Earth-sun attitude sensor: Hardware design and ground tests
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Earth-Sun Attitude Sensor (ESAS) is a proposed instrument which, observing the Earth in the visible spectrum by a CCD camera, would evaluate the Nadir and the Sun rays direction, that is, sufficient data for a complete three-axis attitude estimation. ESAS (and Moon-Sun Attitude Sensor, MSAS) feasibility study was funded by the Italian Space Agency. The ESAS instrument, which does not require tracking (as MSAS does), could have a large use in all of the satellites having an axis constantly pointed to the Earth (e.g., typical Sun and Geo synchronous satellites). This paper analyzes the ESAS operating modes (in terms of terminator visibility, Sun blinding, and Sun eclipsed by the Earth), and then proposes a ground test equipment (which includes a two-axis equatorial mount, a GPS receiver, a notebook, and two digital video cameras), to quantify the instrument accuracy. Since MSAS and ESAS use the same basic idea, this can be accomplished by the Moon observation and with two different ground tests. These tests, which will be soon performed, are: 1) a comparative test, where the attitudes estimated by the Moon and the star observations are compared, and 2) a differential test, where the attitudes evaluated in different times, would differ by a known rigid rotation matrix (associated with the Earth spinning).
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