In the vast expanse of space exploration, precision and accuracy are paramount. Among the myriad of technologies aiding in this endeavor, Sun Sensors emerge as essential instruments, providing crucial data for spacecraft orientation and navigation. Let’s delve into the essence of Sun Sensors and their pivotal role in navigating the cosmic seas.
The solar sensor is an optical attitude sensor that determines the orientation of the solar vector in star coordinates by being sensitive to the orientation of the solar vector, thereby obtaining the orientation information of the spacecraft relative to the sun. The Sun is chosen as the reference target because the apparent angular radius of the Sun’s disk is almost independent of the spacecraft orbit and is very small. Therefore, for most applications, the Sun can be approximately regarded as a point light source. This simplifies sensor design and attitude determination algorithms. Moreover, the high brightness and high signal-to-noise ratio of the sun make detection easier to implement.
The composition of a solar sensor mainly includes three aspects: optical head, sensor part and signal processing part. The optical probe includes an optical system and a detection device, which uses photoelectric conversion function to obtain real-time attitude angle information of stars relative to the sun. The optical head can use slits, small holes, lenses, prisms, etc.; the sensor part can use photocells, CMOS devices, code disks, gratings, photodiodes, linear array CCD, area array CCD, APS, SMART and other devices; Part of the signal processing solution can use discrete electronic components, microcontrollers, programmable logic devices, etc.
Generally, solar sensors can be divided into “0-1” type, analog type and digital type according to the way they work:
(1) Sun appearance sensor (0-1 type sun sensor). It uses a digital signal 1 or 0 to indicate whether the sun is within the field of view of the sensor;
(2) Analog solar sensor. The output signal it produces is a continuous function of the star’s vector orientation (solar angle) relative to the sun;
(3) Digital sun sensor. It provides a discrete coded output signal whose output value is a function of the measured solar angle. The characteristics of this sensor are: large field of view, high precision, long life and reliability. It has been widely used in various types of spacecraft.
Working principles and characteristics of various solar sensors
The “0-1” solar sensor, also known as the solar discovery detector, can produce an output signal as long as there is the sun, and can be used to protect instruments and position spacecraft or experimental instruments. Its structure is also relatively simple. There is a slit on the sensor, and a photocell is attached to the bottom. When the satellite searches for the sun, once the sun enters the detector’s field of view, the photocell produces a step response, indicating that the sun has been discovered. A continuous step signal indicates that the sun is within the sensor’s field of view. Generally speaking, the rough attitude determination of the satellite is completed by a “0-1” type solar sensor, which is mainly used to capture the sun and determine whether the sun appears in the field of view. The “0-1” type solar sensor should be able to cover the entire celestial sphere, and all sensors should work at the same time. Although this kind of sensor is relatively simple to implement, it is more susceptible to interference from external light sources. For example, the sunlight signal reflected by the ball and the sunlight reflected by the solar sail can easily interfere with this sensor. Therefore, the sensor’s filter is able to filter out occasional electrical pulses.
Analog solar sensors, also known as cosine detectors, often use photovoltaic cells as their sensing devices. Its output signal strength is related to the incident angle of sunlight. The relationship formula is: where, Iθ—the difference between the normal direction of the solar beam and the photovoltaic cell. angle. 0—The short-circuit current of the photocell; Almost all analog solar sensors work on the entire celestial sphere. Their field of view is generally around 20°~30°, and the accuracy is around 1°. The threshold for judging the occurrence of solar signals should not be high. The threshold is 80% (usually 50%) of the solar signal. As shown in Figure 1. This kind of accuracy is okay for communication satellites, but for earth observation satellites, the accuracy is too low. Therefore, current communication satellites mainly rely on this analog solar sensor.
The implementation principle of the analog solar sensor is simple, but its accuracy cannot meet the increasing requirements of the satellite attitude control system. Moreover, the analog solar sensor is easily interfered by other light sources such as the earth’s reflected light, which causes errors in the attitude measurement results. , Therefore, digital solar sensors have been greatly developed. Moreover, digital solar sensors can meet increasingly higher requirements for light weight, low power consumption, high precision, and modularity. Digital solar sensors are sensors that calculate the angle of sunlight by calculating the deviation of the sun’s rays from the center position on the detector. There are two main types: CCD and APS. CCD solar sensors include linear CCD digital solar sensors. Sensors and area array CCD solar sensors, while APS digital solar sensors are mainly area array. Currently, there are many digital solar sensor products using CCD. The field of view of digital solar sensors is generally around ±60°, and its accuracy can reach ≤0.05°. The principle is to use sunlight to illuminate the CCD detector through a slit, and calculate the angle of the sunlight by calculating the position of the solar image deviating from the center of the CCD. Its working band mostly uses the visible light band of 0.4mm~1.1mm. Although the digital solar sensor has a large field of view, only a small section of it is actually used. In actual work, it only focuses on detecting the main area close to the optical axis, and only detects the two sides far away from the optical axis in a smaller amount of time. Detection; In addition, in order to avoid interference from sunlight reflected by solar panels, etc., the solar sensor will also filter out occasional strong signals; finally, digital solar sensors generally need to be added in front of the CCD. Filters are used to attenuate the intensity of sunlight so that it does not work in a saturated state.
Sun Sensors are sophisticated devices designed to detect the position of the sun relative to a spacecraft’s orientation. By measuring the intensity and direction of sunlight, these sensors provide valuable data for maintaining spacecraft attitude, ensuring proper orientation for communication, solar power generation, and scientific observations.
At the heart of Sun Sensors lies a complex interplay of optics, electronics, and computational algorithms. These sensors typically consist of photodiodes or photovoltaic cells that convert sunlight into electrical signals. Advanced algorithms then analyze these signals to determine the sun’s position relative to the spacecraft, enabling precise adjustments to maintain proper orientation.
The utilization of Sun Sensors offers numerous advantages in space exploration endeavors. Their accuracy and reliability make them indispensable for spacecraft attitude control, ensuring that missions remain on course and objectives are achieved. Moreover, Sun Sensors provide a cost-effective solution for spacecraft navigation, leveraging the abundant and predictable nature of sunlight.
The versatility of Sun Sensors extends far beyond traditional spacecraft navigation. These sensors find applications in a wide range of space missions, including satellite communications, Earth observation, and interplanetary exploration. Whether used to stabilize satellites in orbit, align solar panels for maximum efficiency, or navigate spacecraft to distant planets, Sun Sensors play a crucial role in expanding humanity’s reach into the cosmos.
Continual advancements in technology have propelled the evolution of Sun Sensors, enhancing their capabilities and expanding their potential applications. Modern sensors feature improved sensitivity, reduced size and weight, and enhanced onboard processing capabilities, allowing for greater precision and efficiency in spacecraft operations.
As humanity’s exploration of space continues to evolve, the importance of precision in navigational technologies cannot be overstated. Sun Sensors stand as a testament to our commitment to exploring the mysteries of the universe with clarity and accuracy. Whether guiding spacecraft to distant worlds or enabling scientific observations of celestial phenomena, these sensors remain indispensable tools in humanity’s quest for knowledge and discovery.
In the boundless expanse of space, where the sun shines as a beacon of light and energy, Sun Sensors serve as our guiding stars, illuminating the path towards exploration and discovery. Through their precision and reliability, these sensors empower us to navigate the cosmic seas with confidence and precision, paving the way for new discoveries and scientific breakthroughs.
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