Recently, as the development of advanced science technology and military unmanned system is continuing at an increasing pace, the pattern of warfare in the future is also changing. The warfare situation in the marine environment is expected to show a different pattern from the existing naval warfare due to introduction of unmanned underwater vehicles [1] [2] . A lot of advanced countries are spurring on the development and commercialization of UUVs [3] . UUVs are typically submarines that are capable of autonomous or remote operations. UUVs can carry out underwater missions that have been very dangerous to humans onboard or that require a very long mission time. The purpose of UUVs is generally classified into intelligence, surveillance and reconnaissance (ISR), mine countermeasure (MCM), and anti-submarine warfare (ASW). The main reason for the development of this UUV is that there are many military benefits to be gained from asymmetric forces like submarines. There is little chance of being detected by enemies and there are fewer acoustic and magnetic signals because all missions are carried out in underwater. The vessels can be made at a significantly lower cost than the manned submarines, and because it’s unmanned, the size of the vessels can be made significantly smaller as well. Additionally, UUVs can be sent to the hostile and dangerous missions without fear of losing human lives. It can also relieve human operators from monotonous and dull missions [4] . However, as it performs missions in underwater, it must operate in a harsh environment under the high ocean current and heavy hydraulic pressure. Despite, the US and other advanced countries of military forces are increasingly developing and operating UUVs capable of the precise underwater navigation [5] . Due to many merits that those UUVs provide, the use of such systems is expected to rise rapidly in the near future [6] [7] [8] [9] [10] . The technology associated with the development of UUVs is also increasingly getting sophisticated. In this regard, the analysis of the UUV development needs to be conducted, which suggests the technological advancement of the systems. However, such analysis has not been properly conducted, according to our initial study [11] [12] [13] [14] [15] . The novelty of this paper lies in the fact that the major UUV development has been investigated, and then provides the insights towards the trends of the technological development and the most important aspects of the core technological advancement.

Currently, the United States is most actively developing and operating the marine unmanned systems. Recently, the development trends of advanced countries have greatly emphasized the concept of autonomous operations for ISR, MCM, and ASW. Below is the status of UUV development by major countries [6] - [15] .

In this section, we derive the required functions and the technologies of UUVs from the UUV development in major countries. The UUV design should be evaluated according to each mission through a detailed comparative analysis. In addition, to improve operability, it is important to have the advanced onboard equipment and to standardize the interface technology and modularization among the equipment. The UUVs can be categorized by the hull size, types of energy source and operation range. Those determine the endurance of the UUVs, which can be very important for the reconnaissance mission. The onboard sensors and mission equipment determine the type of missions that the UUVs can carry out. Since the UUVs are operating in deep waters where the constant connection between the remote ground operators can be highly difficult, the precise navigation capability becomes the critical issue. The autonomy level also determines the UUVs flexibility in terms of when the situations change too rapidly for the remotely situated human operators can handle. In this regards, the important information is shown in Table 6. In order for the information to be applied to the UUV design, the detailed requirements should be derived based on this result, which represent the most critical required functions. Those include the efficient energy source, a high level of autonomy, effective launch and recovery system, communications system, and the mission equipment onboard. All of these critical functions need to be developed, in order for the UUVs effectively operate as intended.

In this paper, through the investigation of many UUVs that are currently under development, we have identified the most important aspects of UUVs. Those include the efficient energy source, a high level of autonomy, effective launch and recovery system, communications system, and the mission equipment onboard. All of these critical functions need to be developed, in order for the UUVs to effectively operate as intended. In this regard, the main contribution of this paper is the fact that the most important required functions have been identified, if any UUVs need to be successful. Unmanned underwater vehicles represent a mili- tary equipment that can be used to covertly detect and engage in enemy harbor areas or under the enemy waters. Also, considering the development trend worldwide, it is expected that UUV systems of the higher performance levels shall be developed in the near future. The UUVs will fundamentally alter the

way the war is being fought. Regardless of the shallow waters or deep sea waters of the enemy territory, the UUVs will be increasingly deployed to conduct highly dangerous missions that have never been possible by legacy naval system. The major countries around the world already see the potential of UUVs in the maritime warfare and continue to develop them into a higher level. Such trend will continue in the foreseeable future.

This work was supported by the Agency for Defense Development (ADD) under the Contract No. UD160008DD. The authors wish to express sincere gratitude for the financial support.

Heo, J., Kim, J. and Kwon, Y.J. (2017) Technology Development of Unmanned Underwater Vehicles (UUVs). Journal of Computer and Communications, 5, 28-35. https://doi.org/10.4236/jcc.2017.57003