Local orbital reference system
Bus modes

Attitude control

Unlike an aircraft which uses its attitude to control its trajectory (pitching to go up, banking to turn, etc.) the angular motion of a satellite, which is travelling in a vacuum, has practically no effect on its trajectory (i.e. its orbit).

The satellite's orbit is determined by the initial velocity given to it by the Ariane launcher, and then by the small corrections made regularly by micro-thrusters.

Attitude control (angular orientation) is needed so that the optical system covers the programmed ground area at all times. However, the satellite tends to change its orientation due to torque produced by the environment (drag of the residual atmosphere on the solar array, solar radiation pressure, etc.) or by itself (due to movement of mechanical parts, etc.). Thus the angular orientation has to be actively controlled. Another reason is the need to prevent "blurring " of the scenes acquired.

The attitude is continuously controlled by a programmed control loop: sensors measure the satellite's attitude, the onboard computer then processes these measurements and generates commands which are carried out by the actuator, to ensure correct pointing.

SPOT 4 is "three-axis stabilized", which means that its orientation is fully controlled relative to three axes. One of these directions corresponds to the line between the satellite and the centre of the Earth, also called the "geocentric direction " another is perpendicular to this geocentric axis, in the direction of the satellite's velocity vector; the third is perpendicular to the first two. These three form the local orbital reference system.

orbloc2a.gif (26486 octets)The local orbital reference system is defined at each point of the orbit by three unit vectors. These vectors are derived from the satellite position and velocity vectors:

axes4_5a.gif (18663 octets) satellite axes

Axes Xs, Ys, Zs represent an orthogonal reference frame related to the satellite (satellite axes). Nominal attitude pointing consists of the best possible alignment of this set of axes with the local orbital reference system (while ensuring stability and limiting angular rates about this position).

With perfect geocentric pointing, this gives:

Xs = -T
Ys = -R
Zs =  L

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The following equipment is used for attitude control: