Position-based ad hoc routing algorithms have proved to have decent performance in delivery ratio and end-to-end delay. Position service is essential for such algorithms. In this paper, a distributed position service system, named VDPS, is proposed and evaluated. In the system, each node has a virtual home region (VHR), which is a geographic area. Nodes residing in a node?s VHR will act as its position servers. A node updates its position to the servers, through which other nodes can obtain its position. To reduce the overhead on position management and simplify the operation, in VDPS, a VHR is divided into subregions. A region-based broadcast is used for position update, and a sequential searching is used for position retrieving. Different mechanisms for improving system robustness have been proposed. Mathematical models are built for analyzing system robustness and control overhead. Illustrated results show that VDPS can maintain a high system robustness at a relatively low control overhead.