Gravitational Redshift Test in a Lunar Distant Retrograde Orbit Using a Compact Hydrogen Maser

This letter reports a gravitational redshift measurement experiment using a satellite-based compact passive hydrogen maser (PHM) in a lunar distant retrograde orbit (DRO). In March 2024, the Chinese Academy of Sciences launched the DRO-A/B twin satellites, which entered a DRO in July 2024. This orbit has a geocentric distance of approximately 300,000-450,000 kilometers and a 2:1 resonance ratio. Employing microwave dual one-way ranging (DOWR), satellite-ground time-frequency comparisons were successfully achieved in April 2025 using the PHM aboard the DRO-A satellite. This study validated the in-orbit performance of the compact PHM and supported tests of the Einstein Equivalence Principle. The gravitational redshift measurement result is (8.74 ± 4.17) × 10^-3. As the world’s first fundamental physics experiment to deploy PHMs in a lunar DRO, this study provides significant new engineering approaches for testing gravitational theories in cislunar space.