  Seafloor anchor nodes, which form a geodetic network, are designed to provide
surface and underwater users with positioning, navigation and timing (PNT)
services. Due to the non-uniform distribution of underwater sound speed,
accurate positioning of underwater anchor nodes is a challenge work.
Traditional anchor node positioning typically uses cross or circular shapes,
however, how to optimize the deployment of reference nodes for positioning
underwater anchor nodes considering the variability of sound speed has not yet
been studied. This paper focuses on the optimal reference nodes deployment
strategies for time--of--arrival (TOA) localization in the three-dimensional
(3D) underwater space. We adopt the criterion that minimizing the trace of the
inverse Fisher information matrix (FIM) to determine optimal reference nodes
deployment with Gaussian measurement noise, which is positive related to the
signal propagation path. A comprehensive analysis of optimal reference-target
geometries is provided in the general circumstance with no restriction on the
number of reference nodes, elevation angle and reference-target range. A new
semi-closed form solution is found to detemine the optimal geometries. To
demonstrate the findings in this paper, we conducted both simulations and sea
trials on underwater anchor node positioning. Both the simulation and
experiment results are consistent with theoretical analysis.
