The Fine Sun Sensor (FSS) is designed to deliver exact information about the position of the sun. This information is used for yaw steering of the spacecraft and therefore applied in earth pointing devices and solar array orientation.

The FSS is an analogue sensor, based on the use of a quadrant detector which is capable of measuring the solar aspect angle in two axes. Processing the four quadrant outputs results in the two components of the solar aspect angle.

The applied algorithms cancel out all disturbances that are common to the four quadrants, such as temperature effects, particle radiation degradation, window performance and scale factors. The FSS provides four quadrant output voltage and a sun presence output voltage.