The knowledge of forces and torques exerting on a spacecraft is of great interest for orbit and attitude control, and thus for dimensioning of tanks, momentum wheels, attitude control laws.
PERTURBATIONS software provides an accurate modelling of the main forces and torques exerting on-orbit: air drag, solar radiation pressure, earth radiation pressure, thermal imbalances, magnetic moment, and gravity gradient.
PERTURBATIONS has been developed within SYSTEMA framework. The first computation steps are common to most SYSTEMA applications:
Spacecraft geometry is modelled with simple shapes (rectangles, quadrangles, cylinders, spheres, etc.) hierarchically organised. For geometry definition, a 3D-graphic modeller is available.
3D-geometrical model is enhanced by the definition of physical properties, such as: surface front and back coatings with their associated thermo-optical properties (for radiation pressure), surface temperatures and interaction properties (air drag), embarked magnetic moment (magnetic moment), masses and inertias (gravity gradient).
Definition of spacecraft orbit and pointing requirements is carried out with MISSION module. A complete orbit generator is available in MISSION / ORBIT. The nominal pointing, attitude laws, and mobile parts (e.g. solar array) can be defined within the MISSION / POINTING module. Both modules are completed by means of visualisation of the spacecraft trajectory and attitude.
Then, the computation of some or all of forces and torques is carried out:
PERTURBATIONS input parameters allow to choose environmental models (e.g. solar activity, earth magnetic field model), and ray-tracing accuracy (number of rays to be fired).
Post-processing tool allows the user to visualise forces and torques, as well as cross sections, centre of pressure, etc. A decomposition on orbit harmonics has been added for AOCS purpose.
PERTURBATIONS was developed under an ESA contract, initially for ERS orbit restitution. The air drag module has been used on Gravity Probe B, but PERTURBATIONS has been mostly used for solar pressure analyses (Horizon, Ressat, Hot Bird 7, and all Eurostar 3000 spacecrafts). The determination of solar pressure forces and torques on-orbit is part of mission analysis and AOCS process.
Comparisons with in-flight data have been carried out (ST-1 for solar pressure, SPOT 1) showing good correlation between simulated and actual forces and torques.