Active pixel sensor (APS) based star tracker

Charge-Coupled Device (CCD) based star trackers provide reliable attitude estimation onboard most 3 axis stabilized spacecraft. The spacecraft attitude is calculated based on observed positions of stars, which are located and identified in a CCD image of the sky. A new photon sensitive imaging array, active pixel sensor (APS), has emerged as a potential replacement to CCDs. The APS chips utilize existing Complementary Metal Oxide Semiconductor (CMOS) production facilities, and the technology has several advantages over CCD technology. These include: lower power consumption, higher dynamic range, higher blooming threshold, individual pixel readout, single 3.3 or 5 Volt operation, the capability to integrate on-chip timing, control, windowing, analog to digital (A/D) conversion and centroiding operations. However, because the photosensitivity of an APS pixel is non-homogeneous, its suitability as a star tracker imager has been unknown. This paper reports test results of a 256×256-pixel APS chip for star tracker applications. Using photon transfer curves, a system read-out noise of 7 electrons, under laboratory conditions, has been determined (photogate type). The full well of an APS pixel is determined to be around 450,000 electrons. Utilizing astronomical observations, the sensitivity of APS was measured to 13600 e^-(second.mm²) for a 0^th magnitude star. Centroiding accuracy of the APS was in the order of 1/10 pixel. The dynamic range of the APS was better than 9 magnitudes. These measurements allow us to conclude that the APS is a potential replacement for CCD star trackers