Understanding the mechanisms behind massive earthquakes to predict their magnitude and frequency requires conducting frequent measurements of seafloor movements with centimeter-level precision. However, with the currently available method, such observations call for traveling by ship to the target location, collecting data, and then returning—making it difficult to conduct measurements with sufficient frequency. Under these circumstances, Associate Professor Yusuke Yokota of the Institute of Industrial Science, The University of Tokyo, throughout joint research with HAMA, Inc., has succeeded for the first time in the world in measuring the position of the seabed with centimeter-level precision using an unmanned aerial vehicle (UAV) that lands on the sea surface. This breakthrough technology is attracting attention for enabling more frequent and cost-effective measurements of seabed positions.

Using UAVs to Conduct More Frequent Measurements of the Seafloor

Seismologists and geologists use sound waves to measure the topography and position of the seafloor. This method involves emitting sound waves from the sea surface toward the seafloor and measuring the time it takes for them to reflect back, allowing for calculation of the distance to the seafloor. In particular, to understand the mechanisms of earthquakes occurring along plate boundaries—such as the Nankai Trough earthquake—and to predict their magnitude and frequency, it is necessary to measure minute movements of the Earth’s crust on the seafloor, both frequently and with centimeter-level precision. However, since these sound wave-based measurements are currently conducted primarily by ships, it has been difficult to conduct measurements with sufficient frequency, as traveling to the site by ship, conducting observations, and returning is both time-consuming and costly.

In his search for a new method of measuring positions on the seafloor that could overcome these challenges, Yokota turned to the use of UAVs. He reached out to HAMA, Inc. (hereinafter “HAMA”) upon learning that they were developing a flying boat-type UAV capable of taking off and landing on the sea surface. Yokota explained as follows.

“Previously, I had also conducted marine observations using helicopter-type drones. However, those drones have a limited flight range, so for observations far out in the ocean, we needed an aircraft capable of flying much greater distances. That’s when I learned about HAMA’s flying boat-type UAV, which has fixed wings and flight range of over 100 kilometers, so I decided to contact them. The intended observations required a larger UAV than the existing models, and HAMA willingly agreed to take on the challenge. This led to the start of our joint research in 2020.”

Two years later, in 2022, Yokota and his team became the first in the world to successfully conduct deep-sea observations using a UAV. Subsequent improvements to both the aircraft and the method of analysis led to a major breakthrough in 2025, when they finally achieved centimeter-level precision in measuring positions on the seafloor. Let’s now take a closer look at how these measurements were made possible.

UAV moving across the sea surface (manufactured by Hama Co., Ltd.)

How to Integrate Observation Equipment into Small Aircraft

Measurements of positions on the seafloor are carried out using the GNSS-Acoustic Ranging Combination Method (GNSS-A) regardless of whether by ship or UAV. GNSS stands for “Global Navigation Satellite System,” a general term for systems that determine positions on Earth using signals from artificial satellites. (The term “GPS” specifically refers to the GNSS developed and operated by the United States.) Additionally, acoustic ranging is a technique that calculates distance by measuring the time it takes for sound waves to travel.

GNSS-A combines these two technologies, using GNSS to determine the positions of ships or UAVs on the sea surface and sound waves—which can travel long distances underwater—emitted from ships or UAVs to measure the underwater environment. Integrating these two types of data allows for measurements of positions on the seafloor. Along the Nankai Trough and in other important regions, seafloor acoustic reference stations (seafloor stations) have been installed––with a few dozen kilometers between each––to serve as reference points for making observations. Researchers regularly measure their positions using GNSS-A to monitor movements of the Earth’s crust on the seafloor.

Schematic diagram of GNSS-A. GNSS measures the position of ships or UAVs above sea level, and the position of underwater stations installed on the seabed is measured by the sound waves emitted by those ships or UAVs.

These observations are currently carried out by ship, and Yokota set out to make them also possible using UAVs. The method of observation is essentially the same whether by UAV or ship, and generally speaking, the same equipment is required. However, the challenge was far from simple.

“The process of developing a UAV equipped with observation instruments that could fly 100 kilometers offshore, land on the sea surface to conduct measurements, and then take off again to return required ingenuity in various areas and repeated trial and error. The biggest challenge was figuring out how to mount equipment on the UAV. We asked HAMA to develop an aircraft larger than their existing models, but it was equally essential to miniaturize and lighten the equipment, as well as to carefully design its placement within the aircraft.”

Part of the observation equipment mounted on the UAV. The cylindrical device on the right is a sonar.

The first step taken by Yokota and his team was to minimize the amount of additional measurement equipment to be installed on the aircraft. As an example, motion sensors used on ships to measure sway and tilt are quite heavy. However, since the UAV itself is already equipped with lightweight motion sensors, these could be omitted.

“In addition, sonar equipment—which generates sound waves through vibration—must be of a certain physical size to emit sound waves that can reach long distances, which limits how small they can be made. However, the circuitry that generates the electrical signals to produce the vibrations could be made more compact, so we tried to miniaturize it. Unfortunately, this led to a number of issues. For example, parts that shouldn’t come too close together began to interfere with each other, and we had to come up with ways to prevent that. And challenges unique to the marine environment made resolving these problems far from easy. Also, we had to carefully design the layout of the equipment so as not to adversely affect the UAV’s flight performance. We went through repeated rounds of trial and error on each of these points.”

Meanwhile, HAMA produced prototypes of larger aircraft and repeatedly conducted experiments as they worked to create the required model. To improve the precision of measurements, it was desirable for the UAV to be able to travel over the ocean and conduct observations at multiple locations. Achieving this required the aircraft to be able to take off after completing an observation, land on the water again, and then take off once more to conduct further observations. The aircraft was also designed with functions that would allow it to carry out these operations autonomously.

Refining the Data Analysis Method and Other Factors to Achieve Centimeter-level Precision in Measurements

After much effort, the UAV itself was completed. In 2022, the various instruments were mounted onto the UAV to conduct observations in Sagami Bay, during which distance was successfully measured by establishing communication with a seafloor station located at a depth of approximately 1,300 meters. This marked the world’s first successful attempt at acoustic communication and distance measurement between a UAV on the sea surface and the deep seafloor.

However, at that time, the measurements were insufficiently precise. Further refinements were needed to achieve the centimeter-level precision required for monitoring movements of the Earth’s crust. Yokota continued to make further advancements, not only in miniaturizing the equipment, but also in refining the data analysis method and correcting for errors generated by the instruments.

“We have also been conducting research on the theoretical aspects of GNSS data analysis, and since the 2020s, we have worked on improving the models integrated into our analysis programs. These efforts also helped to make measurements more precise. Additionally, since sonar signals contain errors, we researched technologies to correct these errors and incorporated the findings into our measurements.”

While Yokota focused on improving analytical accuracy, HAMA worked to refine the aircraft—slightly widening the wingspan among other adjustments—and completed the new model. Finally, in 2025, they successfully achieved centimeter-level precision in measurements of seafloor positions.

UAV aircraft (new model of HAMADORI6000 manufactured by Hamadori Co., Ltd.) successfully measures seabed position with centimetre accuracy.

Leveraging Multiple Observation Methods to Unravel the Mysteries of the Seafloor

The UAV-based technology for measuring positions on the seafloor brought to fruition through these efforts is now attracting attention for enabling more frequent and cost-effective measurements.

“The new aircraft developed by HAMA has a flight range of over 330 kilometers and is capable of flying at speeds in excess of 90 km/h, making it possible to quickly reach offshore areas such as the Nankai Trough to make observations. It is also expected to cost only a fraction of what is required for ship-based observations, making it possible to conduct measurements of the seafloor significantly more frequently than is currently feasible. Another major advantage of using UAVs is their ability to quickly reach earthquake sites and conduct observations immediately after they occur.”

However, Yokota does not intend to use only UAVs in conducting frequent observations of the seafloor. In addition to this research, he is also working to develop a method of monitoring the seafloor using small unmanned boats. Deploying multiple unmanned boats over a section of the ocean could enable an operational cycle in which all points are continuously monitored, even while some of the vessels undergo maintenance. In addition, Associate Professor Toshihiro Maki’s laboratory at UTokyo-IIS is researching how to integrate autonomous underwater vehicles (AUVs), which can move autonomously underwater, with UAVs. Yokota explains that combining these various technologies to enable multiple methods of observation will enable measurements to be conducted more frequently, ultimately leading to more advanced monitoring of the seafloor.

“To be honest, I’m not fond of boats. I get terribly seasick (laughs). That’s why I dream of a future where we won’t need to travel to sites by boat and can instead monitor everything about the seafloor from the comfort of an office. The seafloor is often described as a frontier that remains even more mysterious than outer space. I intend to keep overcoming each and every challenge that I face so that the observation technologies I develop can help shed light on some of the seafloor’s mysteries.”

Testing of small unmanned boats

Related Article> Behind the Scenes of the World’s First UAV Seabed Observations——Yusuke Yokota (UTokyo-IIS) × Masata Kaneda and Kento Suzuki (HAMA, Inc.)——

Related Article> Advancing earthquake prediction with an unmanned aerial vehicle

Related Article> Exploring the deep: drones offer new ways to monitor sea floor

Related Link> HAMA Inc.

Featured Researcher
Yusuke Yokota
(Associate Professor, Institute of Industrial Science, The University of Tokyo)
Special Field of Study: Underwater Information System

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