Performance Analysis of Star Sensor with APS Photodetectors

This paper presents a performance analysis for star trackers that adopt Active Pixel Sensor photodetector technology. They have many advantages on Charge Coupled Devices such as low power consumption, single voltage supply needed (3.3V or 5V), increased radiation hardness, simplified qualification procedures, capability of local reset and readout, onchip integrated timing, control, and analog-to-digital conversion. Unfortunately, their noise level is higher than in Charge Coupled Devices. Anyway, this effect can be compensated by taking advantage of their capability to manage different local exposure times. This allows for tracking bright stars at a faster tracking frequency than the one that is selected for Charge Coupled Devices, whereas a slower frequency is set for dim stars. The resulting temporal average of the number of observed stars is closer than the one of single shutter systems to the number of stars observed by Charge Coupled Devices. As a consequence, also the accuracy in attitude determination increases because it is a function of the number of detected stars. An analytical investigation of the attainable accuracy is discussed. All sensor noise sources are considered and their effects are evaluated as a function of the exposure time. The resulting accuracy turns out to be in the range 1-3 arcsec that is the one requested for low cost star sensors.