About Control Systems

What are Control Systems?

The Control Systems discipline encompasses satellite attitude control and fine pointing, formation flying and orbital rendezvous, landing on asteroids and planetary bodies and reentry through Earth's atmosphere.

The general scope of a satellite's trajectory is set by the launcher that hauls it skyward – selected by orbital dynamics experts long in advance of the satellite being built – after which smaller thrusters manoeuvre it into its chosen orbit. Then begins the ongoing task of controlling the spacecraft's pointing direction – known as its attitude – as it proceeds along its orbital path.

The problem is that satellites have their attitude perturbed in various ways, whether by airdrag from the outermost layers of the atmosphere or Earth's gravitational influence. A satellite's own contents can set up undesirable vibrations – liquid 'sloshing' in a fuel tank is one example – while even sunlight itself exerts non-negligible pressure.

The perturbing effects of such forces – called 'torques' – can however be counteracted or desired rotations induced by the satellite's Attitude and Orbit Control Systems (AOCS). The AOCS incorporates sensors to identify the satellite's current attitude ( such as inertial gyroscopes, star trackers or magnetometers) and actuators (including thrusters, reaction wheels or magnetic torquers) to trigger the desired motion.

Why are Control Systems important?

The set tasks of many spacecraft require them to maintain specific pointing angles. Communication satellite antennas need to hold a link with specific ground stations while space telescopes or Earth observing missions line up to pre-set observation targets. Similarly, planetary probes must keep their antennas locked on their homeworld to relay their discoveries back, no matter how tiny Earth shrinks with distance.

So the AOCS is a crucial subsystem – a mission will come to a premature end if it fails. The level of performance required from an AOCS varies from mission to mission: A communications satellite pointing to a ground station might require an accuracy of 0.1 degrees while a space telescope seeking a certain astronomical target needs to be thousands of times more precise. Formation flying and planetary lander missions currently in the planning stages will need even greater precision.

What innovations are involved?

Current AOCS components make up a large amount of spacecraft mass, rely on large moving parts such as spinning mechanical gyroscopes whose motion can perturb the spacecraft platform and are prone to premature breakdown.

The aim of current R&D within this sphere is to reduce the overall size of the AOCS subsystem while enhancing spacecraft controllability and boosting their reliability and operational lifetime, applying micro-electronic mechanical system (MEMS) technology where possible.

New AOCS devices under development include autonomous star trackers capable of reliably identifying spacecraft attitude on an ongoing basis without any additional navigational information. Also being developed are fibre optic gyroscopes which lack any moving parts, instead measuring changes in light passing through a fibre optic loop to detect attitude changes.

Another new technological development are miniaturised micro-thrusters for extremely precise attitude control. Tethers attached to spacecraft are under study as a tool for energy-free manoeuvring but also energy generation. Solar sailcraft are another exotic concept under examination.

Radio and laser communication pointing devices are being considered meanwhile for enhanced telemetry, tracking and command (TT&C) links with Earth.

What applications and missions are they enabling?

Improved performance Control Systems should enable a new generation of spacecraft constellations whose mission success will require mastering formation flying techniques.

ESA's twin-spacecraft Proba-3 technology demonstrator, scheduled for early in the next decade, will pave the way for operational constellation missions: LISA Pathfinder involves a trio of spacecraft seeking to detect gravitational waves, while the Darwin flotilla of spacecraft will combine light together through a technique called interferometry to deliver a combined resolution equivalent to a single giant telescope, in order to detect Earth-sized exoplanets.

Satellite interferometry also has Earth observation applications, with spacecraft constellations capable of measuring millimetre scale shifts in the terrestrial surface – although the technique requires extremely precise satellite control to be successful.

Autonomous navigation systems are being worked on to enable landings on planetary surfaces, due to be carried out in 2016 by the ExoMars rover as well as inertial navigation systems to help traverse the Martian terrain. The planned follow-up Mars Sample Return Mission (SRM) will additionally require autonomous return to orbit and orbital docking abilities.

Control Systems also encapsulates analysis of future Ariane 5 and Soyuz from Kourou launches and re-entry trajectories as well as Next Generation Launcher designs.