Melrose Brown, UNSW Canberra Space; Edwin G. W. Peters, University of New South Wales Canberra; Matthew Dilkes, UNSW Canberra Space; Ryan Jefferson, UNSW Canberra Space; Andrew Lambert, UNSW Canberra Space; Rabbia Saleem, UNSW Canberra Space; Russell Boyce, ; Ed Kruzins, UNSW Canberra Space; Timothy Bateman, UNSW Canberra Space
Keywords: space-based assets, astrodynamics, formation flying, space weather
Abstract:
Close proximity satellite formation flying in Low Earth Orbit (LEO) offers the potential to disaggregate complex missions from large, single satellite platforms, to constellations of small satellites that can offer greater resilience and redundancy for the same cost. Close proximity formation flying is traditionally achieved through closed loop control of active onboard propulsion systems. One solution to avoid the complexity and risk of integrating onboard propulsion is to use differential aerodynamic drag to control the relative motion of the spacecraft without the need for an active propulsion system. The approach involves a drag plate on each satellite that is tilted with respect to the satellites’ velocity direction to change the drag force experienced by the spacecraft relative to one another, creating the ability to affect the along track spacing of the satellite formation
Real-world applications of differential aerodynamic drag typically use deployed solar panels as the drag plates. The spacecraft attitude determination and control system (ADCS) controls the orientation of the solar arrays relative to the velocity direction. An operational example of this approach is found in the Planet Flock constellation. To date, differential drag has been used by satellite operators for coarse control of the spacing between constellation members at large along track distances. Theoretical concepts for using differential aerodynamic drag control for close proximity operations have been reported in the literature, however no on-orbit demonstrations were performed prior to the Australian ‘M2’ 6U CubeSat pair in April 2022, owned and operated by the University of New South Wales (UNSW), Canberra. Using an open “human in the loop” control approach, UNSW Canberra Space successfully reduced the along track separation of M2-A and M2-B from 160km to hold a steady along track separation distance of 2km from 11th-14th April 2022.
The work presented here analyses different concepts for removing the time-intensive human-in-the-loop control approach and replace it with a simple, autonomous, closed-loop control system suitable for real-world close proximity station keeping on the M2 spacecraft. The paper reports on the results from an inter-satellite radio frequency (RF) link experiment conducted on the spacecraft in Q4 2022 and uses the results to inform the performance characteristics of a simulated intersatellite RF ranging capability using the M2 spacecraft UHF transceivers. The performance of the autonomous, on-orbit, closed-loop, control approach is compared with improvements implemented to the existing ground-station based control approach.
The work begins with an analysis of the autonomous on-orbit control system while operating under ideal conditions, where 100% of the orbit is available for differential drag manoeuvres, before exploring off-nominal conditions caused by the temporary loss of attitude control from system resets or pointing schedules to accommodate other mission objectives. Two modes of operation are investigated: a station keeping mode, with the sole aim to minimize the change in relative along track position; and an along track target mode, where the controller seeks to change the along track separation distance to meet a specified value. The effect of uncertainty in the RF-based range measurements for relative orbit control is investigated through simulations and the maximum range of the intersatellite RF ranging sensor estimated from the on-orbit experiments conducted in Q4. Recommendations for the use of the control methodology to support the M2 maritime optical and RF surveillance mission objective are provided.
Date of Conference: September 19-22, 2023
Track: Space-Based Assets