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4. CHALLENGES AND PROSPECTS OF UUV MOTION PLANNING AND TRACKING CONTROL
The motion planning and tracking control of the UUV has a promising future in the maritime projects of
underwater rescue, detection, investigation, tube pavement, creature study, and military strategy. Hence, there
is still a large requirement for thorough and systematic research in this field. In this section, the challenges and
prospects of the UUV motion planning and tracking control for underwater navigation are listed and analyzed.

4.1. Multi­UUV collaboration
In this section, possible future studies on the multi-UUV collaboration are given, mainly divided into the
multi-UUV collaboration considering the environmental effect and heterogeneous vehicles and the hunting
of dynamic targets.

4.1.1. Environmental effect and heterogeneous vehicles
Most researchers discuss the UUV assignment in an ideal underwater environment and regard the vehicle as
a pure particle, which lacks the consideration of the practical condition of the UUV operation. Therefore, the
complex environmental factors such as ocean currents effect, unpredictable seamounts, and various moving
obstacles should be involved in further studies. Moreover, the UUV system contains uncertainty that cannot
beaddressed initially, suchthat theheterogeneousvehicles ofdifferentmodel parameters, navigating velocities,
or safe distance are required to be studied. In addition, the topics of the formation control of heterogeneous
vehicles between UUVs and USVs (or even unmanned aerial vehicles) have become more attractive, as the
USV can help to instruct the UUV in real-time positioning through its less-affected GPS system and efficient
communication above the water surface [129,130] .

4.1.2. Dynamic targets
Most current UUV motion planning and tracking control studies concentrate on tackling static targets, such as
theunderwatersearchofcrashedvehicles, yetthefollowingorhuntingofdynamictargetsforUUVsisacrucial
issue in the military defense for the marine system [131] . The following or hunting of dynamic targets covers the
topics of dynamic task assignment, intelligence of the moving target, path planning while chasing the target,
and containment of the target completed by the UUVs, which are still waiting to be further investigated.

4.2. Efficient underwater positioning and path planning
The developed underwater positioning methods of real-time efficiency are required, as the conventional po-
sitioning and navigating systems such as GPS are not valid due to the poor communication affected by the
underwater environment. In addition, path planning in the underwater condition is the most vital part of
UUV motion planning, where some innovative intelligent planning methods can be applied in this field to
improve efficiency, such as reinforcement learning.

4.3. Robust underwater trajectory tracking
Owing to the complexity and uncertainty of the underwater environment and the nonlinear UUV system, the
robustness of UUV trajectory tracking has to be advanced to guarantee the UUV navigates as desired. There-
fore, the problems that affect the robustness of UUV trajectory tracking such as excessive speed references,
actuator saturation, and thruster damages are worth investigating [132] .

4.4. Real­time underwater recognition
Current approaches that can attain the identity information of the underwater targets are limited due to the
low invisibility and unpredictable obstacles of the surrounding environment; the inefficient communication
through the fluids in the deep-sea area also restricts the real-time recognition of the underwater target, thus
affecting the real-time UUV navigation to unknown targets. Hence, the advanced underwater target detection
techniquessuchasthemulti-sensorinformationinfusionapproachanddeeplearning-basedimagerecognition
can be applied in this field to process and achieve the results in time [133–136] .