Looking Vector Direction Analysis for the Moon-Based Earth Observation Optical Sensor

As the earth's only natural satellite, the Moon has unique advantages as an earth observation platform because of its long observation distance and the vast expanse of the lunar surface. To observe the earth from the Moon, the looking vector direction and its variation regularity need to be investigated. In this paper, we first establish a Moon-based earth location algorithm to reveal the variations of the looking vector. In this algorithm, assuming the sensor points to the earth, the looking vector expressed as azimuth and elevation angles is calculated, and the intersection point between the looking vector and the earth's ellipsoid is solved using the relevant equations. The mathematical expression of the observational scope is also derived so as to determine the looking vector range if the looking vector direction is limited in the earth disk. Based on this algorithm, we describe the earth's motion in the lunar topocentric coordinate system and analyze the looking vector direction characteristics by assuming different positions on the lunar surface. In addition, the earth location error is evaluated by adding the looking vector errors. The results show that sensors at different positions on the lunar surface have different looking vector pointing characteristics and different earth location errors. The earth location error is relatively small in the high-latitude region of the Moon. Combining the restriction of lunar local topography, we suggest that the regions that latitude between 30 °N and 80 °N or 30 °S and 80 °S are the ideal locations for the Moon-based earth observation platform.