How many types of attitude sensor? Why is the star tracker irreplaceable?

Attitude information is an indispensable Reference criterion and a powerful guarantee for spacecraft on-orbit service，so In order to explain the principle and method of Aircraft attitude measurement,  Learn more about the importance of star tracker in aerospace applications and the difference from other attitude trackers.

The measurement of aircraft attitude mainly depends on the measurement of multiple vectors of aircraft coordinate system by attitude sensor, the solution is carried out by the quaternion output of the aircraft. This is also the main function of the trackers and the value of existence.

The five commonly used attitude tracker are as follows:

Sun tracker: The most widely used type of tracker, all the satellites are equipped with sun tracker, direct measurement The vector value of the sun in the spacecraft coordinate system.

Infrared horizon tracker: direct measurement the vector value of the earth in the spacecraft coordinate system, also known as Earth tracker, it’s used in many Earth-related missions.

Magnetometer: Satellite attitude is determined by measuring the Earth’s magnetic field, the accuracy is not high, limited by the orbit height

Inertia sensor: Also known as “inertial navigation system”. It can be used for attitude control and navigation of moving objects, do not need to rely on any outside information to complete the task, diversified, fiber-optic gyroscope has high precision, often used with star tracker.

Star tracker: The orientation of the navigation star in the coordinate system of the aircraft is measured to determine the attitude of the aircraft，The most accurate attitude sensor available，its accuracy can reach the arc second level.

Comparative analysis of attitude sensors:

The advantages and disadvantages of these attitude sensor are not the same, just a simple comparison.

Sun tracker: The frame of reference is the sun. The advantage is low power consumption, low mass, it generally provides protection for solar panels and instruments. The disadvantage is that parts of the orbit may not be visible.

Earth sensor: The frame of reference is the earth. The advantage is that it can be used in low Earth orbit, borderless, easy to analyze. But the disadvantage is that it is generally necessary to scan the horizon, and the orbit and attitude are tightly coupled.

Magnetometer: The reference frame is the geomagnetic field. The advantages are economical, low power consumption, and only low Earth orbit is always available. But the disadvantage is that the accuracy is poor and it is only available for low orbits.

Inertia sensor: The frame of reference is inertia. The advantage is that no external sensor is required, independent of orbit operation, short time and high precision, but the disadvantage is that there is drift, the parts are easy to wear under high-speed rotation, and the power consumption and quality are relatively large.

Star tracker: The frame of reference is star. The Advantages are high precision, arc-second level, independent of orbit operation. But the disadvantage is that the mass is large, complex, expensive, and the star identification time is long.

So, from the current attitude sensor on the satellite as a whole, on high-precision satellite equipment, star sensors are almost inevitable choices, so the next article will focus on how the star tracker can assist the satellite work.