In all previous coverage problems, targets or areas are required to be monitored all the time. But in many applications typically featured as patrolling, target points are only required to be monitored with certain frequency. For example, a set of static sensors are distributed in a forest, surveilling ecological environment. Information collected by those static sensors are gathered by a set of mobile sensors, say, at least once every three hours. One may be wondering what is the minimum number of mobile sensors that are needed to accomplish such a task, and how to design trajectories for those mobile sensors. Motivated by such a consideration, Cheng et al. proposed the following dynamic coverage problem.