A Global-Shutter Centroiding Measurement CMOS Image Sensor With Star Region SNR Improvement for Star Trackers

Abstract

A star tracker is a critical sensor for determining and controlling the attitude of a satellite. It utilizes a complementary metal–oxide–semiconductor (CMOS)-active pixel sensor to map the star field onto the focal plane. Starlight is measured and star centroids are calculated to estimate attitude knowledge. In this paper, we present a CMOS image sensor for star centroid measurement in star trackers. To improve sensitivity to low-level starlight, the capacitive transimpedance amplifier pixel is used as the detector. To improve centroiding accuracy, the proposed sensor architecture allows star pixels, pixels that are above a star threshold, to cluster together. The mean value of all the pixels in this cluster is calculated. The star signals are then amplified in relation to this mean value. This increases the signal-to-noise ratio in star regions in line with their starlight intensity. An adaptive region-of-interest readout architecture is also proposed, which reports only star regions instead of the entire frame. The proof-of-concept chip, containing a $128 \times 128$ pixel array, was fabricated using AMS 0.35- $\mu \text{m}$ CMOS Opto process. Each pixel has a size of $31.2 \times 31.2~\mu \text{m}^{2}$ . The measurement results show that centroiding accuracy increases with higher centroiding gain. Within a limited exposure time, the relative centroiding accuracy can surpass that of a commercial image sensor by more than 1%.