How does the IMAGE satellite know where to point?

The instruments are located at fixed positions on the spacecraft relative to the spin axis and the spin plane. The Neutral Atom Imagers and the Ultraviolet Imagers point out from the axis of the satellite and are in the spin plane. All of this fixes the pointing of the instruments with respect to the spacecraft's coordinate system. To point the instruments to specific places in the sky is a bit trickier.

The Attitude Control System consists of a three-axis magnetometer to measure the orientation of the earth's local magnetic field. This orientation information is computed 'on the fly' from minute to minute, and is only a crude measure of the orientation good to a few degrees in accuracy. There are two, Horizon Crossing Indicators (HCI) which determine the spacecraft spin angle and rate by looking for transits of the Earth's horizon every spacecraft revolution. A Fan Sun Sensor (FSS) determines the Sun's attitude, and from this the course's spin-axis orientation is found. In addition, the frequent radio contacts between the satellite and ground provide velocity and distance information which is used to update the orbit ephemeris so that scientists can keep track of exactly where the satellite is with respect to the earth. All of this information together, allows a mathematical 'pointing' model for each instrument field of view to be created from second to second. This information is then used to 'tag' the data so that, on the ground, a map of the sky can be created from the instrument detector outputs.

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