RTK-GPS Positioning in Japan by Virtual Reference Station (VRS) System with GPS-Based Control Stations

The accuracy of a few centimeters can be easily obtained in 2drms of horizontal positions by RTK-GPS positioning, as the range from a user GPS antenna to the satellite is determined by the phase measurement of the carrier waves. It is necessary, however, to provide the carrier phase data from a reference station located at a known position, to a user receiver. It is hard to prepare expensive GPS receiver for RTK-GPS. RTK-GPS users in Japan have desired the development of a more flexible, wider and less expensive carrier phase data dissemination service. While, Geographical Survey Institute (GSI) of Japan has established the network of as many as 947 GPS-based control stations (CS) covering all over the Japanese Islands, in order to observe crustal movement for seismic prediction and research. The service for RTK-GPS carrier phase data dissemination has been operated experimentally since October 1997 from several GPS-based CS. And since December 2000, it has been operated experimentally by Virtual Reference Station (VRS) system using a several stations, known as FKP (flachen-korrectur-parameter; Geo++ in Germany) system in Tokyo area. The authors have investigated RTK-GPS positioning at a fixed point in National Defense Academy (NDA) of Japan and in Tokyo University of Mercantile Marine (TUMM), both out of the VRS network area. First, the carrier phase data from several GPS-based CS are transmitted to the data control center. And they are converted into FKP data there~ And the data are transmitted to user receiver via a cellular phone with the data rate of 9,600 bps. The authors evaluated their validity by applying the data to RTK-GPS positioning. First, the authors measured the Age of the VRS system before positioning. The system fixes the positions from a combination of the data transmitted by the reference station and those by user receiver collected at the same time. The Age is measured to be a little over 1 s with 0.08 s in S.D. The Age of a cellular phone is about 0.2 s that is included in the total Age. Sometimes value of the Age exceeds 8 s by the trouble of the cellular phone. It is confirmed 2drms of Horizontal positions and S.D. in altitude are smaller than several centimeters respectively. The nominal positioning accuracy of ordinal RTK-GPS is 2 cm plus the length of baseline times 2ppm, within a radius of about 10 km from the reference station. Although the sites to which base line length from the nearest GPS-based CS exceeds 30 km and locate out of GPS-based CS network area for VRS, the remarkable accuracy is obtained comparable to the conventional RTK-GPS positioning. Next, the authors measured the time to fix the ambiguity. They put off the GPS antenna cable from GPS receiver and then put it to the receiver again in order to make the cycle slip of the ambiguity perfectly. The time is measured to be about 2 minutes to recover for RTK-GPS fix. Further, the authors tried positioning while onboard a ship under navigation. They measured the time to fix the ambiguity after the ship passed below Rainbow Bridge in Tokyo Port. The time is measured to be around 7 minutes to recover RTK-GPS fix positioning. These results show that the present VRS experiment system operates satisfactorily even outside of the area.