The Optical Solar Sensor (OSD) has been designed to determine the direction towards the center of the Sun's visible disc.

The direction to the Sun is computed in the instrument coordinate system relative to the pixels' position on the linear CCDs illuminated with the light passed through the incoming coding mask.

The OSD coding mask has three groups of slits with three slits in each group. This mask configuration provides for reliability increase as well as for the required accuracy. In order to identify the groups and slits in the groups distances between the groups and particular slits have been made different.