This paper presents the hierarchical sensor network system for robust localization. This system consists of parent nodes with a low priced LI GPS receiver and child nodes equipped with an acoustic ranging device. Relative positions between child nodes are estimated based on acoustic ranging through the inverse Delaunay algorithm. This algorithm localizes all the nodes simultaneously, thus, the accumulation of the error in the localization is suppressed. Relatively localized child sensor nodes are given global coordinates with the help of GPS on parent nodes. Field experiment was conducted with three GPS parent nodes and twenty-one child nodes (MOTE).