A micro-digital sun sensor (μDSS) is a sun detector which senses a satellite’s instant attitude angle with respect to the sun. The core of this sensor is a system-on-chip imaging chip which is referred to as APS+. The APS+ integrates a CMOS active pixel sensor (APS) array of , a 12-bit analog-to-digital converter, and digital signal processing circuits. The μDSS is designed particularly for microsatellite applications, thus low power consumption is the major design consideration. The APS+ reduces power consumption mainly with profiling and windowing methods which are facilitated by the specific active-pixel design. A prototype of the APS+ which is designed in a standard 0.18-μm CMOS process is presented. The APS+ consumes 21 mW at 10 fps, which is 10 times less than the state of the art. In order to improve noise performance, a reset noise reduction method, quadruple sampling (QS), is implemented. QS reduces the effect of the reset noise compared to the conventional delta double sampling method, even in a 3-transistor active pixel structure. The APS+ obtains an accuracy of 0.01 deg with the QS method.