The main technical challenges faced by USV in acquiring long-term meteorological and oceanographic observations in the deep ocean include improving its long-term persistence and increasing its survivability under extreme sea conditions, especially during the passage of typhoons/hurricanes (US DOD, 2013). Therefore, the IAP USV team developed two types of long-term unmanned autonomous marine meteorological observation USVs, which are the solar-powered USV and the diesel-powered unmanned semi-submersible vehicle (USSV). From 2017 to 2020, the USV and USSV have completed dozens of sea tests and trials, and have sailed thousands of kilometers in total. The world's first rocketsonde was launched from the USSV in 2017 (Chen et al., 2019b). The IAP USV is a long-term, rugged, and unmanned autonomous navigation vessel (Fig. 1). The USV can automatically deploy, collect and transmit observations, and can complete unmanned measurements under extreme sea conditions. Details about the USV design and performance are provided in Table 1. The vessel can carry a payload of 20 kg for an automatic weather station (AWS), sea surface temperature (SST) and seawater salinity sensors, wave sensors, and pyranometers. China’s Beidou communication satellites allow for the measured data to be automatically transmitted to the ground control center, where high temporal resolution temperature, humidity, air pressure, wind direction, wind speed, SST, seawater salinity, and total shortwave radiation can be obtained in real-time. The technical specifications of meteorological and hydrological sensors are shown in Table 2.
Figure 1. The IAP solar-powered USV conducted sea trials in a sea area near Tanmen Port of Hainan Island in June 2020.
Technical parameters The solar-powered USV Observation modes Navigation mode, positioning mode, drifting mode Weight 80 kg Size 4.6 m×2.0 m×1.2 m Speed 3–5 knots Endurance 30 days Positioning accuracy 10 m Payload 20 kg Communication Beidou satellite communication Data interface USB/RS232 Power Solar energy/24 V Power consumption 400 W Sea states 6–7 Communication frequency 1–60 mins (adjustable) Observation parameters Temperature, humidity, air pressure, wind direction, wind speed, SST, salinity, and total shortwave radiation
Table 1. Main technical parameters of the solar-powered USV.
Observation parameters Technical specifications (range and accuracy) Temperature −25~55°C, 0.2°C Air pressure 850~1150 hPa, 0.5 hPa Relative humidity 0~100%, ±3% Wind speed 0~40 m s−1, 1 m s−1 Wind direction 0~360°, 2~5° Total shortwave radiation 0~2000 W m−2, 3% Seawater temperature −5~35°C, 0.01°C Conductivity 0~80 mS cm-1, 0.01 mS cm-1
Table 2. Technical specifications of the meteorological and hydrological observation parameters
Compared with traditional meteorological and oceanographic observation platforms such as buoys, the main technical advantages of the USV are as follows:
(1) Automatic deployment, collection, and transmission of data.
(2) Powered by solar energy, the USV can carry out long-term and long-range continuous meteorological and oceanographic observations.
(3) The semi-submersible structure minimizes the effect of waves on vehicle motion, making the USV very stable and allowing it to survive and collect data under extreme sea conditions (sea state 6–7).
(4) Real-time acquisition and Beidou satellite transmission of high temporal resolution (up to one minute) observations.
The main components of the USV system are the USV hull, USV control system, power system, communication system, meteorological and hydrological observation loads, auxiliary unit, and a ground control center. A block diagram of the system is shown in Fig. 2. The meteorological and hydrological sensors were calibrated before and after the observation experiment. The USV can be self-righting, semi-submerged, and has a high waterproof grade, and can work under severe sea conditions (sea state 6–7). The onboard control system can preset and change the navigation route and observation mode via presets and real-time commands issued with the communication system from the ground control center. The communication system transmits control commands and observation data in real time through the Beidou satellite array. The USV platform can carry different types of meteorological and oceanographic observation instruments according to the observation mission. The ground control center monitors the working state of the USV in real time, sends control commands, and receives and stores observation data.
|Technical parameters||The solar-powered USV|
|Observation modes||Navigation mode, positioning mode, drifting mode|
|Size||4.6 m×2.0 m×1.2 m|
|Positioning accuracy||10 m|
|Communication||Beidou satellite communication|
|Power||Solar energy/24 V|
|Power consumption||400 W|
|Communication frequency||1–60 mins (adjustable)|
|Observation parameters||Temperature, humidity, air pressure, wind direction, wind speed, SST, salinity, and total shortwave radiation|