With the recent development of satellite communication and the diversified demands for communication, there has been increasing demand to utilize the position of the mobile object as the information. This idea is proposed and has been developed in various countries of the world as the radio determination satellite system (RDSS). This paper outlines the hybrid communication/positioning system using two geostationary satellites. The system has been developed by the Communication Research Laboratory in collaboration with the R & D Center for Radio System, together with the result of basic experiments. Two kinds of experimental systems with different modulation/demodulation methods are constructed. One is the method based on the offset QPSK (called SCPC), and the other is based on the spread-spectrum (called SS). The former transmits the narrowband signal for distance measurement, in addition to the communication signal. Consequently, this method can easily be introduced into the SCPC satellite communication system presently used in practice, although the accuracy of the positioning is slightly deteriorated. Compared to the former, the latter has the advantage that a highly accurate positioning can be realized since a high-speed positioning signal is employed using a wide transmission bandwidth. However, the system configuration becomes complex. A field experiment is conducted for each system using the Engineering Test Satellite V (ETS-V) and the Inmarsat satellite. The positioning accuracy of 2 to 8 km for low altitude and 1 to 2 km for middle altitude is obtained in SCPC. The position accuracy of 400 to 600 m is obtained near Japan by SS. [ABSTRACT FROM AUTHOR]