Attitude Control Subsystem Design of the Stable and Highly Accurate Pointing Earth-imager

Abstract

As technology improves, increasingly higher resolution payload can be achieved using Cubesats for Earth observation. The diffraction limit prevents the resolution to up to a few meters for these missions and are confined to Very Low Earth Orbits (VLEO). At these altitudes, strong disturbances act on the system, limiting its lifetime and the pointing capabilities of CubeSats. As a solution, the Department of Space Engineering at the Delft University of Technology
has proposed a 6 unit CubeSat named the Stable and Highly Accurate Pointing Earth-imager (SHAPE) orbiting at a Sun synchronous VLEO which uses a momentum wheel to passively stabilize the system against the external environment within a competitive cost of less than 500 000e. By utilizing the dual-spin stabilization concept, composed of a stable platform and a spinning rotor, it is expected to perform pointing missions of less than 1 degree.

In this thesis, the SHAPE concept has been revisited and further developed based on the work of Kuiper and Dolkens to conduct whether these types of missions are feasible within the aspect of the Attitude Determination and Control Subsystem (ADCS). This thesis covers the base of this subsystem approached from a top-down methodology; designed from the final nominal mission mode to the detumbling mode on a system level.

The ADCS design will consist of a momentum wheel which has been determined to have an angular momentum of 1 Nms. This value is based on a prediction of the worst-case atmospheric density of the next solar cycle. The design
point, at which the momentum wheel has been sized, has been taken at 90% of SHAPE’s lifetime after several design iterations. Hereby, the last 10% of the mission has been partially forfeited with degraded performances due to the
exponential increase in disturbances acting on the spacecraft at lower altitudes. As therefore, the mass and size of the momentum wheel has been reduced with 41% and 20%, respectively. To re-align the angular momentum vector within
the 1 degree pointing requirement, a set of magnetorquers with a dipole moment of 0.5 Am2 has been chosen due to their low power consumption, mass, cost, and high reliability while capable of producing sufficient torque. Also, a
damper is to be integrated as it provides the system asymptotic reduction of the transverse momenta, thus increasing the image quality without expenditure of additional power.

To reach the nominal mission observation state, several momentum wheel spinup strategies have been investigated. Based on a trade-off between three spinup concepts, it was concluded that the major axis spinup is most suited. This
type is initiated after the spacecraft as a whole has attained an angular momentum equivalent to that of the desired end value of the momentum wheel. Then, a constant rotor torque is applied, providing a momentum transfer from the platform to the rotor. The disadvantage of this spinup procedure is that the system’s solar panels are aligned parallel to the orbital plane, meaning that power cannot be generated and batteries are required during the spinup. Despite this, it was found that after completing the spinup, the transverse angular momenta was minimized to marginal values in contrast to the other spins. The inclusion of the passive damper during the major axis spin further improves the ability of reducing the transverse momentum as the damper’s dissipative energy property adds an asymptotic attraction at the point of lowest energy, located at the spin axis near the end of the spinup.

The all-spun state is achieved using a set of thrusters. This choice was taken as the magnetorquers was found not to deliver sufficient torque. From the detumbling analysis, it was concluded that the magnetorquers are able to reduce
the tumbling rates with magnitudes of up to 35 deg/s to mean motion values in less than an orbit using a static gain B-dot controller.

Imperfections in the momentum wheel can cause static and dynamic imbalance, imparting internal disturbances to the system which affects the image quality detrimentally. Therefore, isolators are to be integrated within the momentum
wheel suspension subsystem. If these disturbances can be negated using isolators, it can be expected that the pointing error will stay within one degree and attitude stability can be achieved at least until the design altitude of 280 km.
However, the analysis and design of the isolators have yet to be done and thus the attainability of the pointing and image quality requirements are still inconclusive.