9:15 AM - 9:30 AM
[SCG48-02] Evaluation of UAV-based GNSS-A observation system specifications through a sea trial
Keywords:GNSS-A, Acoustic ranging, Seafloor geodetic observation, UAV, Seaplane
The Japan Coast Guard operates a seafloor crustal movement observation network, SGO-A (Seafloor Geodetic Observation - Array), in the seas around Japan. SGO-A uses GNSS-A (Global Navigation Satellite System - Acoustic ranging combination) technology, which can detect crustal deformation of the seafloor with the centimeter level. The data obtained from SGO-A are extremely important for understanding the mechanisms of a huge earthquake that can generate a tsunami.
GNSS-A detects crustal deformation of the seafloor by determining the position of a seafloor station through repeated acoustic ranging between a sea surface platform such as a ship or buoy and a seafloor station previously installed on the seafloor. When the seafloor station receives an acoustic signal from the sea surface platform, it sends back the received signal as it was about one second later. This signal round-trip is used to measure the distance between the sea surface platform and the seafloor station.
Currently, GNSS-A observations are mainly conducted by research vessels, but research is being conducted on the use of a seaplane-type UAV (Unmanned Aerial Vehicle) as an alternative sea surface platform (Yokota et al., 2023). A seaplane-type UAV can move at a speed of 80 km/h or more (about 4 times faster than a ship) as a plane, and can take off, land, and move on the ocean. This capability makes it suitable for a GNSS-A observation. Utilizing a UAV for a GNSS-A observation could significantly reduce the consumption of resources, including money, energy, and time. Additionally, it could enhance the frequency and timeliness of observational data, thanks to the rapid deployment of observation equipment and faster data collection.
In November 2023, a preliminary acoustic ranging experiment between a UAV and a pseudo seafloor station on the bottom of the tank was conducted at Institute of Industrial Science (IIS) Ocean Engineering Basin at Kashiwa Campus, The University of Tokyo. We confirmed that acoustic ranging is feasible under ideal conditions and to evaluate characteristics such as a UAV attitude that enable acoustic ranging. The UAV was fixed to a moving cart while floating on the surface of the water so that it could be moved horizontally, and the UAV was repeatedly moved horizontally to perform acoustic ranging with a pseudo seafloor station at a depth of approximately 4 m. The test results showed that acoustic communication was established at an acoustic emission angle of approximately 30 to 40 degrees. The sonar mounted on the UAV is protected by a polycarbonate board to protect it from impact of water landing. In order to evaluate the effect of the presence or absence of this board, the same experiment was conducted without the board, but there was no effect of the presence or absence of the board on the acoustic ranging established ejection angle or the quality of the acoustic signal.
Based on the results of the above preliminary tank experiment, a sea trial was conducted off the coast of Ito in January 2024. At the actual observation point of SGO-A, communication with the actual seafloor station was conducted by the observation system onboard the UAV. While stable acoustic communication was possible when the UAV was stationary, communication was not possible when the UAV was underway due to bubbles generated near the sonar. Therefore, we removed the polycarbonate board, which structurally causes bubbles, and modified a part of the UAV body. As a result, it successfully communicated with a seafloor station albeit unsteadily while navigating.
As described above, the UAV-based observation system successfully communicated with the seafloor station while navigating. On the other hand, future issues, such as the generation of bubbles near the UAV chassis that interfere with acoustic signals, were also identified.
GNSS-A detects crustal deformation of the seafloor by determining the position of a seafloor station through repeated acoustic ranging between a sea surface platform such as a ship or buoy and a seafloor station previously installed on the seafloor. When the seafloor station receives an acoustic signal from the sea surface platform, it sends back the received signal as it was about one second later. This signal round-trip is used to measure the distance between the sea surface platform and the seafloor station.
Currently, GNSS-A observations are mainly conducted by research vessels, but research is being conducted on the use of a seaplane-type UAV (Unmanned Aerial Vehicle) as an alternative sea surface platform (Yokota et al., 2023). A seaplane-type UAV can move at a speed of 80 km/h or more (about 4 times faster than a ship) as a plane, and can take off, land, and move on the ocean. This capability makes it suitable for a GNSS-A observation. Utilizing a UAV for a GNSS-A observation could significantly reduce the consumption of resources, including money, energy, and time. Additionally, it could enhance the frequency and timeliness of observational data, thanks to the rapid deployment of observation equipment and faster data collection.
In November 2023, a preliminary acoustic ranging experiment between a UAV and a pseudo seafloor station on the bottom of the tank was conducted at Institute of Industrial Science (IIS) Ocean Engineering Basin at Kashiwa Campus, The University of Tokyo. We confirmed that acoustic ranging is feasible under ideal conditions and to evaluate characteristics such as a UAV attitude that enable acoustic ranging. The UAV was fixed to a moving cart while floating on the surface of the water so that it could be moved horizontally, and the UAV was repeatedly moved horizontally to perform acoustic ranging with a pseudo seafloor station at a depth of approximately 4 m. The test results showed that acoustic communication was established at an acoustic emission angle of approximately 30 to 40 degrees. The sonar mounted on the UAV is protected by a polycarbonate board to protect it from impact of water landing. In order to evaluate the effect of the presence or absence of this board, the same experiment was conducted without the board, but there was no effect of the presence or absence of the board on the acoustic ranging established ejection angle or the quality of the acoustic signal.
Based on the results of the above preliminary tank experiment, a sea trial was conducted off the coast of Ito in January 2024. At the actual observation point of SGO-A, communication with the actual seafloor station was conducted by the observation system onboard the UAV. While stable acoustic communication was possible when the UAV was stationary, communication was not possible when the UAV was underway due to bubbles generated near the sonar. Therefore, we removed the polycarbonate board, which structurally causes bubbles, and modified a part of the UAV body. As a result, it successfully communicated with a seafloor station albeit unsteadily while navigating.
As described above, the UAV-based observation system successfully communicated with the seafloor station while navigating. On the other hand, future issues, such as the generation of bubbles near the UAV chassis that interfere with acoustic signals, were also identified.