How does a spacecraft maintain its attitude?

Why maintain a posture?

The correct attitude is necessary for the normal operation of the spacecraft. For example, when satellites communicate or observe the earth, the antenna or remote sensor must be pointed at the ground target; when the spacecraft performs orbit control, the engine must be aligned in the required thrust direction; when the spacecraft re-enters the atmosphere, the braking heat protection surface is required Quasi head-on airflow. These are required to establish and maintain a certain attitude of the stars.

How to measure posture?

Based on celestial body

sun sensor

The sun is the strongest luminous object visible from our spacecraft, and it can be approximately regarded as a point light source. By sensitive to the incident angle of sunlight, the angle between the sun’s line of sight and a certain body axis of the spacecraft can be measured. Sun sensors are most commonly used to determine attitude and are used in almost every spacecraft.

star sensor

Similar to the solar sensor, a star can also be used as a reference to sensitively measure the angle between a certain reference axis of the spacecraft and the line of sight of the star, and compare it with the angular position parameters of the star in the ephemeris to determine Spacecraft attitude. The measurement accuracy of the star sensor is very high, but the star brightness is required to be higher than +2 visible magnitude stars.

Taking the earth as reference position

The Earth is the brightest celestial body other than the sun that can be observed by a near-Earth spacecraft, so the Earth has become one of the most important benchmarks for spacecraft. However, since the earth is a vast observation target for near-Earth spacecraft and is not a definite reference direction, in actual engineering, the local vertical line or the local horizon where the spacecraft is located is usually used as the reference direction.

An infrared horizon is a sensor that uses the earth’s own infrared radiation to measure the attitude of a spacecraft relative to the local vertical line or the local horizon. It is referred to as a horizon. The working band of infrared horizon instruments is generally chosen to be the narrow carbon dioxide infrared band of 14 to 16 μm. The reason is that the infrared radiation intensity of carbon dioxide in the atmosphere 25 to 50 km above the earth’s surface in the 14 to 16 μm band decreases rapidly with increasing altitude, so an infrared horizon working in this narrow band can obtain extremely The clear outline of the earth helps improve measurement accuracy. At the same time, the infrared horizon is not sensitive to the sunlight reflected by the spacecraft itself and can work normally no matter day or night, so it is widely used in engineering.

Taking the inertial space as the reference orientation

The fixed axis and precession properties of the gyroscope are used to maintain the spatial datum. The fixed axis property means that when the gyroscope is not acted upon by an external moment, the gyro axis remains in the same direction relative to the inertial space; the precession property means that when the gyro is acted upon by an external moment, the gyro axis will tend to the external moment vector along the shortest path. , the precession angular velocity is proportional to the magnitude of the external torque. This precession characteristic is also true when the input and output quantities are interchanged, that is, when the gyro has a precession angular velocity input, the gyro will produce a torque output.

Accelerometer

The accelerometer is an inertial sensor used to measure the absolute acceleration component along the accelerometer input axis of the accelerometer mounting point on the spacecraft. Although accelerometers are not currently widely used for attitude stabilization and control of spacecraft, they are an important device in spacecraft navigation systems.

Based on the ground station as the reference orientation

Radio frequency sensors are often used by communication satellites because the ground transmitting stations of communication satellites can serve as radio beacon sources for the sensors. On the other hand, both radio frequency sensors and communication antennas require pointing control, so the two are structurally integrated, which can avoid pointing caused by bending deformation caused by non-integrated structures like other sensors (such as infrared horizons). error. RF sensors have high accuracy. The principle of the radio frequency sensor to determine the attitude of the spacecraft is based on the measurement of the angle between the axis of the spacecraft antenna and the line of sight of the radio wave.

Based on the geomagnetic field

A magnetometer is a sensor that measures the attitude of a spacecraft based on the Earth’s magnetic field. The magnetometer itself is used to measure the strength of the magnetic field in the space environment. Since the magnetic field strength at every point around the Earth can be determined in advance by the Earth’s magnetic field model, the attitude of the spacecraft relative to the Earth’s magnetic field can be determined by comparing it with the information measured by the magnetometer.

In an actual spacecraft attitude control system, the various sensors introduced above generally cannot meet the requirements when used alone. Instead, multiple attitude sensors need to be used in combination to form an attitude measurement system. There are three main reasons: First, because only two attitude angles can be obtained at most with respect to the same datum, complete attitude information cannot be obtained if the sun sensor, star sensor, infrared horizon, etc. are used alone; second, due to various Sensors all have limitations. For example, solar sensors cannot work in the shadow of the earth, inertial sensors such as gyroscopes drift, and star sensors have a small field of view that makes it difficult to capture target stars initially. The third reason is due to the long life of the spacecraft. The working characteristics require the sensor to reliably provide high-precision attitude information for a long time, so the redundancy of the attitude sensor becomes an important issue that must be considered. It is precisely because of the above three reasons that spacecraft are often equipped with a variety of sensors to correct and supplement each other’s measurement information, learn from each other’s strengths, back up each other, and give full play to their respective advantages.

In addition, the accuracy of the spacecraft attitude control system depends on the accuracy of attitude determination, including the accuracy of the attitude sensor. As the accuracy requirements for spacecraft attitude control systems are getting higher and higher, the accuracy requirements for sensors are generally five times or one order of magnitude higher than the system accuracy. Therefore, improving the measurement accuracy and information processing accuracy of spacecraft sensors will become a key issue in the future. The key issue.

How to adjust your posture?

The core of maintaining posture is how to generate torque. The actuators that can be carried by spacecraft mainly include thrusters, flywheels and geomagnetic torquers.

Thruster

It is one of the most widely used actuators for spacecraft control. It uses mass jet discharge to generate reaction thrust according to Newton’s second law, which is why this device is called a thruster or jet actuator. When the thruster is installed so that the thrust direction passes through the center of mass of the spacecraft, it becomes an orbit control actuator; when the thrust direction does not pass the center of mass, a torque relative to the center of mass of the spacecraft will inevitably be generated and it becomes an attitude control actuator.

According to the different forms of energy required to generate thrust, mass displacement thrusters can be divided into cold gas thrusters, hot gas thrusters and electric thrusters. Among them, the working medium consumed by cold-gas thrusters and hot-gas thrusters needs to be carried by the spacecraft from the ground, which is limited and cannot be replenished in orbit; while the electric energy consumed by electric thrusters can be replenished by solar cells in orbit, and the consumption of working medium is greatly reduced. Therefore, electric thrusters have become an important development direction for long-life, high-precision spacecraft thrusters in the future.

Flywheel

According to the principle of “momentum conservation”, the momentum moment of the high-speed rotating rigid body installed on the spacecraft is changed, thereby generating a control torque proportional to the change rate of the rigid body’s momentum moment, which acts on the spacecraft to cause its momentum moment to change accordingly. This The process is called momentum exchange. The device that realizes this momentum exchange is called a flywheel or flywheel actuator. The flywheel actuator can only be used for attitude control of the spacecraft.

Magnetic torque converter and other actuators

In addition to the two main types of actuators, thrusters and flywheels, there are other forms of actuators in spacecraft. They use environmental fields such as magnetic fields and gravitational fields to interact with the spacecraft to generate torques to achieve attitude control, such as magnetic torque, gravity gradient torque, solar radiation torque, aerodynamic torque, etc. These moments are generally relatively small and are related to factors such as orbital altitude, spacecraft structure and attitude. Among them, the magnetic torquer is the most common one.

The interaction between the magnetic properties of the spacecraft and the environmental magnetic field can produce a magnetic moment. When the two are perpendicular to each other, the magnetic moment is maximum; when the two are parallel to each other, the magnetic moment is zero. For an Earth-orbiting spacecraft, the magnetic moment always exists as long as the spacecraft has a magnetic moment. If it is not used as a control torque, it will become a disturbance torque. The energized coil installed on the spacecraft is the simplest magnetic torque device. The magnetic moment generated by the energized coil interacts with the earth’s magnetic field to generate a control torque to achieve attitude control.

magnetic torques, the most commonly used ones are gravity gradient torques, etc. The magnetic moment is inversely proportional to the cube of the orbital height. The lower the orbital height, the greater the magnetic moment. Therefore, the magnetic torque as a control torque is more suitable for low-orbit spacecraft, the gravity gradient torque is suitable for medium-altitude orbiting spacecraft, the solar radiation torque is suitable for high-orbit spacecraft such as geosynchronous orbit satellites, and the aerodynamic torque is also suitable for low-orbit spacecraft, but the last two This kind of torque is rarely used as a control torque. A device that uses environmental torque to generate control torque can be called an environmental actuator.

For the actuators used in spacecraft control, high reliability, long life, and high precision are the basic requirements, which are directly related to the life and accuracy of the control system. Among the several actuators introduced above, flywheels, thrusters, magnetic torquers and gravity gradient torque actuators are the most commonly used. The control accuracy of flywheels and thrusters is high, while the control accuracy of environmental actuators is low, so flywheels and thrusters have become the main actuators for spacecraft control.