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This paper investigates a formation control technique for low-Earth-orbit nanosatellites based on the differential aerodynamic drag and lift. An innovative method of using only the yaw angle deviat...

This paper investigates an attitude control method for the CubeSat using a moving mass actuator to solve the problem of the strong aerodynamic disturbance in low Earth orbit. The rotational and translational equations are derived for the CubeSat with three moving masses, and their dynamic effects are analyzed. A magnetorquer is used to prevent the underactuation of the attitude control system. The movement of moving masses is slowed down by using a discrete double-loop Proportion Integral Differential control method, thereby reducing the fast time-varying additional disturbance. A nonlinear observer is used for the precise estimation of the slow time-varying disturbance. Notably, the ideal attitude control torque is allocated to two actuators by using the proposed control allocation algorithm. Numerical simulation indicates that the attitude convergence accuracy is up to ±0.1° despite the uncertain dynamics, unknown disturbances, and dynamic effects. The results verify the feasibility of the proposed control method.

This paper addressed a strongly nonlinear problem caused by status mutation in CubeSat attitude estimation system. The multiple fading factors are employed to make different state channels have separate adjustment ability, which enhances the tracking performance for status mutation. The second-order difference transformation is adopted to improve the approximation accuracy of state posterior mean and covariance. Therefore, a multiple fading second-order central difference Kalman filter (MFSCDKF) is formed for CubeSat attitude estimation system in the presence of status mutation. Compared with the single fading factor first-order central difference Kalman filter, the proposed one can improve tracking performance and estimation accuracy simultaneously. Simulation results based on real telemetry data from the on-orbit CubeSat NJUST-1 verify the effectiveness of the proposed MFSCDKF.

Any vehicle propelled by solid rocket motors (SRMs) must include an attitude control system capable of dealing with the torque generated by thrust misalignment. In order to expand the application of SRMs on CubeSats, an attitude control system utilizing moving mass actuators is discussed. The present research develops an eight-degree-of-freedom simulation model of a 2U CubeSat with two moving mass actuators. That model also considers the influence of propellant combustion processes. By analyzing the model disturbance source and systematic coupling, the key layout parameters are designed and a simplified control model is proposed. The controller is derived based on a combination of backstepping and sliding mode techniques. An orbit maneuver from 300 km circular orbit to 300 and 500 km elliptical orbit using this attitude control system is verified.

A finite-time controller based on optimal path planning was developed for the agile CubeSat attitude maneuver. This achieved a stable attitude large-angle maneuver just by using momentum wheels and a magnetorquer, which is unlike other larger satellites that need a control moment gyroscope or thruster. The proposed optimal path planning algorithm was based on a quintic polynomial with three path segments in order to decrease angular velocity in the attitude maneuver process as far as possible to improve system stability under the condition of satisfying CubeSat's limited control capability, and there is also an improved differential evolution (DE) algorithm for the optimization of path planning, in which the variation and crossover coefficients all obey the beta distribution. The finite-time controller was based on the nonsingular terminal sliding mode, and an improved PD-type sigmoid function was proposed to replace the sign function. Finally, the physical simulation platform is built by using the NJUST-2 as an object, and the results revealed that the finite-time controller based on the Beta DE path planning can decrease angular velocity in the attitude maneuver process as far as possible to improve system stability under the condition of satisfying the CubeSat's limited control capability and then achieves a stable attitude large-angle maneuver just by using momentum wheels and a magnetorquer.

VELOX-II that has been launched on 16th December 2015 is the second CubeSat built by the Satellite Research Centre (SaRC) of Nanyang Technological University (NTU). However, according to the operation data of VELOX-II in orbit, it can be observed that there is a large steady state error and the error becomes the most crucial factor for the pointing error. In this paper, Model Predictive Control (MPC) with integration was proposed in order to offset the steady state error. At first, the MPC with integration was based on a state space model with an incremental input aiming to add an integral item. Secondly, a quadratic cost function was designed to derive the control law of MPC with integration. Finally, a series of analysis and simulations were implemented for VELOX-II’s MPC, PD, Sliding Mode Control (SMC) and MPC with integration, respectively. The theory analysis and simulation results revealed that the steady state error is from environmental and actuator’s disturbances, and it can not be offset by VELOX-II’s MPC, PD and SMC controllers, but can be eliminated by the MPC controller with integration; comparing with PID, the MPC has much improved attitude tracking performance, especially in the aspect of time delay; different with SMC, the MPC with integration is easy to be come true on the 8051 chip because of the smaller amount of computations and codes; VELOX-II using the MPC controller with integration can achieve a higher pointing accuracy.

This paper proposes high-accuracy and reliable attitude measurement methods exclusive for CubeSat with restrictions of low cost, limited space, and low power consumption. The attitude measurement unit is equipped with Commercial Off-The-Shelf (COTS) components including Micro-Electro-Mechanical System (MEMS) gyro and two simultaneously operating star trackers (STR) to enhance the measurement accuracy. The Multiplicative Extended Kalman Filter (MEKF) is used to estimate the attitude of CubeSat, and four kinds of attitude estimation layouts are put forward according to the idea of weighted average of two quaternions from two STR and different architectures of information fusion. Using the proposed methods, the attitude measurement unit can continuously provide accurate and reliable attitude knowledge for attitude control unit when the CubeSat is running in orbit. Numerical simulation is performed to verify the effectiveness of the proposed methods, and it offers a reference for CubeSat developers from the perspective of engineering application.

In this paper, an improved strong tracking unscented Kalman filter based on multiplicative modified Rodrigues parameters (MRPs) is proposed for satellite attitude estimation. The multiplicative MRPs are superior to additive MRPs in terms of attitude representation, especially when attitude angles are large. By minimizing the loss function in Wahba’s problem, a novel method of weighted average for MRPs is derived to replace the simple procedure. The generation of Sigma points, update of state variables, and calculation of covariance matrices are all different from the existing literature to maintain the multiplicative property of MRPs. Simulation results by raw telemetry data from the on-orbit CubeSat Enlai-1 demonstrate the excellent performance of the proposed filter under large attitude angles.

The orbital manoeuvre of a CubeSat is an essential ability in the process of constellation deployment and other missions. Therefore, an attitude control method based on piece-wise affine (PWA) MPC was proposed for CubeSats during orbital manoeuvres. Then, a solid thruster with high thrust that was previously applied only to large and traditional satellites was innovatively applied to CubeSats for orbital manoeuvres. However, the solid thruster with high thrust has a large eccentric torque from the thrust deviation, so it will cause rapid attitude rolling and result in orbital manoeuvre mission failure. Hence, PWA-MPC strategy with the disturbance observer was designed, which can solve the eccentric torque problem and achieve high accuracy attitude control by a hybrid actuator composed of momentum wheels and a cold gas thruster. In addition, during the orbital manoeuvre process, propellant consumption results in a continuous change in the moment of inertia of a CubeSat. In order to offset this change, the guide rail of the cold gas thruster was innovatively designed to move the cold gas thruster in the working process of the solid thruster. Finally, real-time hardware-in-loop simulation results show that the PWA-MPC strategy has higher attitude stability than those of other MPC strategies in the orbital manoeuvre process of a CubeSat and deserves to be applied.

The rapid attitude manoeuvre is a control challenge for a CubeSat in low magnetic environments, which include lunar orbits, geosynchronous (GEO) orbits or other higher Earth orbits. Therefore, a rapid attitude manoeuvre control method based on the segmented sliding mode was proposed for CubeSat in a low magnetic environment. The segmented sliding mode control algorithm was applied to the CubeSat attitude manoeuvre control, where the segmented sliding mode surface was designed to guarantee fast convergence to the origin point of the whole attitude manoeuvre process, and the hyperbolic tangent function was used in the reaching law of the controller to alleviate reaction saturation of the wheels. In addition, innovative control strategies were proposed for the hybrid actuators of the reaction wheels and microthrusters, and a guide rail of the microthruster was innovatively designed to move the microthruster to offset the change in moment of inertia caused by propellant consumption. Finally, hardware-in-loop simulation results reveal that our method has a stronger anti-saturation ability and faster manoeuvre speed than the traditional control method, and the operational life cycle of the thrusters can be accordingly increased.