What are Sun Sensors?

Sun sensors are navigational instruments that are used in space by a spacecraft to determine the direction and position of the sun to the position of the spacecraft. 

In a typical sun sensor, a thin slit is present at the top of a rectangular chamber which can be of different sizes and allows light to enter and fall on an array of photo detective cells at the bottom of the chamber. The chamber casts an image of a thin line on the chamber bottom. The cells at the bottom measure the distance of the image from a centerline and determine the refraction angle by using the chamber height.

Since these photo detective cells work on the principle of a photoelectric effect they convert the incoming photons into electrons and hence voltages which are in turn converted into a digital signal. When two sensors are placed perpendicular to each other, the direction of the sun about the sensor axes can be calculated. There are various types of sun sensors, each one best suited to the characteristics and requirements of the satellite.

Sun sensors are widely used in spacecraft attitude determination systems, orbit control, and attitude control of spacecraft. They are also used for solar array pointing, gyro updating, fail-safe recovery, meteorological systems, tracking systems, and navigation systems.