Chief of Crustal Deformation Research Division
Geography and Crustal Dynamics Research Center
Email: [email protected]
GEONET (GPS Earth Observation Network) which consists of 1,200 GPS permanent stations is the largest GPS network of the world. This article summarises briefly the history, the system, applications and perspective of the network
EONET (GPS Earth Observation Network) is the nationwide GPS array of Japan, which consists of 1200 GPS permanent stations, and is the largest GPS network of the world at current. This network, operated by Geographical Survey Institute, has been used for crustal-deformation monitoring and geodetic control. Recently, real-time capability is added to the network system to quicken the response of crustal deformation monitoring and to enhance the function as a social infrastructure for real-time positioning services and other applications. In this article, we summarise briefly the history, the system, applications, and perspective of the network.
Evolution history of the network
Construction of GPS permanent network by GSI started in 1993 with 110 stations in the Tokai and South Kanto region. This network was specialised for crustal deformation monitoring at the area. On the other hand, nationwide GPS array of 100 permanent stations was constructed in 1994 and the operation of the network started on 1 October of the year.
Three days later, the East off Hokkaido Earthquake (4 October, 1994, M8.1) took place and it took only two days to detect the coseismic displacement up to 40 cm by the GPS network system. This is revolutionary fast, precise and effective compared with the traditional survey method. As the ability of the network was proved also by other succeeding seismic/volcanic events including the 1995 Kobe earthquake, GSI integrated the two network systems into one and have added stations gradually to monitor more details of crustal deformation.
GEONET consists of about 1,200 GPS observation sites and the central station at an office of GSI in Tsukuba. The average spacing of observation stations is about 20km. A station pillar of five metre tall with the base of two metre depth is settled at each site. The pillar is made with stainless steel and equipped with a GPS antenna in the radome at the top. Choke ring antenna is used at most of the sites. A dual-frequency GPS receiver, communication devices, backup battery, tilt metre etc. are installed in the body of the pillar.
Most of the sites, where broadband line is available, have on IP-connection through secure IP-VPN (Internet Protocol Virtual Private Network) and dual-frequency signals of carrier phase and code of GPS satellites observed in 1 Hz are transferred in real time to the central station. The data are also decimated into 30 second sampling interval and kept in the receiver memory for several days. The saved data are transferred in case of failure of data communication. For the sites where broadband line is not available, data are observed with 30 second sampling interval and transferred by telephone line or satellite communication every 3 hours.
The central station controls the operation of all observation sites, data communication, data management and archive, analysis, etc. The real-time 1 Hz data can be analyzed by an RTK-type software. 1 Hz data are saved in hard disk for two weeks and then discarded. 1 Hz data saved in the disk are soon decimated into 30 second interval, and archived in the database after conversion to a common format. The archived 30-second data are used in the routine analyses.
Three kinds of the routine analyses are carried out; the quick analysis, the rapid analysis, and the final analysis. The quick analysis is carried out in near real-time, every 3 hours with 6 hours data window. The rapid and final analyses produce daily solutions with 24 hours data and more precise than the quick analysis. The IGS ultra rapid products is used in the quick and rapid analyses. They contain the GPS satellites orbit information calculated and predicted by global GPS network by the International GPS Service (IGS) and are much more precise than broadcast orbit. The final analysis is carried out about two weeks later with the IGS final products, which is the most precise ephemeris data available for GPS study. The solutions of the all three analysis types are stored in the database.
The solutions of the analyses can be displayed on the monitor screen and printed out in several ways. For example, we can plots horizontal/vertical displacement vectors in a map or time series graph of baseline components with selecting stations, baselines, and period. The results are also provided on the web page: https://mekira.gsi.go.jp/ENGLISH/index.html