What is a sun sensor?

Introduction to solar sensors

As we all know, the sun sensor is the most widely used type of sensor in the aviation field, and all satellites are equipped with sun sensors. In the rapidly developing society led by science and technology, the prospect of solar sensors is very good. Today we will explain the content of the sun sensor, and focus on explaining the working principle and characteristics of various sun sensors.

The sun sensor (sun sensor) is an optical attitude sensor that determines the orientation of the sun vector in star coordinates by sensing the orientation of the sun vector, thereby obtaining the orientation information of the spacecraft relative to the sun. The sun was chosen as the reference target because the apparent angular radius of the sun’s disk is almost independent of the orbit of the spacecraft and is small, so the sun can be approximated as a point source of light for most applications. This simplifies the sensor design and attitude determination algorithm. Moreover, the high brightness and high signal-to-noise ratio of the sun make detection easier to achieve.

The sun sensor is suitable for the orbit control and attitude control of the spacecraft; to protect the normal operation of highly sensitive instruments, such as star sensors and earth sensors; to provide the attitude control reference signal on the star, and to control the solar panels of the spacecraft position.

Structure of the sun sensor

The composition of the solar sensor mainly includes three aspects: the optical head, the sensor part and the signal processing part. The optical probe includes an optical system and a detection device, which uses the photoelectric conversion function to obtain the attitude angle information of the star relative to the sun in real time. The optical head can use slits, small holes, lenses, prisms, etc.; the sensor part can use various devices such as photocells, CMOS devices, code discs, gratings, photodiodes, linear array CCDs, area array CCDs, APS, and SMART; Part of the signal processing scheme can use discrete electronic components, single-chip microcomputers, programmable logic devices, etc.

Classification of solar sensors

Generally, solar sensors can be divided into “0-1″ type, analog type and digital type according to the way they work:

(1) The sun appears sensor (0-1 type sun sensor). It uses a digital signal 1 or 0 to indicate whether the sun is in the field of view of the sensor;

(2) Analog sun sensor. It produces an output signal that is a continuous function of the vector orientation (solar angle) of the star relative to the sun;

(3) Digital sun sensor. It provides a discrete coded output signal whose output value is a function of the measured sun angle. The characteristics of the sensor are: large field of view, high precision, strong advantages in life and reliability, and have been widely used in various types of spacecraft.

Working principle and characteristics of various solar sensors

0-1” type sun sensor

The “0-1” sun sensor is also called the sun discovery detector, that is, as long as there is the sun, it can generate an output signal, which can be used to protect the instrument and position the spacecraft or experimental instrument. Its structure is also relatively simple. There is a slit on the top of the sensor, and a photocell is pasted on the bottom. When the satellite searches for the sun, once the sun enters the field of view of the detector, the photocell will produce a step response, indicating that the sun has been found. A sustained step signal indicates that the sun is within the sensor’s field of view. Generally speaking, the satellite’s rough attitude determination is accomplished by a “0-1” sun sensor, which is mainly used to capture the sun and determine whether the sun appears in the field of view. The “0-1” type sun sensor should be able to cover the entire celestial globe, and all sensors work simultaneously. Although this 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, the sunlight reflected by the solar panel, etc. are likely to interfere with this sensor. Therefore, the sensor’s filter is able to filter out occasional electrical pulses.

Analog sun sensor

The analog sun sensor is also called a cosine detector. It often uses a photocell as its sensor device. Its output signal strength is related to the incident angle of sunlight. angle. 0—the short-circuit current of the photocell; almost all analog sun sensors work on the whole sphere, and their field of view is generally about 20°~30°, and the accuracy is about 1°. The threshold for judging the occurrence of solar signals is not high 80% (usually 50%) of the sun signal is the threshold. Such accuracy is not bad for communication satellites, but for satellites observing the earth, the accuracy is too low. Therefore, current communication satellites mainly rely on this analog sun sensor.

digital sun sensor

The realization principle of the analog sun sensor is simple, but its accuracy is difficult to meet the increasing requirements of the satellite attitude control system, and the analog sun sensor is easily interfered by other light sources such as the earth’s reflected light, which will cause errors in the attitude measurement results , Therefore, the digital sun sensor has been greatly developed. Moreover, the digital sun sensor can meet the increasingly higher requirements of light weight, low power consumption, high precision, and modularization. The digital sun sensor is a sensor that calculates the angle of sunlight by calculating the deviation of the position of the sun’s rays on the detector relative to the center. There are mainly two types of CCD and APS, and the CCD sun sensor includes a linear CCD digital sun sensor. Sensors and area array CCD sun sensors, while APS digital sun sensors are mainly area array. At present, there are many digital sun sensor products using CCD. The field of view of the digital sun sensor is generally about ±60°, and its accuracy can reach ≤0.05°. The principle is mostly to use sunlight to shine on the CCD detector through a slit, and calculate the angle of sunlight by calculating the position of the sun image away from the center of the CCD. Its working band mostly adopts the visible light band of 0.4mm~1.1mm. Although the field of view of the digital sun sensor is very large, only a small section of it is really used. In actual work, it only focuses on the main area close to the optical axis, and the two sides far away from the optical axis are only used less frequently. In addition, in order to avoid the interference of sunlight reflected by solar panels, etc., the sun sensor will also filter out the occasional strong signal; finally, the digital sun sensor is generally 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.