This article intends to give to the reader an overview of the state of the art attitude sensors used for the modern missions.

Attitude determination uses a combination of sensors and mathematical models to collect vector components in the body and inertial reference frames, typically in the form of a quaternion, Euler angles or rotation matrix.
The main sensors available on satellite area are:

• Earth sensors
  There are different earth sensors available on the market like horizon crossing indicators and horizon scanners. The sensors use the difference of the infrared light emitted by Earth and the one emitted by the deep space in order to detect the horizon, and then they return the vector to the approximate center of the Earth.

• Sun sensor
  The sun sensors are available in two forms: coarse sun sensors (used usually to determine the attitude of the satellite in a safe mode when the accuracy is not an important issue) and fine sun sensors (used whenever a big accuracy is requested).

• Star sensor
  They are the most accurate sensors used for attitude determination at the present. It is important that they deliver a full attitude determination, meaning that they don’t need another vector measurement since the measurement of the stars in the field of view already provides an attitude solution.
  The process consists in taking a picture of the sky, comparing this picture with a star map stored on board of the spacecraft and based on some specific algorithms identifying the stars found and generating an attitude solution.
  Future notes will give the details of the specific star sensors topics.

• Magnetometer
  They measure the vector and the intensity of the magnetic field at the current point in orbit of the satellite. As the spacecraft is carrying a database with an accurate magnetic field model, then if the orbital position has been identified, the vector can be used for attitude determination. However, the quality of the vector measurement depends on the quality of the magnetic field model stored in the onboard computer as well as on the current events from ionosphere (like magnetic storms).

• Gyro
  They are used to measure the angular velocities and not directly the attitude of the satellite.