A decentralized controller for target tracking and obstacle avoidance for a multi-agent system is proposed. The controller is based on a navigation function which is composed of a goal function, as the D-optimality criterion of the Fisher information matrix, and a constraint function, which generates repulsive force to the agents near the obstacle. The multi-agent system is modeled as a graph and each agent moves independently by the control input calculated based on the information accessible for the agent. It is proved that the multi-agent system converges to the critical points where the navigation function achieves its global minimum by acquiring maximum information of the target while avoiding collision with the target. An exemplary simulation study is given for a multiple unmanned aircraft system.