Formation flying of the ANSER cluster using differential-drag control

Formation flying control (FFC) is an essential building block in any mission involving the joint operation of several satellites in orbit. An example is the fractionated hyperspectral imager distributed among the three 3U cubeSats (Leader-S, Follower 1 and Follower 2) of the Advanced Nanosatellite Systems for Earth observation Research (ANSER ® ) mission developed by the National Institute of Aerospace Technology (INTA) in Spain. One of the goals of the ANSER mission is to control the relative position between the cubeSats of the cluster by using the aerodynamic drag and lift available in low-Earth orbits. The satellites include two elements to perform this kind of control: a set of deployable flaps (three hinged flaps per side), achieving a theoretical ratio of 21 between the maximum and minimum cross-sectional areas; and a complete attitude determination and control system (ADCS), that allows for an accurate control of the three Euler angles to perform FFC. This work focuses on the use of differential drag to control the along-track separation between the two Followers that were launched together in October 2023. The original Leader was also launched with the Followers but it was not correctly deployed in orbit due to a failure. Therefore, a new Leader-S was developed and launched in January 2025. The FFC strategy and the in-orbit formation-flying results for more than one year of mission only with the Followers are reported and discussed. During the commissioning phase (uncontrolled formation flying), the two Followers naturally drifted apart a distance of about 12,000 km in the along-track direction ( ∼ 1 / 4 of an orbit), which was then reduced to about 100 km by using differential-drag control and kept roughly constant by regularly commanding station-keeping maneuvers. As a by-product of this long FFC campaign, a simplistic aerodynamic model (with 2 discrete states: maximum and minimum drag) of the two Followers, ready to be introduced in a high-fidelity orbit propagator, was experimentally identified. Moreover, a comparison between the experimental and model-based orbital decay rates and time derivative of the distance between satellites revealed significant differences. This highlights that in-orbit past data is more appropriate than low-fidelity models for the planning of differential-drag maneuvers. This experimental work has advanced the field of FFC in cubeSats demonstrating that large ratios between the maximum and minimum cross-sectional areas, fine ADCS and the use of past in-orbit kinematic data enable highly responsive, adjustable and precise FFC when using differential aerodynamic forces. • Formation flying of the ANSER cluster of CubeSats for joint operation of the payloads. • Aerodynamic drag control through deployable flaps and three-axis attitude control. • Differential-drag methodology based on past in-orbit kinematic data. • In-orbit results of the formation establishment from 10,000 km to 100 km of distance. • In-orbit results of the formation keeping at inter-satellite distances of 100–200 km.