Wide dynamic range CMOS image sensor for star tracking applications
Date of Issue2015
School of Electrical and Electronic Engineering
VIRTUS IC Design Centre of Excellence, Nanyang Technological University
Recent trends in space technologies highlight the importance of star trackers. Star trackers are optical-electronic devices that measure starlight directions. They are the most accurate sensors for 3-axis satellite attitude estimation. Current generation star trackers generally use CMOS image sensors as their star sensor thanks to this type of sensor’s inherent advantages of low power, low cost and, more importantly, their ability to integrate system-on-a-chip technology. This high degree of integration reduces process complexity and improves efficiency in attitude data processing. It also allows smaller, lighter star tracker designs, a distinct advantage in miniaturized space platforms. Conventional CMOS image sensors, however, do not meet design specifications for use in a star tracker, where several important limitations and design challenges have to be overcome. Most off-the-shelf CMOS image sensors are not space qualified to withstand the space radiation environment. Not only can radiation effects significantly exacerbate a sensor’s performance degradation, but they can also permanently damage the sensor itself. Radiation tolerance is therefore a critical prerequisite for such charge-sensitive devices destined for use in space applications. A wider dynamic range is also required if a wide range of star brightness is to be detected accurately, while active pixel sensors in deep submicron CMOS technologies fall short of meeting aggressive high dynamic range requirements due to their supply voltage limitations. Moreover, high-sensitivity pixels and low-noise readout circuits capable of achieving a high signal-to-noise ratio are essential for high centroiding accuracy. This thesis focuses primarily on the above-mentioned design aspects of CMOS image sensors. Its main contributions can be separated into three parts. Firstly, radiation-induced impact on active pixel sensors were investigated. Different radiation hardening techniques were validated using silicon implementations to outline proper design guidelines for CMOS image sensors to be used in space applications. Secondly, two novel wide dynamic range CMOS image sensor architectures were proposed. One of them allows adaptive integration time for each pixel and expedites the readout using multiple readout channels. The other applies a dual-exposure charge subtraction strategy to extend dynamic range, and, accordingly, a new high-sensitivity capacitive transimpedance amplifier pixel was proposed. Finally, with very large scale integration (VLSI) architectures to improve centroiding accuracy in mind, a global-shutter CMOS image sensor with star-region SNR improvement was proposed.
DRNTU::Engineering::Electrical and electronic engineering::Integrated circuits