Deorbiting Algorithms Development for CubeSats using Propulsion and Sails

CubeSats are becoming increasingly popular within the scientific and commercial community, as they provide relatively cheap and quick access to space. However, as their launching rates increase rapidly, the concern that they may have a negative impact in the space debris problem also increases. This calls for the development of novel deorbiting technologies for CubeSats. In a response to this need, this thesis presents two new attitude controllers and deorbiting algorithms, which enable ionic thrusters, as well as drag sails, in order to accelerate CubeSat orbital disposal. These algorithms are designed with nanosatellites capabilities in mind, requiring minimum attitude determination and control. Their efficacy is demonstrated through numerical models in all cases.

In the first approach, a geomagnetic field tracker controller is presented. This controller aligns the thrusting carrying axis of the satellite with the local magnetic field vector. The only sensors and actuators required are magnetometers and magnetorquers respectively. A suitable deorbiting algorithm is also presented, which is activated once the CubeSat is tracking the geomagnetic vector. This algorithm determines the portions of the orbit in which thrust must be applied, and it only requires a model of Earth's magnetic field. This approach is simulated with ionic thrusters, obtaining deorbiting rates between 0.35 km/day and 50 km/day, depending on the type of engine used. Proof of stability is provided through Floquet theory, while robustness analysis is executed through Monte Carlo simulations. This approach has advantages such as minimum sensing and actuating requirements, and it doesn't require movable parts nor deployables, minimizing the probability of failures in orbit.

In the second approach, a gyroless spin-stabilization controller is proposed. This algorithm fixes the thrusting carrying axis of the CubeSat in the inertial frame. Just as the first approach, this controller only requires magnetometers and magnetorquers. Once the satellite is stabilized, an orbit sampling algorithm is introduced. This algorithm is able to determine the portions of the orbit where to apply thrust, using only Global Positioning System inputs. This approach is simulated with electrospray thrusters, achieving deorbiting rates in the order of 45 km/day. Stability analysis is provided through Lyapunov theory, while Monte Carlo simulations are used to prove the robustness of the algorithm.

The attitude stabilization phases of both approaches are very flexible, in that they can work with a variety of thrusters, as well as non propulsive technologies. Dragsails are often proposed as means for deorbiting CubeSats, however, there is a gap in the literature when it comes to their stabilization in orbit. Therefore, the efficacy of these stabilization approaches when used in conjunction with drag sails is analysed. In the case of the geomagnetic field tracking algorithm, deorbiting rates in the order of 19 km/day are attained. In the case of the gyroless spin-stabilization algorithm, deorbiting times of up to 12.5 km/day are achieved.

These algorithms provide convenient means for CubeSat deorbiting, contributing to space debris mitigation efforts. They require minimum hardware and software capabilities, and because of this the probability of failures is low, and they provide excellent deorbiting rates.