Sun sensors are widely used in spacecraft attitude determination subsystems to provide a measurement of a sun vector in a spacecraft coordinate. Two categories of sun sensors commonly used in the past are an analog sun sensor and an optical coding based digital sun sensor. On the basis of the photovoltaic effect of a photocell, the analog sun sensor outputs analog current. Its resolution and accuracy of determination is too low to meet the requirement of wide field of view (“FOV”) and high accuracy in spacecraft attitude determination. Compared with the analog sun sensor, the optical coding based digital sun sensor has higher accuracy, but its heavy photo detector and complicated control of accuracy makes it too large compared to the size of a micro spacecraft or a small rover, and therefore limits the improvement of the accuracy of determination.
A better solution is a charge couple device (“CCD”) or a CMOS APS based sun sensor. The CCD and CMOS APS based sun sensors are capable of achieving higher accuracy and avoid the drawbacks mentioned above. Further, with the development of CMOS APS technology, its performance exceeds that of CCD sun sensors, especially in power and anti-radiation. The availability of APS technology (in which detection and an A/D circuit are integrated on a single chip) and micro electro mechanical systems (“MEMES”) technology has led to the development of a miniature sun sensor for attitude determination. At the Jet Propulsion laboratory at the California Institute of Technology, there was an ongoing multiyear research of micro sun sensors utilizing MEMES technology. In the project, a silicon wafer mask with multiple apertures was placed on the top of a CMOS APS focal plane array. Sunspots from the apertures were formed on the focal plane when sunlight illuminated on the mask. The attitude angle was capable of being derived by analyzing the precise location of the sunspot centroid on the detector. Although the sun sensor can achieve higher accuracy, the centroid calculation and conversion into attitude angle function are done in spacecraft flight computer, which increases the load of the computer substantially, and basically, the computer has to read all pixels, which limits the update rate of the sun sensor. The sun sensor has concentrated on the optical and accuracy aspects, not on the integration of optical system with process electronics. The research of APS based sun sensor has also been carried out at a number of studies in China. But most of these studies have concentrated on design of the optical system of the sun sensor.
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