A positioning performance analysis of the RESSOX synchronization network for the Japanese Quasi-Zenith Satellite System, QZSS, is presented. A hardware-in-the-loop experiment setup has been developed to study concrete effects on positioning when a real space-born OCXO is employed in the atomic clock-less RESSOX architecture. This study focuses on a positioning performance analysis of QZSS used in conjunction with GPS. Particular focus is given to the effects of faulty synchronization on positioning, specifically when, because of unavoidable communication interruptions, the QZSS satellite clock has to function without remote control. Results show that the recently proposed RESSOX phase error compensation method can guarantee enough time accuracy even for communication interruptions of the order of one hour. The relationship between QZSS plus GPS positioning accuracy and QZSS clock quality is discussed.