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USING THE EARTH'S UMBRA TO MAINTAIN ORBITAL PERIOD
FARRELL,J.F.
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Abstract
A method is described whereby the umbra of the earth may be utilized to obtain an on-boar indication of the orbital period of a satellite. It is found that, for certain angles of inclination, the shadow of the earth provides an accurate timing mechanism and that, by measuring orbital passage through a period of several revolutions, qualitative information regarding orbital elements can be developed within the satellite. Optimum results are obtained when the orbital plane lies in the ecliptic plane; however, certain other orbital inclinations can be tolerated. Oblateness of the earth is considered and found to be of no consequence. (Author)