Variation of Pointing Vector Direction for Moon-Based Observations.

The variation of pointing vector direction for Moon-based Earth observations are analyzed in this paper. The pointing vector direction is parameterized as two angles, namely azimuth and elevation angle. We first establish the observation geometry model to calculate the pointing vector in Moon-centered Moon-fixed (MCMF) coordinate system. Then, the azimuth and elevation angles are defined and their theoretical expressions are deduced respectively. Besides, different observation periods are considered. It is found that the pointing vector direction shows regular variations over one orbital period and 18.6 years. The combination of azimuth and elevation angles can characterize variations of pointing vector direction. They mainly depend on the lunar sub-Earth points. The elevation angle ranges from 83 degrees to 97 degrees and the azimuth angle varies from -8 degrees to 8 degrees in an 18.6-year period. These characteristics will give valuable guidance for application of Moon-based platform.