In marine environments, autonomous USVs are increasingly deployed to the missions that require a very long patrol time in dangerous areas. It is very advantageous to utilize such unmanned system due to the facts that those vessels are being operated without humans onboard, while the vessels can be manufactured at a considerably lower cost. Because of those inherent benefits of unmanned systems, there has been a steadily growing interest all around the world [1] [2] . In recent years, the military USVs have been actively developed around the world and major missions are being expanded not only for marine research but also for military purposes such as target tracking, surveillance, reconnaissance, mine sweeping, and anti-submarine warfare [3] . In many ways, the USVs are very different from the manned counterparts. The manned vehicles are constantly monitored and stationed by the operators, who actually feel and sense the surroundings and the workings of the vehicles. Therefore, the manned systems have no problem in terms of responding to the changing situations [4] [5] . In this regards, the control of the manned vehicles can be instantaneous and immediate. However, the unmanned vehicles (USVs) are completely different [6] . The USVs are remotely operated without any human operators onboard. This means that any changes in the surroundings of USVs must be sensed by the onboard sensing devices, and electronically transmitted to the remotely situated human operators. The human operators, in turn, monitor the sensor signals, and internally construct the 3D images of what the USVs are facing. The sensitivity of the sensing devices and how the sensed information is graphically presented to the human operators can greatly influence the way in which the remote vehicles are responding to the changing environments [6] [7] . Therefore, the USV itself and the ground control station (GCS) where the remotely situated human operators are controlling the USV have to be concurrently designed and considered in terms of how to reach the intended functions and performance levels for the USVs [8] . In this regards, according to our initial study, the majority of the USVs that have been developed by many nations resemble each other as to the vehicle shape, the operating methods, and the missions that are being carried out [9] [10] . The purpose of this paper is, therefore, to investigate the recently technological development of USVs and identify the concept of USV operations. Such analysis has not been properly addressed, hence the study results will provide good insights towards the development of the USVs in the complex marine environment [9] [10] [11] [12] [13] .

The main differences between the USVs and the manned surface vehicles include the following. 1) The overall mechanical workings of both vehicles are almost identical. However, the USVs must have onboard communication devices that can constantly transmit the various signals (i.e., system status data and sensor signals) to the GCS. 2) The USVs must have a ground control station for remote operations. 3) Most recent USVs are equipped with intelligent and autonomous functions for navigation and mission execution. The main missions that can be performed by USVs in military are classified into intelligence, surveillance and reconnaissance (ISR), mine countermeasure (MCM), anti-submarine warfare (ASW), special operations support, rescue operations, and electronic warfare [14] . In fact, the application areas are increasing every year, and the trend will continue in the foreseeable future. In this paper, we classified development cases based on ISR, MCM, ASW missions. We analyzed most countries, which include USA, Israel, Singapore, and other countries that are actively developing advanced marine unmanned vehicle systems. We identified the trends of the technological advances, while suggesting the future directions for development.

In case of US, a research level of the USV technology has been advanced significantly but most countries still lack technology to commercialize it [25] . In this study, the technology fields to be suggested through our analysis are largely autonomous system, collision avoidance, launching and recovery, weapon operation and common infrastructure technology for operation. In addition, to improve operability, it is important to upgrade the onboard equipment and to standardize the interface technology and modularization among the equipment. The USVs can be categorized by the size, fuel and operation range. Those determine the endurance of the USVs, which can be very important for reconnaissance mission. The onboard sensors determine the type of missions that the USVs can perform. The autonomy level also determines the USVs flexibility in terms of when the remote signals are lost or when the situation changes too rapidly for the remotely situated human operators can handle. In this regards, Table 6 shows the important design factors closely related to the required performance in USV development.

In this study, we derived factors and functions to be considered in the USV design through analyzing development cases of USV in major countries. The demand of USVs which can be autonomously operated without the operator intervention has been increasing dramatically. The growing interests lie in the facts that those USVs can be manufactured at much lower costs, and can be operated without the onboard human operators, while can be sent to the hostile or quite dangerous areas without the fear of losing human lives. The utilization and the deployment of such vessels will continue to grow in the future and the future will show the vessels with much higher autonomy levels. In such cases, the USVs can be operated at a much longer time without the intervention of human operators, which can fundamentally change the way the maritime warfare has been conducted. Additionally, the most important trend is the intelligence and autonomy levels of the USVs. With the advent of the AI (artificial intelligence, i.e.,

deep learning), previously impossible task such as recognizing enemy vessels through the camera images can be automated. In the future, the capability of the USVs will be highly dependent on the technology levels of AI and the autonomy along with the types of onboard sensing devices.

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) Analysis of Design Directions for Unmanned Surface Vehicles (USVs). Journal of Computer and Communications, 5, 92-100. https://doi.org/10.4236/jcc.2017.57010