A NASA formation-flying experiment, StarFOX, showcases the potential of autonomous navigation for satellite swarms. This experiment involves four cubesats, known collectively as Starling, which calculate their orbits by combining visual images from star trackers with robotics algorithms.
"Such a visual-navigation system on a swarm of satellite can be used to navigate around Earth," said Simone D’Amico, Stanford University associate professor of aeronautics and astronautics and founding director of Stanford’s Space Rendezvous Lab, to SpaceNews. "Since we don’t use GPS, it can be used to fly around the moon or around Mars with an increased level of autonomy."
Mission operators communicate with Starling, launched on a Rocket Lab Electron in July 2023, as a single entity. StarFOX is one of four experiments assessing communications, navigation and autonomy technologies for future swarms. The success of Starling has led NASA to extend the mission until December 2025.
Previous vision-based navigation tests involved one observer satellite and one target, with the observer knowing the target’s initial location. StarFOX has demonstrated that four satellites can navigate autonomously “without a priori information and without maneuvers to improve navigation accuracy,” D’Amico explained.
The key to StarFOX's success lies in a set of algorithms called angles-only Absolute and Relative Trajectory Measurement System (ARTMS). ARTMS combines image-processing and initial orbit determination algorithms with an algorithm that refines swarm state estimates over time.
"We have learned what accuracies are achievable using typical star trackers used on smallsats," D’Amico shared. “And we have learned how we can improve that performance, for example, by exchanging these visual measurements that are taken by multiple observers."
Starling satellites exchange data via inter-satellite links. “If multiple observers see a common target in the field of view, exchanging these measurements and fusing them into our algorithms can determine more accurately and faster the orbit of that object,” D’Amico said. He added that the data exchange also enhances each satellite's knowledge of its own orbit.
StarFOX also highlighted the significance of rigorous testing. “We learned the importance of complete, hardware-in-the-loop testing done on the ground before flight,” D’Amico stated. "In order to make it within the constraints of budget and time, we did individual unit tests and sacrificed some of the integrated tests that would have given us the full confidence on the execution of the experiments."
Early in the Starling mission, flight software crashed. A NASA Ames Research Center software engineer addressed the issue by allocating additional memory, using an engineering model on the ground.