Bepi Colombo, a future ESA mission to the planet Mercury, presents several key mission challenges- especially in terms of mass restrictions, power limitations and a harsh thermal and radiation environment. In order to pave the way for this mission a technology development programme was put in place with the goal of developing and demonstrating the critical technologies for the mission in time for the start of the main spacecraft design phase. Among the critical technologies identified as requiring development was the Autonomous Star Tracker (ASTR) which was required to be of extremely low mass and power while still being highly autonomous, very robust to solar flares and capable of high accuracy. A contract was let in January 2002 with Galileo Avionica, for the development, manufacture and test of the required ASTR. The developed sensor makes full use of novel technologies such as Active Pixel Sensors (APS), Multi Chip Modules (MCM) and Titanium Microcasting to achieve the required 3.5 arcsec (1 sigma) performance fully autonomously, and very robustly, in a single self contained 1.1 Kg, 3.7 Watt package. A test model of the sensor was manufactured and subjected to extensive testing to demonstrate all key aspects of the design. This paper describes the design of the sensor with emphasis on the new technology used and the algorithms required for robust operation of an ASTR using APS detectors. The paper goes on to describe the tests performed and the results achieved and concludes with a discussion on the steps now required to achieve full flight qualification. In addition to the main subject of the paper and in order to set the STR development described herein in the context of the wider APS based STR development in Europe, an overview and current status of the other ESA funded APS based STR developments in Europe is given by means of an introduction.
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