The importance of unmanned surface vehicles (USVs) and unmanned underwater vehicles (UUVs) has recently increased as various marine resources, such as petroleum and gas, are gaining importance (Bentley, 2002; Nicholson and Healey, 2008). Although massive amounts of resources are present in the oceans, marine exploration is difficult to perform due to ocean currents and weather conditions (Hou, 2013). Particularly, the operation of a UUV in water poses difficulties that are not encountered on land, such as difficulties in supplying power, communication issues, low visibility, and water pressure. Recently, studies on platforms that combine USVs and UUVs have been performed to resolve such difficulties. Sarda and Dhanak proposed a hybrid surface and underwater vehicle (HSUV) that assists in launching and recovering vessels using tether cables (Sarda and Dhanak, 2017). However, this platform is limited in terms of power supply and real-time transmission of large amounts of data. Zwolak et al. proposed a similar platform that can additionally acquire large amounts of data (Zwolak et al., 2017); however, this platform faces difficulties in real-time data transmission and power supply for long-term operation. In addition, it is difficult to obtain the real-time location of a UUV.

To resolve the aforementioned limitations, an HSUV that combines a USV and a UUV, retaining their advantages, is proposed in this study. The USV used is a stable catamaran designed to easily accelerate (Molland et al., 1994). The UUV used is also a catamaran that receives power from the USV, liberating space that would be occupied by batteries and increasing the payload capacity. Sensor data of the UUV can be verified in real time on land through a tether cable, and control commands can be transmitted. The power supplied from the USV enables UUV to be operated for a long period of time using its function for real-time communication with land. In addition, images from the UUV can be acquired in real time on land. The real-time relative locations of the USV and UUV can be identified by utilizing ultra-short base line (USBL) sensors.

The system components, kinetics, and control algorithm of the developed HSUV are described in Sections 2, 3, and 4, respectively. Kinematic performance and control test results from offhshore tests are described in Section 5.

Fig. 1 shows the structure of the HSUV constructed for this study. Operation console commands are transferred to the USV and UUV via a hub through wireless bridges, and USV and UUV are connected through a tether cable. The total weight of the HSUV, excluding the operation console, is 350 kg.

The dynamics of the HSUV is based on the Newton–Euler equations and vectors. The USV moves along the x-axis and y-axis; hence, its motion can be expressed as surge, sway, and yaw. The notation associated with each axis is presented in Table 2. Considering the Coriolis centripetal force and attenuation acting on the USV, the dynamic equations can be expressed as Eqs. (1)–(3). The detailed definitions for the parameters and states are described in Fossen (2011) and Cho et al. (2020).

Here, m is the mass, I is the mass moment of inertia, and τ is the thrust of two rear thrusters attached to the USV.

Eqs. (4)–(6) represent the motion of the UUV. The UUV follows the considerations for USV, but surge, sway, and yaw are considered as relative locations and relative direction angles. The subscript R in Eqs. (4)–(6) was used to avoid confusion with the equations of the USV. The symbol on the right side of sigma represents the sum of environmental disturbance and thrust.

Offshore tests were performed on the relatively calm coast of the Korea Maritime and Ocean University to evaluate the performance of the proposed HSUV and its control system. The offshore tests include accuracy tests of the GPS, which is the key sensor on land, and communication tests. Based on these tests, route tracking tests of the USV, DP, and performance tests of the side scan sonar were performed.

Tests for the communication distance of the platform were performed prior to the main tests. The communication equipment used in the unmanned vessel has a high effective transmission rate of 360 Mb/s and additionally uses an omnidirectional antenna.

The test results indicated that data are effectively received and transmitted up to 890 m in the absence of obstacles between thewireless communication devices. Fig. 10 shows a picture of the offshore tests performed.

In this study, the structure and control system of the HSUV were designed and constructed and basic performance tests were performed. The sensor data of the USV and UUV can be verified in real time through the tether cable, and the UUV can be used for a long period of time. The individual sensor performance tests were performed, followed by offshore tests using the controller algorithms constructed in Section 4 to verify the performance. The maximum error of the WPs of the USV was 2.60 m, and the average DP error was 2.03 m, which indicates the excellence of the control system. In addition, the side scan sonar of the UUV was used in the same sea area to explore the bottom of a river. The river, surrounding terrain, and fish search information could be monitored in real time at the land station by utilizing the real-time communication system.