Attitude simulation software to support Brazilian space missions

Abstract

Cost reduction on system tests with hardware in the loop during qualification phase of the onboard attitude control software imply in increasingly simulation realism. Attitude simulation software not only has to mimic the dynamical behavior of the spacecraft, but also shall be easily configurable, to adapt to different orbit, attitude pointing requirements, environment disturbances, sensors, actuators, and to run in real-time among other minor requirements. As the requirements for accuracy and stability for attitude pointing has increased in many modern missions, so do the complexity of the simulation algorithms and necessity for dynamical realism. Rigid body dynamics can be considered excessively simple to model a satellite with several reaction wheels, non-rigid structures, unbalanced solar arrays, fuel and liquid sloshing, not to mention crew motion and vehicle docking. This work presents the efforts being carried out to fulfill the requirements of the Brazilian space missions, nominally Multi Mission Platform, with a simulation environment capable to accomplish from early definition and mission analysis phase up to acceptance tests of the attitude control software. Development is being conducted in order to achieve a high degree of realism and range of applicability of the simulation software, compatible with the expected cost reduction of the simulation hardware (ordinary PCs). The kinematics and dynamic models to support AOCS (Attitude and Orbit Control Subsystem) simulation and testing with or without hardware-in-the-loop for Brazilian space missions are presented. Normally the dynamic equations are expressed in angular momentum components, due to the high complexity of these equations when derived in terms of the angular velocity. This, of course, is particularly important in satellites composed by several articulated rigid bodies as solar panels, robotic arms, space booms, deployable antennas, etc. Nevertheless, the inverse of the inertia matrix shall be calculated on the run, since it varies in time. This can be somewhat slow, mainly considering the high degree of freedom in dynamics of satellites with articulated panels and reaction wheels. In this case a more complex but fast set of equations in terms of the angular velocity is the best choice. This work describes the attitude dynamic equations expressed in angular velocities for satellites with appendages (solar panels, telescopes, directional antennas, etc.) and reaction wheels. They can be easily extended, however, to include nutation dampers and robotic arms. As usual, this formulation requires the knowledge of the acting torque in the connection joint, which is not always understood, modeled or known. So the dynamic model presented in this work uses the angular acceleration instead of the torque at the joint, which simplifies and reduces the order of the differential equations. Simulation results are presented, emphasizing the difference between the behavior of rigid body and articulated rigid bodies.