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Attitude and Orbital Control Systems (AOCS)

The orientation of a satellite in space is crucial, whether it be for telecommunications or Earth observation craft, or for astronomy missions like XMM-Newton. ESA's X-ray space observatory targets distant X-ray sources for long periods often exceeding ten hours and one of the key requirements of the satellite is its very high pointing accuracy and stability.

This high precision of control in orbit is the responsibility of the satellite's Attitude and Orbit Control System (AOCS). This major subsystem has been provided by Matra Marconi Space (MMS) UK and constitutes the principal contribution from the United Kingdom to the XMM-Newton mission.


Attitude and Orbit Control System (AOCS) (courtesy of MMS)

While orbiting the Earth in its highly elliptical orbit, XMM-Newton is steered to point its telescope towards targets selected by astronomers. The 3.8 tonne satellite slowly turns towards these celestial objects at a rate of 90 degrees per hour.

The pointing accuracy of the 10 m long XMM-Newton is 0.25 arcsec over a 10-second interval. "In terrestrial terms, this is equivalent to someone in Bristol looking at a melon on Saint Paul's cathedral in London using a handheld telescope and seeing it without the slightest jitter" explains Mike Backler, the MMS Project Manager for XMM-Newton.

Reaction wheel

Reaction wheel (courtesy of MMS)

XMM-Newton can change its orientation by means of two sets of four small thrusters (second set is for redundancy) that use hydrazine gas. But, for precise pointing, reaction wheels mounted on the spacecraft are the primary means to control its attitude. There are four reaction wheels, three of which are for nominal operations and one for redundancy.

On the basis of data supplied by a star-tracker telescope, which recognises the different star formations in its field of view, the AOCS computers calculate the appropriate momentum of the reaction wheels for precision manoeuvring of the spacecraft during observations or to change the target. The maximum rotation speed of the reaction wheels is 4000 rpm. For redundancy XMM-Newton carries a second star-tracker telescope.

The fully redundant AOCS represents some 100 kg of electronics: a dozen black equipment boxes, containing the computers, reaction wheels, Sun and inertial sensors, power supplies and associated cabling. Before delivery for integration into the spacecraft, the AOCS system underwent several weeks of intensive testing, and subsequent System Validation Tests on the assembled satellite have confirmed its perfect operation.

Fail-safe approach

The design of the AOCS has been driven by a fail-safe approach. All eventual failures must be detected. The principal component of the AOCS is its flight Failure Detection and Correction Electronics unit. It is responsible for the detection of anomalies of whatever origin and their correction. Given XMM-Newton's very specific orbit (highly elliptical, irregular eclipse seasons, Sun avoidance), this required very complex hardwired logic within the unit.

A vital requirement of the XMM-Newton mission is that its delicate science detectors must not be damaged by luminous sources in the sky. The telescope's longitudinal axis is prevented from ever coming within 70° of the Sun's direction. Likewise it will never point to within 42.5° of the Earth or 22° of the Moon during scientific observations. In addition, a solar aspect angle within the range 70-110 must be maintained at all times. Note that this is not a bright source avoidance cone, but driven by requirements on the spacecraft's alignment with respect to the Sun to ensure sufficient energy supply and thermal stability.

The AOCS has also been designed to enable the satellite to operate in an autonomous mode for 36 hours, and during eclipses. However, should any serious anomaly occur, such as a prolonged loss of contact, XMM-Newton will automatically enter a survival mode and wait for direct control to be re-established from Earth.

The Attitude and Orbit Control System entered into play seconds after Ariane 5 released XMM-Newton. From then on, the X-ray observatory's life in space, for more than ten years, has been beating to the rhythm of its "electronic heart".

Last Update: 19 September 2011

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