We propose a sensor positioning scheme for a wireless sensor network consisting of beacons as well as collaborative anchors (CA) to help sensors within a prescribed service area to locate themselves. We assume a robust performance is achieved in the sense that a prescribed location accuracy requirement is fulfilled within the service area. Under the assumption that the time-of-arrival and location estimators adopted achieve the associated Cramér-Rao bound, the performance of the location scheme is derived and analyzed. A ranging energy optimization problem is proposed, and a practical algorithm is presented. The effectiveness of this algorithm is illustrated by numerical experiments.