High accuracy sun sensor using CCDs

Abstract

Introduction A new type of two-dimensional sun sensor with the accuracy of a few arcseconds and the field of view (FOV) of 2 degrees by 2 degrees has been developed. The sensor consists of a pair of periodic pattern reticles and associated linear-array-chargecoupled-devices (linear CCDs). The solar aspect is measured by detecting the phase of a periodic image pattern on each CCD, which is formed by the solar light passing through the reticle. The sun sensor will be flown on Solar-A mission of ISAS in 1991, the attitude of which satellite will be controlled better than few arcseconds stability to obtain fine solar X-ray images by an X-ray mirror telescope which is being developed jointly by ISAS and NASA. Recent scientific sapceborne missions requires precision attitude pointing to satisfy their objectives. Thus, sun sensors are required to have higber accuracy. For example the Solar Maximum Mission (SMM) utilized the pointing sun sensor with an accuracy in linearity of f20 arcseconds (0.0055 degrees) within the range of f 0.5 degrees1. Few years ago, the authors developed a CCD sun sensof with accuracy of 0.025 degrees and the field of view of 100 degrees by 100 degrees, and employed them on the X-ray astronomy satellite GINGA launched in 1987 and the exospheric research satellite EXOS-D scheduled for launch in early 1989. With the start of the Solar-A Project, a high energy solar physics mission to be launched into orbit in 1991 by the Mu-3SII-6 vehicle, the need has arisen to develop a more accurate sun sensor system that can satisfy the sun-pointing attitude control requirement; 0.01 degrees absolute pointing accuracy with shortand mid-term attitude stabilities of better than 1 arcsec/sec and 5 arcsec/min, respectively. The authors, therefore, has started the development of the sun sensor described IMAGE below, using the same type of linear array CCDs as before but based upon the new principle of operation to realize the required accuracy and stability.