  Underwater localization is of great importance for marine observation and
building positioning, navigation, timing (PNT) systems that could be widely
applied in disaster warning, underwater rescues and resources exploration. The
uneven distribution of underwater sound velocity poses great challenge for
precise underwater positioning. The current soundline correction positioning
method mainly aims at scenarios with known target depth. However, for nodes
that are non-cooperative nodes or lack of depth information, soundline tracking
strategies cannot work well due to nonunique positional solutions. To tackle
this issue, we propose an iterative ray tracing 3D underwater localization
(IRTUL) method for stratification compensation. To demonstrate the feasibility
of fast stratification compensation, we first derive the signal path as a
function of glancing angle, and then prove that the signal propagation time and
horizontal propagation distance are monotonic functions of the initial grazing
angle, so that fast ray tracing can be achieved. Then, we propose an sound
velocity profile (SVP) simplification method, which reduces the computational
cost of ray tracing. Experimental results show that the IRTUL has the most
significant distance correction in the depth direction, and the average
accuracy of IRTUL has been improved by about 3 meters compared to localization
model with constant sound velocity. Also, the simplified SVP can significantly
improve real-time performance with average accuracy loss less than 0.2 m when
used for positioning.
