In order to clarify the principle and method of spacecraft attitude measurement, further understand the importance of star trackers in aerospace applications and the difference from other attitude sensors, Here briefly introduces the situation of the main attitude sensors on spacecraft.
The spacecraft attitude description usually uses the orientation relative to a specified coordinate system, the orientation relative to the celestial inertial coordinate system is the most common one. In order to complete this process, it need to know the direction of one or more direction vectors relative to the spacecraft coordinate system and the designated coordinate system, and these vectors are known relative to the designated coordinate system. The most commonly used vectors include the sun vector, the center of the earth vector, the navigation star vector, and the earth’s magnetic field vector. As long as the attitude sensor measures the direction of these vectors relative to the coordinate system of the sensor itself or the coordinate system of the spacecraft, it can further calculate the attitude of the spacecraft in the specified coordinate system. The following is an introduction to the characteristics of different attitude sensors.
The sun sensor is a sensor that directly measures the vector of the sun in the spacecraft coordinate system. The sun sensor is an optical sensor, and it is the most commonly used attitude sensor on satellites, and it is used on almost all satellites. The sun is the brightest star near the Earth, it can be easily separated from other light sources such as background stars, and the distance between the sun and the Earth makes the sun a point relative to the Earth’s satellite, which greatly simplifies the design of the solar sensor. At the same time, the vast majority of satellites need solar energy as energy, and the sails need to face the sun; some devices or instruments need to avoid the sun to prevent stray light and heat. On these occasions, the sun sensor is very important. There are many design methods and styles of sun sensors, but essentially there are three types: analog sun sensors, sun appearance sensors, and digital sun sensors.
Infrared horizon sensor
The infrared horizon sensor is a sensor that directly measures the vector of the earth in the spacecraft coordinate system, and it is also an earth sensor. It can directly calculate the attitude of the spacecraft relative to the Earth, so it has been widely used in many Earth-related tasks. applications such as satellite tracking and data relay. The earth cannot be treated as a point like the sun, especially for low-earth orbit satellites, the earth occupies 40% of the satellite’s field of view, so only detecting the appearance of the earth is not enough for satellite attitude, so most Earth sensors are used to check the horizon. The horizon sensor is an infrared sensor used to detect the contrast between the warm earth’s surface and the cold space. The difficulty lies in the selection of the sensor’s threshold for distinguishing the earth and its surroundings. However, this threshold will vary greatly due to influences such as the Earth’s atmosphere and reflected light from the sun. The traditional earth sensor adopts the scanning method, has moving parts, and has insufficient life and reliability; at present, the static infrared earth imaging sensor in the large market has made great progress.
Magnetometer
Magnetometer is a commonly used vector measurement sensor on spacecraft, which has the following characteristics:
1. Can measure the direction and strength of the magnetic field at the same time;
2. Low power consumption and lightweight;
3. Within a certain orbit range, the measurement results are trustworthy;
4. Able to work in a wide temperature range;
5. No moving parts;
However, the Earth’s magnetic field model is only an approximate description of the Earth’s magnetic field, and if it is used as the measurement basis of the magnetometer, it will bring large errors, so the magnetometer is not a high-precision attitude sensor.
The star tracker is the highest-precision attitude sensor, and its accuracy can reach the arc-second level. Its working process is as follows:
1. First measure the vector of the navigation star in the spacecraft coordinate system
2. Obtain the vector corresponding to the inertial coordinate system through star map identification
3. By comparing the vector relationship of the corresponding navigation stars in the two coordinate systems, the transformation matrix from the inertial coordinate system to the spacecraft coordinate system is obtained, that is, the attitude of the spacecraft in the inertial coordinate system.
Star trackers have a wide range of applications. At earth orbit satellites or deep space probes, on large spacecrafts or small satellites, star trackers are almost always used for high-precision attitude determination.
Inertial Sensors
Inertial sensors are sensors that use accelerometers and inertial sensors to measure position and attitude relative to an inertial system, also known as an inertial navigation system. The inertial sensor uses the accelerometer to measure the motion acceleration of the carrier, and calculates the immediate position of the carrier through calculation; the inertial sensor is used to realize the reference coordinates required for navigation and positioning calculations.
The inertial sensor can complete the navigation task without relying on any information from the outside world because it does not have any optical or electrical contact with the outside world, so it has good concealment, and its work is not limited by weather conditions. However, inertial sensors generally have high-speed rotating parts, which are easy to damage and drift and cannot work independently for a long time with high precision. Generally, systems such as star sensors are required to correct them.
Attitude Sensor Comparison Table
| Name | Reference System | Advantage | Shortcoming |
| Sun Sensors | Sun | Low power consumption, low quality, the sun is clear and bright, generally necessary for solar panels and instrument protection | May not be visible in some orbits, with an accuracy of about 1′ |
| Earth Sensors | Earth | Always available for low-Earth orbit, undefined boundaries, easy analysis | Generally, the horizon needs to be scanned, the accuracy is about 0.1º, and the orbit and attitude are tightly coupled |
| Magnetic strength | geomagnetic field | Economical, low power consumption, low Earth orbit always available | Poor accuracy, greater than 0.5º, only available for low orbits. The satellite needs to be magnetically balanced |
| Star Trackers | Star | High precision, arc second level, independent of orbital movement, available at any position in space | Massive and complex. The price is slightly expensive, and the star recognition time is long |
| Inertial Sensors | Inertia | No need for external sensors, independent of orbital movement, good short-term accuracy | There is drift, high-speed rotating parts are easy to wear, relatively large power consumption and mass |
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