The Korea Research Institute of Standards and Science (KRISS) and the Korea Astronomy and Space Science Institute (KASI) have agreed to join forces to contribute to the redefinition of the time unit "second," scheduled for around 2030.
The two institutions signed a memorandum of understanding on "Convergence Research Cooperation on Ultra-Precision Optical Clock Comparison Using SLR" at KRISS headquarters in Daejeon on Monday.
Under the agreement, the two institutions will cooperate on joint use of key research equipment including SLR and atomic clocks, joint research on SLR-based optical clock comparison measurements including the ACES-ELT mission, and exchange of research personnel and training of specialists.
The two institutions will launch an international joint project to precisely compare the atomic clock on the International Space Station (ISS) with Korea's ytterbium optical clock.
The "optical clock," which is more than 100 times more precise than the cesium atomic clock currently used to define the "second," has emerged as the next-generation time standard. Comparing and verifying the performance of optical clocks worldwide has become a key task for the redefinition of the "second."
However, satellite-based methods such as GPS lack precision, while optical fiber networks face physical limitations in connecting continents, making precise comparison research between optical clocks worldwide difficult.
The European Space Agency (ESA)'s "Atomic Clock Ensemble in Space (ACES)" mission emerged to address this challenge. The project, which has been pursued since the 1990s, finally completed the installation of an ultra-precision atomic clock ensemble on the ISS in April 2025. ACES is an international space project that uses the atomic clock system mounted on the ISS to verify Einstein's theory of relativity and precisely test fundamental physical laws.
In particular, ACES enables time comparison between space and ground at a precision of 10^-17, 10 times more precise than existing satellite methods, making it the optimal alternative for verifying optical clock performance worldwide beyond technical and geographical constraints.
Korea is participating in the European Laser Timing (ELT) project, a laser-based time comparison method within the ACES mission. ELT involves firing lasers toward the ISS, a manned facility. It requires strict approval procedures from ESA for human safety.
Korea has become the second country in the world, after Germany, to obtain ESA approval and join the ACES-ELT project. Korea and Germany have the geographical advantage of offset ISS observation time zones, and are expected to provide key data needed to verify the stability of the ISS atomic clock.
To participate in the international joint project "ACES-ELT," the two institutions last year connected KRISS's "ytterbium optical clock (KRISS-Yb1)" and KASI's "Sejong Satellite Laser Ranging (SLR) system" via a dedicated optical fiber network. This enables KRISS's precise time signal to be transmitted to the Sejong SLR, then launched into space on a laser and directly compared with the ISS atomic clock.
"Through international joint research, we are now able to verify the performance of optical clocks, which had been difficult due to technical limitations," KRISS President Lee Ho-seong said. KASI President Park Jang-hyun said, "It is very meaningful that government-funded research institutes are pursuing convergence research by utilizing their respective technologies."
