We have proposed synchronized spread spectrum multiple access (synchronized SS-CDMA) short message communication using Quasi-Zenith Satellite System (QZSS) as a safety confirmation system during large disaster. In synchronized SS-CDMA, each user calculates the distance to the satellite using satellite positioning signal precisely in order to achieve high density user multiplexing. Accordingly, it is necessary to investigate the influence of positioning data error on time synchronization accuracy at the satellite. In this paper, we measure the positioning data error of the Global Positioning System (GPS)/QZSS receivers. Then we show that the error satisfies the required time synchronization accuracy between users for high density user multiplexing.