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Abstract: For tracking problem，general literatures just compute the time of catching up with object or under the known position，where tracker catches up and the ratio of velocity between them is calculated. Obviously，it is more important to obtain analytic solution. This paper obtains the general expression about the question by establishing and solving 2nd-order ODE. It deduces the same conclusions with other literatures under the known onditions at last.

Key words: tracking problem, movement locus, 2nd-order ODE, step-down order , analytic solution