We have been developing a remote synchronization system for the onboard crystal oscillator (RESSOX) of the quasi-zenith satellite system (QZSS) since 2003. QZSS is a three-satellite navigation/positioning system conceived to improve the positioning performance (satellite availability and position accuracy) of the presently available global positioning system (GPS) in areas where high-rise buildings and mountains reduce the number of visible GPS satellites in Japan. The orbital planes of QZSS satellites are inclined (at 43 degrees) from the equatorial plane, although the semi-major axis is the same as the geostationary satellites. RESSOX is conceived as a very precise radio-controlled clock. The target of RESSOX is synchronization within 10 ns between the ground station time standard and the onboard QZSS crystal oscillators. We have achieved a synchronization error within 2 ns in the ground experiments.