Solar Orbiter is part of ESA’s “cosmic vision” science programme exploring planets and life, the solar system, fundamental laws and the universe.
Via an intricate series of gravitational flybys of Venus and Earth, Solar Orbiter will reach an elliptical solar orbit with a closest approach inside the orbit of Mercury, and with successive orbits tilting further away from the plane of our solar system.
The aim is to investigate how the Sun creates and controls the vast region of surrounding space called the heliosphere, within which the solar wind causes well-known effects such as the Northern lights, and space weather events on Earth such as geomagnetic storms.
What is AOCS and Why Does it Matter?
To achieve its mission, the Solar Orbiter carries a number of in-situ and remote sensing instruments. Given the heat and radiation environment of the mission, the Attitude & Orbit Control Subsystem (AOCS) is a critical development since the spacecraft must never depoint more than a small amount from the Sun, in order that its sunshield can protect the spacecraft. These stringent safety requirements,coupled with the demands of the science instruments, drive challenging AOCS performance for a range of mission activities.
At the highest level, the AOCS is required to maintain the spacecraft pointing, or “attitude”, to the required accuracy. While space may appear to be a very benign environment, in practice disturbances, both internal and external, act on the spacecraft and perturb the attitude. Examples include disturbance torques arising from solar radiation pressure, gravity gradient, aerodynamic forces, magnetic moments, and errors arising from thruster firings to deliver force.
Each of the AOCS modes of operation for Solar Orbiter uses a different subset of the available sensors & actuators depending on the specific challenges of the mode. All the algorithms must be designed and tuned in order to achieve the mode performance requirements, and proven to be sufficiently stable and robust. This must be performed for a spacecraft with multiple flexible appendages and sloshing fuel, the properties of all of which are uncertain.
Under the leadership of the overall mission prime contractor, Airbus Defence & Space, the AOCS itself is being delivered by a team of ADS in the UK (AOCS prime contractor) Terma in Denmark (responsible for implementation of the on-board flight software) and Tessella. Tessella’s responsibilities cover the design, tuning, simulation, performance assessment and verification support for the AOCS algorithms for all the spacecraft modes of operation.
Some particular challenges faced were:
- Controlling attitude and rate during momentum offloading by thrusters, due to the ever present risk of over-exciting the spacecraft’s flexible appendages.
- Tuning the attitude estimation algorithms for differing phases to balance accuracy against settling time of the gyro calibration. In particular, pointing stability over a minute must be no more than a few tenths of an arcsec – like the size of a tennis ball 25km away!