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

This letter presents 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 from approximately 300,000 to 450,000 kilometers and a 2 : 1 resonance ratio. In April 2025, utilizing microwave dual one-way ranging (DOWR), the experiment successfully achieved satellite-ground time-frequency comparisons using the PHM aboard DRO-A. This work validated the in-orbit performance of the compact PHM and supported tests of the Einstein Equivalence Principle (EEP). The gravitational redshift measurement result is (8.74 ± 4.17) × 10^-3. As the world’s first fundamental physics experiment deploying PHMs in a lunar DRO, it provides significant new engineering approaches for testing gravitational theories in cislunar space.