RTK-GPS Positioning in Japan by GPS-Based Control Station via DMCA Mobile Radio Communication System

RTK-GPS is a real time positioning system of the accuracy of one cm order with carrier phase measurement. Many applications are now under examination in Japan. The most indispensable for the positioning procedure are carrier phase data from reference stations. There is at present a mobile radio communication system operating in Japan, called DMCA (Digital Multi-channel Access) system. It can be used as a medium of dissemination of the carrier phase data. It is an 8 kbps radio system for many users on TDM/TDMA mode. Geographical Survey Institute (GSI) of Japan has established the network of as many as 947 GPS-based control stations all over the Japanese Islands for seismological researches. They are available for the system of RTK-GPS positioning through the medium of DMCA. Experiments have been operated since October 1997 for RTK-GPS data dissemination from several GPS-based stations via DMCA both around Tokyo metropolitan region and in the Kansai district. The Trimble CMR (Compact Measurement Record) data for RTK-GPS positioning are transmitted all day long through MCA base stations which have the service area of 20-30 km in radius. The authors tried RTK-GPS positioning first at a fixed point in Tokyo University of Mercantile Marine. The data were received from Nerima GPS-based Control Station (base line length: 17 km) by three Trimble 4000SSi GPS receivers with a single GPS antenna. It is confirmed that their performance is within the specification described by the manufacturer. Each pattern of distribution of the positions calculated by the three receivers is quite similar to each other. The ‘Fix’ solution rate, or the rate of fixed positions with the flag of ‘Fix’, is 87.7 % for 145,259 fixed positions which are calculated every second for 40 hours. The data transmission delay measures normally 1.6-1.7 s. The communication line of DMCA has originally been designed to quit sending the data every 1.5-5.0 minutes for several, often several dozen seconds, depending on the areas and the periods of time. It is widely desired that the system should be improved for continuous positioning. Another experiment was carried out comparing the positioning by two GPS receivers and a single fixed antenna which receive the data for 3.3 hours from two different GPS-based control stations, at Nerima and Kawasaki, respectively 17 and 16 km away from the user GPS antenna. The rate of ‘Fix’ solution is 93.9 % and 52.4 %, respectively. The difference between their average positions is 3.28 cm in latitude, 1.44 cm in longitude and 0.91 cm in altitude. 2drms against the average points is 2.63 cm and 3.60 cm, and S.D. in altitude is 2.08 cm and 2.83 cm, respectively. The low rate of ‘Fix’ solution by Kawasaki GPS-based Control Station is supposed to be due to the excess of data transmission delay over 2.0 s, which is observed approximately every other second. It seems that the transmission delay should be within 2 s in order to get ‘Fix’ solution by Trimble 4000SSi’s. There must be some technical problems to be solved in the communication system from Kawasaki GPS-based CS through Shinyokohama MCA Base Station. Next, they tried the positioning on board a ship navigating in Tokyo Bay. They confirmed that it works well.