GPS/Acoustic (GPS/A) seafloor positioning is a novel method for observation of a regional seafloor crustal movement, and is now most promising in the field of seafloor geodesy. Compared with land-based GPS observation, however, it has critical problems against on-line, continuous, and long-term observations as well as the limited accuracy of repeated positioning. It has recently been pro-posed that GPS/A positioning with five precision transponders can improve the accuracy of seafloor positioning with limited observation time. Since GPS/A positioning on a moored buoy combined with the new positioning method can be a solution to cope with the important problems, a stand- alone observation system is under development in preparation for a possible future application to the cabled seafloor observatory project DONET funded by the MEXT, Japan. It can be a gateway to semi-real-time seafloor positioning based on continuous GPS/A observation.