Wakayama, Mar 04 (News On Japan) - A green beam of light piercing the night sky from Wakayama Prefecture is helping scientists determine Japan’s exact position on Earth with centimeter-level precision, as the Japan Coast Guard continues decades of satellite laser measurements at the Shimosato Hydrographic Observatory.
About 800 kilometers above Earth, a soccer-ball-sized satellite speeds through space at roughly 7 kilometers per second. Though invisible to the naked eye, its path can be tracked using telescopes that guide a straight green laser beam into the night sky. By measuring the time it takes for the laser light to reflect off the satellite and return to Earth, researchers calculate the precise distance between the ground station and the satellite.
This work is carried out around the clock at the Japan Coast Guard’s Shimosato Hydrographic Observatory in Nachikatsuura, Wakayama Prefecture. The facility’s main mission is to collect fundamental data used in creating nautical charts.
The observatory traces its origins back to 1944, when its predecessor, the Katsuura Geomagnetic Observatory, was established to help determine ship positions through celestial observations and to measure geomagnetism for understanding errors in magnetic compasses. The facility adopted its current name in 1954, and satellite laser ranging (SLR) — which determines distances by directing lasers at satellites — began there in 1982.
By accumulating measurements with an accuracy of about one centimeter, researchers have been able to determine the precise position of the Japanese archipelago and contribute data for the creation of detailed maritime charts.
Satellites targeted by the observatory vary widely in altitude, ranging from about 500 kilometers to as high as 20,000 kilometers, and also differ in size. Staff members input predicted satellite orbits into computers and carefully adjust the laser’s angle in increments as small as one-thousandth of a degree while operating the system.
In the early years, limited computer performance meant that only up to nine satellites could be observed in a single night. Today, that number has grown to around 40. In 2023, the observatory achieved a cumulative total of 50,000 observations, and last year it recorded the highest annual number of successful measurements, with this year already on pace to surpass that record.
Tomohiro Uchida, a 43-year-old technical specialist at the facility, explained that each satellite requires a different setup. "Because satellites have different characteristics, we adjust the optimal settings for each one. We also decide which satellites should take priority and take cloud cover and weather conditions into account," Uchida said.
The Earth’s surface is constantly shifting due to the movement of tectonic plates and earthquakes. Accurate positional data has become an essential infrastructure in modern society, with precise latitude and longitude information supporting technologies such as autonomous vehicles.
However, high-precision positioning can now also be achieved through satellite navigation systems, including Japan’s domestically developed Quasi-Zenith Satellite System known as Michibiki and the United States’ GPS.
The Shimosato observatory has also participated in the International Laser Ranging Service, a global network through which countries share satellite laser ranging data. In 2023, however, a new facility operated by the Japan Aerospace Exploration Agency in Tsukuba, Ibaraki Prefecture, began conducting similar SLR observations.
"Alternative methods are emerging, and that reflects the direction of the times," said observatory director Mitsuhiro Suzuki, 64.
Satellite laser ranging operations at Shimosato are scheduled to end at the end of December, and the observatory itself is expected to close in March next year. Until then, the meticulous work of guiding a straight green beam into space to measure the Earth with extraordinary precision will continue without pause.
Source: 産経ニュース
