Overview of Spacecraft Attitude Sensors – Analysis of Star Tracker

The attitude description of spacecraft usually adopts a direction relative to a specified coordinate system, and the most commonly used direction is relative to the celestial inertial coordinate system.Such as star tracker.
To complete this process, it is usually necessary to know the direction of one or more directional vectors relative to the spacecraft coordinate system and the specified coordinate system, and these vectors are all known relative to the specified coordinate system. The most commonly used vectors include: sun vector, Earth center vector, navigation star vector, and Earth magnetic field vector.
As long as the attitude sensor measures the orientation of these vectors relative to the sensor’s own coordinate system or the spacecraft’s coordinate system, it can further calculate the spacecraft’s attitude in the specified coordinate system.

A solar sensor is a sensor that directly measures the vector of the sun in the spacecraft coordinate system.
Sun sensors belong to optical sensors and are the most commonly used attitude sensors on satellites, almost used on all satellites.
The sun is the brightest star near Earth, which can easily be separated from other light sources such as background stars. Moreover, the distance between the sun and Earth makes the sun a point relative to Earth’s satellites, which greatly simplifies the design of sun sensors. At the same time, the vast majority of satellites require solar energy as their energy source, requiring the sails to be facing the sun; Some devices or instruments also need to avoid the sun to prevent stray light and heat generation. In these occasions, solar sensors are very important.
There are many design methods and styles for sun sensors, but they essentially include three types: analog sun sensors, sun appearance sensors, and digital sun sensors.

Infrared horizon sensor is a sensor that directly measures the vector of the Earth in the spacecraft coordinate system, also known as an Earth sensor.
It can directly calculate the attitude of spacecraft relative to the Earth, so it is widely used in many Earth related tasks, such as tracking and data relay satellites (TDRSS), geostationary orbit operational environment satellites (GOES), land satellites (IANDSAT), and so on.
The Earth cannot be treated as a point like the Sun, especially for low Earth orbit satellites, where the Earth accounts for 40% of the satellite’s field of view. Therefore, detecting the presence of the Earth alone is not enough for satellite attitude determination. Therefore, most earth sensors adopt a method of checking the horizon.
The horizon sensor is an infrared sensor used to detect the contrast between the warm Earth surface and cold space. The difficulty lies in the selection of sensitivity values for sensors to distinguish between the Earth and its surroundings.
However, due to the influence of the Earth’s atmosphere and sunlight reflection, this threshold will undergo significant changes. Traditional Earth sensors use scanning methods and have moving components, which result in insufficient lifespan and reliability; Currently, the static infrared type with a large field of view
The Earth imaging sensor has made significant progress, such as the Shanghai Institute of Technology and Physics in China using a 14-16um spectral band area array CCD to achieve complete Earth imaging and achieve high measurement accuracy.

Infrared horizon sensor is a sensor that directly measures the vector of the Earth in the spacecraft coordinate system, also known as an Earth sensor.
It can directly calculate the attitude of spacecraft relative to the Earth, so it is widely used in many Earth related tasks, such as tracking and data relay satellites (TDRSS), geostationary orbit operational environment satellites (GOES), land satellites (IANDSAT), and so on.
The Earth cannot be treated as a point like the Sun, especially for low Earth orbit satellites, where the Earth accounts for 40% of the satellite’s field of view. Therefore, detecting the presence of the Earth alone is not enough for satellite attitude determination. Therefore, most earth sensors adopt a method of checking the horizon.
The horizon sensor is an infrared sensor used to detect the contrast between the warm Earth surface and cold space. The difficulty lies in the selection of sensitivity values for sensors to distinguish between the Earth and its surroundings.
However, due to the influence of the Earth’s atmosphere and sunlight reflection, this threshold will undergo significant changes. Traditional Earth sensors use scanning methods and have moving components, which result in insufficient lifespan and reliability; Currently, the static infrared type with a large field of view
The Earth imaging sensor has made significant progress, such as the Shanghai Institute of Technology and Physics in China using a 14-16um spectral band area array CCD to achieve complete Earth imaging and achieve high measurement accuracy.

Star sensors are devices that determine the attitude of spacecraft by measuring the direction of the navigation star in the spacecraft coordinate system.
The working process is as follows: first, measure the vector of the navigation star in the spacecraft coordinate system, and then identify it through the star map to obtain its corresponding vector in the inertial coordinate system; By comparing the vector relationships of the corresponding navigation stars in two coordinate systems, the transformation matrix from the inertial coordinate system to the spacecraft coordinate system can be obtained, which is the attitude of the spacecraft in the inertial coordinate system.
Star sensors are the highest precision attitude sensors, with an accuracy of up to angular second level. However, traditional star sensors are expensive, large in size and quality, high in power consumption, and the computer on the star sensor is complex in both hardware and software, making design and debugging difficult. In addition, compared to other sensors, star sensors are more susceptible to interference. Many stray lights in space may cause the failure of star sensors. The installation of star sensors should avoid interference from the light emitted or reflected by celestial bodies such as the sun, Earth, and moon, and also consider issues such as not being interfered by the satellite itself. Despite the aforementioned drawbacks of star sensors, the characteristics of high-precision absolute attitude measurement make them widely used in spacecraft such as satellites.Star sensors can provide angular second or even sub angular second level pointing accuracy, enabling three-axis absolute attitude measurement of space vehicles, while Earth sensors, magnetometers, sun sensors, etc. can only provide measurement information for one or two reference directions; The navigation star referenced by the star sensor coordinates is relatively evenly distributed throughout the entire celestial sphere, so a star sensor with appropriate sensitivity and field of view can detect the navigation star almost at any direction, providing three-axis attitude information, which is incomparable to other attitude sensors.
From the current overall situation of attitude sensors on satellites, star sensors are almost an inevitable choice for high-precision satellite equipment.

The inertial sensor uses an accelerometer to measure the motion acceleration of the carrier, and calculates the real-time position of the carrier through calculation; Use inertial sensors to achieve the reference coordinates (navigation coordinate system) required for navigation and positioning calculations. Inertial sensors can complete navigation tasks without relying on any external information, as they do not have any optical or electrical contact with the outside world, so they have good concealment and are not limited by meteorological conditions. However, inertial sensors generally have high-speed rotating components that are prone to damage and drift, making them unable to work independently and with high accuracy for long periods of time. Therefore, systems such as star sensors are generally required to correct them.

The comparison of the star trackermentioned above is as follows：

From the current overall situation of attitude sensors on satellites, star tracker are almost an inevitable choice for high-precision satellite equipment.