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Attitude and Orbit Control

As Venus Express employs fixed, high-gain communications antennas and a propulsion configuration that uses a single main engine, a high level of attitude manoeuvrability is required. Attitude manoeuvres are performed:

  • Between the nadir pointing observation phase of the orbit and the Earth communication phase (see "Operational Orbit" details in the left hand navigation)
  • To reach specific attitudes necessary for other science observations (for example, to enable SPICAV to make occultation observations)
  • For optimisation of reaction wheel off-loading by selection of the most suitable attitude for this operation

Spacecraft attitude measurement is performed using Star Trackers and gyros, ensuring the availability of data in almost any attitude. The main constraint on attitude measurement is that the Star Trackers cannot provide data when the Sun or Venus is in or near their fields of view.

Reaction wheels are used for almost all attitude manoeuvres, providing flexibility and accuracy and reducing fuel consumption. The angular momentum of the wheels is managed from the ground using desaturation manoeuvres as required.

AOCS Hardware

The sensor components of the Attitude and Orbit Control System (AOCS) comprise:

  • Two Star Trackers (STR)
  • Two Inertial Measurement Units (IMU)
  • Two Sun Acquisition Sensors (SAS)

Each Star Tracker has a circular field of view of 16.4 degrees and is capable of making measurements using stars of magnitude 5.5 or greater. Three axis attitude data is derived, with at least three stars always present in the field of view. The STRs incorporate a star pattern recognition function and can autonomously perform attitude acquisition. Both STRs are mounted on the face of the spacecraft most protected from solar illumination, with an angle of thirty degrees between their fields of view.

Each Inertial Measurement Unit employs three ring laser gyros and three accelerometers, aligned on three orthogonal axes. The gyros are used during attitude acquisition, for roll rate control, during the observation phase, to ensure that the required pointing performance is achieved, and during trajectory corrections, where they serve as backups for the other sensing systems. The accelerometers are used during trajectory corrections to make accurate measurements of change of velocity.

Each Sun Acquisition Sensor comprises solar cells mounted on a pyramid, which is then mounted on the exterior of the spacecraft main body. The SASs are used for pointing the spacecraft in Sun Acquisition Mode, during attitude acquisition or re-acquisition.

The AOCS uses a Reaction Wheel Assembly, comprising four reaction wheels in a skewed configuration, which enables most nominal mission operations to be performed with any three of the four wheels. By speeding up or slowing down a reaction wheel, the AOCS is able to produce a torque about the axis of rotation of that wheel and so cause the spacecraft to rotate about that axis. A change to the speed of three of the wheels allows a torque to be generated about any chosen axis.

The AOCS controls the spacecraft propulsion system, firing the 10 N thrusters to perform attitude changes that cannot be accomplished using the reaction wheels, and small trajectory corrections. The main engine (415 N thrust) is used to perform major trajectory changes.

The AOCS also provides the control inputs to the Solar Array Drive Mechanisms (SADM), which change the orientation of the Solar Arrays.

AOCS Modes

The AOCS has several sets of operating modes for different parts of the mission operations:

  • Attitude acquisition and reacquisition
  • Routine scientific mission operations
  • Orbit control

Attitude acquisition or attitude reacquisition is accomplished using two modes. First, the Sun Acquisition Mode, using data from the Sun Acquisition Sensors, points the spacecraft X axis and the Solar Arrays towards the Sun. Then, Safe/Hold Mode completes the acquisition by establishing three-axis pointing with the main High Gain Antenna directed towards the Earth.

Routine scientific mission operations are all conducted in Normal Mode, which is also used for cruise pointing during the journey to Venus and during the attitude changes needed to orient the spacecraft before and after orbit control manoeuvres.

The trajectory correction or orbit control manoeuvres are performed through three modes:

  • Orbit Control Mode (OCM), used for small trajectory corrections performed with the 10N thrusters
  • Main Engine Boost Mode (MEBM), for trajectory corrections performed with the 415N main engine
  • Braking Mode (BM), specifically designed for the aerobraking phase, if such a phase were necessary to achieve the final orbit, using the force produced by the air-drag when passing through the Venus atmosphere at orbit pericentre
  • Thruster Transition Mode (TTM) is used to achieve a smooth transition between the thruster controlled Modes (OCM and BM) and the reaction wheel controlled Normal Mode.

Last Update: 24 May 2007

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