This letter investigates how to place the received-signal-strength (RSS) sensors to improve the static target localization accuracy in the three-dimensional (3-D) space. By using the A-optimality criterion, i.e., minimizing the trace of the inverse Fisher information matrix (FIM), a new optimal RSS sensor placement strategy is developed when sensors can be placed freely in the 3-D space. The smallest reachable trace of Cramér–Rao lower bound, i.e., the inverse FIM, is derived with the corresponding optimal sensor-target geometries. Besides, a resistor network method and a special configuration strategy are proposed to quickly determine the optimal geometries. The findings are concluded in three remarks, which are used to evaluate and improve the estimation accuracy. Simulation examples verified these findings.
