mini传媒

Published on December 24, 2019
Job titles and other details are as of the time of publication.
(The interview was conducted in Japanese and was thereafter translated into English.)

How can the sea level rise of three millimeters per year be calculated?

My research specializes in space geodesy and precise orbit determination. Numerous artificial satellites have been launched since the Soviet Union launched Sputnik I, the world's first satellite, in 1957. Satellites play an important role in human life as indispensable tools for communications, broadcasting, weather monitoring, positioning, and earth observation.

I conduct studies on technology for measuring satellite orbits with an accuracy of only a few millimeters or centimeters. I pride myself on the research for calculating satellite orbits with data from this measurement technology helping satellites fulfill their intended objectives.

Mapping and navigation systems are among the familiar examples of how this technology contributes to humanity, and one of the latest hot topics is the observation of the global environment. Media reports say the sea level is rising three millimeters a year due partly to the melting of polar ice caused by global warming in recent years. Satellites measure the height of the ocean surface, but the height of three millimeters cannot be calculated without first accurately measuring the position of the satellites. Measurement data accumulated in normal times can help calculate the degree of change in the aftermath of a disaster. This effort contributed to calculating a ground shift up to five meters during the 2011 Great East Japan Earthquake. With research institutes around the world constantly measuring environmental changes, the significance of my research lies in making more accurate measurements.

Bringing together the advantages of various positioning methods

Now, let me briefly explain how we measure the distance to satellites. Satellite laser ranging (SLR), a field which I am researching, is a technology for calculating the distance to a satellite by irradiating a laser beam emitted from a pulse laser device on Earth through an optical telescope to a retroreflector (corner-cube reflector) attached to a satellite and measuring the time taken to receive the reflected light using a stopwatch. Large SLR equipment costing hundreds of millions of yen is out of reach for Hitotsubashi University and other schools, so I conduct research activities in collaboration with SLR-equipped research facilities at the National Institute of mini传媒 and Communications Technology, where I previously worked, as well as the Japan Aerospace Exploration Agency and the Japan Coast Guard.

The distance to satellites is measured using not only SLR but also VLBI (very long baseline interferometry), DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), and GNSS (global navigation satellite system: generic term for satellite positioning systems such as GPS in the United States, GLONASS in Russia, Galileo in the European Union, and Quasi-Zenith Satellite System in Japan). Each piece of equipment has its own strengths and weaknesses, and complements the others. For example, GNSS uses radio waves and is suitable for measuring a small range, but not for measuring the distance to satellites and other global-scale measurements, while SLR laser beams have the advantage of minimizing various errors and are suitable for global-scale measurements.

Developing an international cooperation framework as a major key

International cooperation is essential for measuring the extensive range of satellites. I am working to develop a global cooperation framework as the chair of the governing board at the International Laser Ranging Service (ILRS), which was formed in 1997 with the participation of ranging stations, analysis centers, and data centers around the world. Managing the international program is not an easy task due to the conflicting interests among countries, but I can take advantage of many aspects of Hitotsubashi University’s knowledge to operate international joint projects.

Built on top of this global framework is the Global Geodetic Observing System (GGOS), the flagship observation system of the International Association of Geodesy (IAG). The GGOS integrates the three basic geodetic quantities of Earth’s geometry, gravitational field, and rotation to precisely observe and monitor the movements. This system maintains global reference frames that serve as the foundation for all Earth sciences and their applications.

Japan adopted its own geodetic system until 2002, but has since shifted to the World Geodetic System in light of global technological developments and other factors. The 400-meter discrepancy between the two systems highlights the risk of maritime collisions for vessels using the Japanese and global geodetic systems and symbolically points to the importance of global standards.

Challenge of setting up numerous observation stations across the globe

Thanks to ILRS and other initiatives, a plenary session of the United Nations General Assembly in 2015 recognized the importance of a global geodetic coordinate reference system and adopted a resolution recommending that UN member states work together to maintain the system. Accurate positioning information is becoming increasingly important for global environmental monitoring, universal mapping, and cross-border sea and air traffic. The demand is growing for the GGOS to develop and contribute as future infrastructure technology.

Japan’s space geodetic technology is considered among the best in the world. The country has seven observation stations operated by six organizations – the National Institute of mini传媒 and Communications Technology (Koganei and Kashima), National Astronomical Observatory of Japan (Mizusawa), Geospatial mini传媒 Authority of Japan (Ishioka), Japan Coast Guard (Shimosato), Japan Aerospace Exploration Agency (Tanegashima), and National Institute of Polar Research (Syowa Station, Antarctica). Japan disseminates excellent research outcomes as well.

One major challenge ahead is setting up more observation stations to increase the accuracy of geodetic measurements. There are few such facilities around the equator and in the southern hemisphere where many emerging and developing countries unable to afford costly equipment are located. I will work to develop lower-cost equipment by downsizing.

Developing numeracy skills and global perspectives

What makes researchers pursue their interests is the following. In space geodesy, it is extremely difficult to limit errors to within a few centimeters due to the impacts of various factors, such as atmospheric conditions, gravity, and sunlight. Still, researchers are delighted when they discover a model that can provide satisfactory explanations for the observed values.

This research is an interdisciplinary field of study spanning geodesy, space engineering, geophysics, astronomy, and even social sciences. It is a unique field covered by Hitotsubashi University, which specializes in social sciences.

I can list three points to explain how meaningful it is for Hitotsubashi University students to study in my seminar. First, students in a liberal arts-focused university can understand how people in scientific fields find pleasure in their studies. This is useful when working with science professionals. Second, the seminar students have a chance to develop numeracy skills. For example, when sociology students examine minorities, they cannot grasp the actual status without evidence-based discussions, such as whether the minorities represent 0.2 percent, 2 percent, or 20 percent. My seminar teaches students to acquire these numeracy skills. Third, the seminar helps them cultivate global perspectives through the hands-on experience of building international cooperation. Space geodesy is a sensitive area closely linked to military affairs, and provides great opportunities to explore ways of promoting international cooperation while examining its role. My seminar students can also develop reasoning skills for identifying and overcoming various challenges in light of the current global situation.