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The first two indigenously developed communication satellites of India at Geosynchronous orbit (GEO) Insat-2A and Insat-2B have an oscillatory type earth sensor for providing the pitch and roll reference to the momentum biased, 3 axis stabilized control system. The sensor consists of an oscillatory resonant scan mechanism which provides a East-West scan field of +/- 25 degrees. The excellent on orbit performance of the above sensor and also its advantages such as simplicity and long life have given rise to explore the possibility of using such a sensor for low earth orbit (LEO) mission where the subtended angle is large, i.e. nearly 120 degrees. This paper presents a design approach and test results of a germanium wedge prism adapter which separates the scan in the North-South direction by +/- 45 degrees from the existing +/- 6.1 degrees in the GEO sensor and also extends the scan field in the East-West direction from +/- 25 degrees to beyond +/- 60 degrees providing the four horizon cross over points for the computation of pitch and roll errors of the spacecraft. Thus with a marginal change in the optics and a marginal reduction in the signal, all the advantages of the GEO sensor such as long life, higher update frequency, lower weight can be made use of for the LEO mission. A comparative study is also made with the conical type scanning horizon sensor using a motor driven mechanism with the above sensor and the advantages outweight the disadvantages. The development model results show the technical feasibility and confirm the advantages of the sensor for LEO application.
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