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In lunar landing missions, it is very important to closely estimate the horizontal velocity of a descending spacecraft for achieving a successful and safe lunar landing. The purpose of this paper is to present a novel, vision-aided approach for the accurate, efficient, and robust estimation of such horizontal velocity. Our algorithm processes images from a downward-looking camera, as well as attitude...
Two characterization theorems for the class of context-free grammatical families are given. The characterizations are expressed in terms of the family of regular sets, the family of linear sets, and the following language-family operators: union, concatenation, the full semi-AFL operator, the full AFL operator, and a new ternary substitution operator.
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