Star sensor performance degradation

Radiation damage of CMOS image sensors

In order to analyze the reasons for the degradation of star sensor performance and the reduction of attitude measurement accuracy caused by harsh spatial radiation environment, some scholars have conducted in-depth research on the impact of radiation damage to complementary metal oxide semiconductor (CMOS) image sensors in radiation environment on the degradation of star sensor performance. This method reveals the transfer mechanism from the degradation of CMOS image sensor device parameters to the degradation of star sensor system parameters by establishing the correlation between spatial radiation and the radiation damage sensitivity parameters of CMOS image sensors, as well as the performance parameters of star sensors.

Co- γ  The irradiation test shows that the decrease in signal-to-noise ratio of the system after irradiation leads to a decrease in the sensitivity of the star sensor to detect magnitude. The signal-to-noise ratio serves as a bridge between the CMOS image sensor and the star sensor system. The proton irradiation test shows that when the irradiation fluence is greater than 3.68 × At 1010 p/cm2, it is no longer possible to correctly extract the centroid of the star point. The research results lay a certain foundation for the prediction and correction technology of in orbit attitude measurement errors of star sensors, and can also provide a theoretical basis for the design of high-precision star sensors.

Star sensors generally consist of optical systems, imaging systems, data processing systems, and data exchange systems. The imaging system is an important component of the star sensor, and its performance determines the detection ability of the star sensor. The imaging system of star sensors mainly consists of charge coupled devices (CCD) or CMOS image sensors, which are used as cameras to capture images of the starry sky. Compared with CCD, CMOS image sensors have advantages such as high integration, low power consumption, comprehensive electrical functions, window selection and random readout, which are more in line with the requirements of miniaturization, lightweight, and low power consumption of space devices.

Currently, most star sensor products have adopted imaging systems based on CMOS image sensors.

Similar to other silicon based semiconductor devices, CMOS image sensors used in space inevitably face the threat of natural space radiation environment. The high-energy charged particles in the space radiation environment can produce cumulative radiation effects (ionization total dose effect, displacement damage effect) and single particle effects on the device, leading to the degradation of performance parameters such as dark current, dark signal non-uniform noise, and light response non-uniform noise, Even if the function fails. Professional scholars have found that the spatial radiation damage of CMOS image sensors can lead to performance degradation phenomena such as reduced signal-to-noise ratio of star map data, decreased accuracy of star point centroid positioning, and decreased sensitivity of star magnitude detection in star sensors based on image sensors working in space after irradiation, which affects the working accuracy and effective life of star sensors. However, there is a lack of in-depth research on the mechanism of degradation phenomenon, The impact of radiation on the performance parameters of star sensors has not been quantitatively evaluated.

In practical applications, there are many instances where the spatial radiation damage of CMOS image sensors leads to performance degradation and even malfunctions of star sensors. For example, in 2016, Nature magazine reported that the Japanese Hitomi astronomical satellite malfunctioned due to radiation damage to the imaging system when passing through the SAA region. The satellite urgently activated a gyroscope to replace the star sensor to calculate the spatial orientation, but the gyroscope reported an error in the spatial orientation, resulting in the ultimate failure of the satellite mission. Although high-energy particles in the SAA region themselves should not be fatal, the star sensor problems caused by them have caused a series of chain failures on the Hitomi satellite.

In order to ensure the accuracy and reliability of star sensors, some scholars have conducted in-depth analysis of the degradation mechanism of the sensitivity of star sensors to star magnitude detection caused by the changes in dark current noise and dark signal non-uniformity noise after irradiation of CMOS image sensors. They have quantitatively evaluated the impact of radiation on the performance parameters of star sensors, laying a certain foundation for the research of prediction and correction techniques for satellite attitude measurement errors in orbit.

The sensitivity of star size detection in star sensors is mainly affected by the optical system, detector noise, signal-to-noise ratio threshold, and integration time. In the static single star simulator experiment, only CMOS image sensors irradiated with different total ionizing doses are replaced each time, while the optical system, signal-to-noise ratio threshold, and integration time remain unchanged. Therefore, the main influencing factor on the star sensor’s star magnitude detection sensitivity is detector noise (CMOS image sensor noise),

Detector noise

This includes dark current noise, fixed mode noise, readout noise, optical response non-uniformity noise, and dark signal non-uniformity noise.

Read out noise belongs to transient noise, which is the random fluctuation of signal level caused by various noise sources in the circuit channel (column amplifiers, programmable gain amplifiers, and analog-to-digital converters);

Fixed mode noise is caused by the output difference between pixels. Under a fixed integration time, fixed mode noise is basically a constant, which mainly comes from two aspects: on the one hand, there is a mismatch between the intra pixel transistor or column level transistor during the manufacturing process. For 4T CMOS image sensors, the transmission gate transistor and source level follower transistor have a significant impact; On the other hand, it is the dark current within the pixel. The fixed mode noise caused by transistor mismatch can be eliminated through correlated double sampling (CDS), while the sources of dark current are diverse and the generation mechanisms are different. Fixed image noise caused by dark current cannot be completely eliminated. Under no illumination conditions, fixed mode noise can be characterized by dark signal non-uniformity noise (DSNU); Under lighting conditions, the evaluation is conducted through non uniform noise in light response (PRNU).

Through the analysis of previous experimental data, it can be concluded that the sensitivity of star size detection in star sensors is mainly affected by the dark current noise and dark signal non-uniformity noise of CMOS image sensors. With the increase of the cumulative radiation dose, the dark current noise and dark signal non-uniformity noise of CMOS image sensors increase, and the signal-to-noise ratio (SNR) of star sensors decreases, leading to a gradual decrease in star size detection sensitivity.

The dark current noise and dark signal non-uniformity noise of CMOS image sensors are the main factors leading to the degradation of star sensor performance. By establishing a relationship between the noise of CMOS image sensors and the star sensor’s star magnitude detection sensitivity through the system signal-to-noise ratio, a prediction method for the performance parameter degradation of star sensors after irradiation is finally obtained. The experimental results indicate that an increase in the noise of CMOS image sensors (dark current noise and dark signal non-uniformity noise) leads to a decrease in the signal-to-noise ratio of the system. However, a decrease in the signal-to-noise ratio of the system can also lead to a decrease in the sensitivity of the star sensor to detect magnitude.

Proton irradiation test found that when the irradiation dose is greater than 3.68 × At 1010 p/cm2, it is no longer possible to accurately extract the centroid of the star point. The proton displacement damage of the CMOS image sensor has a more serious impact on the performance degradation of the star sensor, but further analysis of the relevant mechanism is needed. The research conducted has analyzed the impact of space radiation on the performance parameters of star sensors from a system level perspective, laying the foundation for the research on prediction and correction techniques for on-orbit attitude measurement errors of star sensors.