Star Sensors and Space Satellite Operations

When working in a natural space environment, the reliability of star sensors can be reduced due to the influence of high-energy radiation. The electromagnetic radiation generated by the impact of high-energy radiation mainly includes X-rays and γ Radiation. Among them, the situation of high-energy particle radiation is quite special, which can be basically divided into three categories based on the radiation source: the Earth’s radiation belt, solar cosmic rays, and galactic cosmic rays.

1. Earth’s radiation zone

The Earth’s radiation belt, also known as the Van Allen radiation belt, poses the greatest radiation threat in Earth’s orbit. It is the proton, electron, and small amount captured by the Earth’s magnetic field α Composed of spatial radiation particles such as particles. According to the changes in high latitude, it can be divided into two types: inner radiation zone and outer radiation zone.

(1) The orbital height of the inner radiation zone is approximately 600km to 10000km. Considering the anomalous distribution of the geomagnetic field, the lower boundary of the inner radiation zone is located about 200km above the South Atlantic, which is a well-known area of geomagnetic anomalies in the South Atlantic. The corresponding internal radiation band is mainly composed of protons, electrons, and a small amount of α Particle composition, which is not sensitive to solar activity, and the energy distribution of protons is mainly 0.1-400 MeV, while the energy distribution of electrons is 0.04-7 MeV.

(2) The range of orbital heights corresponding to the outer radiation zone is relatively wide, ranging from 10000km to 60000 km, and even expanding to 130000 km. The center position of the orbit is about 20000 km to 25000 km. The majority of the outer radiation zone is composed of electrons, and the energy of electrons is mostly 0.04-4 MeV, while the corresponding proton energy is relatively low. External radiation is easily affected by solar activity, and when the local magnetic field is disturbed, the intensity and position information corresponding to the external radiation zone will undergo significant changes.

2. Galactic cosmic rays

The Milky Way cosmic rays are located outside the solar system and are composed of high-energy particles from all directions. The vast majority of high-energy particles are composed of protons (88%), followed by relatively few α Particles, while other particles with higher charges have a content approximately two orders of magnitude less than protons. Its energy range is approximately 0-1010 GeV, and it is distributed in free space more than 50 km above the ground, with a flux of approximately 4 cm-2 · s-1.

3. Solar proton events

The high-energy solar radiation mainly includes the solar wind and X-rays generated during solar flare eruptions γ Rays and high-energy particle streams. The high-energy particle flow generated during solar flares can be referred to as solar cosmic rays. Due to the fact that the main component is high-energy protons and there are very few particles with a charge greater than 3, such flares are also known as solar proton events. The energy of solar cosmic rays ranges from 10 MeV to 10 GeV, with an intensity of approximately 105cm-2 · s-1.

Due to the radiation effect on the detector, star sensors are prone to performance degradation and abnormal operation. At present, the commonly used detectors for star sensors are CCD or APS. Although the detectors are diverse, fundamentally speaking, all detectors are integrated circuits built using semiconductor optoelectronic effects, with the main difference being the charge collection method. So different detectors will produce various abnormal phenomena after being affected by space radiation. From the analysis of the impact mechanism of space radiation on star sensors, the main effects brought by space radiation include displacement damage, total dose effect, and transient effect. Among them, displacement damage and total dose effect belong to cumulative effects, and transient noise caused by transient effect can easily cause abnormal operation of star sensors.

Transient effect refers to the phenomenon where charged particles enter the sensitive layer of an imaging element, and the energy of the charged particles is absorbed. Ionization effect can cause the generation of electron hole pairs (as electron hole pairs generally only appear briefly and only exist in the current cycle without affecting the next cycle, they can be referred to as transient effect). The emergence of transient effects often causes a series of noise and pseudo star interference in star sensor imaging systems. Due to the interference of noise and pseudo star points, the success rate of star sensor recognition will be greatly reduced, and the attitude effectiveness will decrease in tracking mode, even leading to exiting the tracking mode and conducting new star point recognition. The shape characteristics of transient noise can be obtained from ionization energy loss and track length at corresponding pixels. The ionization energy loss of charged particles can be obtained through the incident energy characteristics of the particles and the linear energy transfer density (LET, LinearEnergy Transfer). Generally, particles with an atomic weight greater than 1 exhibit a straight line characteristic; Due to the extremely small electron mass, the scattering of electrons during the incident process also needs to be considered. Usually, at room temperature, it takes approximately 3.65 eV to generate an electron hole pair through ionization. Therefore, the energy transferred, the electron track length at the corresponding pixel, and the charge collection efficiency can be used to determine the amount of charge generated by the transient effect of the particle. By analyzing the energy spectrum and particle flow rate of the current particles in the star sensor, the degree to which the star sensor is affected by transient effects can be quantitatively analyzed.