as the Starling Formation-Flying Optical Experiment (StarFOX), successfully navigated four small satellites working together using visual data from onboard cameras to calculate their orbits. The findings from the initial StarFOX test were presented at the Small Satellite Conference in Logan, Utah. D'Amico highlighted the decade-long challenge his team faced. "Our team has been advocating for distributed space systems since the lab's inception. Now it has become mainstream. NASA, the Department of Defense, and the U.S. Space Force have all recognized the value of multiple assets working in coordination to achieve objectives that would be difficult or impossible for a single spacecraft," he said. "Advantages include improved accuracy, coverage, flexibility, robustness, and potentially new objectives not yet imagined." Navigating the swarm autonomously presents significant technological challenges. Current systems rely on the Global Navigation Satellite System (GNSS), which requires frequent contact with terrestrial systems. Beyond Earth's orbit, the Deep Space Network is relatively slow and not easily scalable for future missions. Additionally, neither system can help satellites avoid "non-cooperative objects" like space debris.

The swarm requires a self-contained navigation system that offers high autonomy and robustness, D'Amico explained. Such systems are made more attractive by the minimal technical requirements and financial costs of today's miniaturized cameras and other hardware. The cameras used in the StarFOX test are proven, relatively inexpensive 2D cameras called 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 combines visual measurements from single cameras mounted on each satellite in the swarm. Similar to a mariner navigating the high seas with a sextant, the field of known stars in the background is used as a reference to extract bearing angles to the swarming satellites. These angles are then processed onboard through accurate physics-based force models to estimate the position and velocity of the satellites with respect to the orbited planet, whether Earth, the moon, Mars, or other planetary objects. StarFOX employs the Space Rendezvous Lab's angles-only Absolute and Relative Trajectory Measurement System (ARTMS), which integrates three new space robotics algorithms. An Image Processing algorithm detects and tracks multiple targets in images and computes target-bearing angles—the angles at which objects, including space debris, are moving toward or away from each other. The Batch Orbit Determination algorithm estimates each satellite's coarse orbit from these angles. Finally, the Sequential Orbit Determination algorithm refines swarm trajectories with new images over time, potentially feeding autonomous guidance, control, and collision avoidance algorithms onboard.