3 0 0 0 OA GPS連続観測システム“GEONET” とその展望
- 著者
- 今給黎 哲郎 大瀧 茂 熊木 洋太 畑中 雄樹 松村 正一 村上 亮 山際 敦史
- 出版者
- 日本測地学会
- 雑誌
- 測地学会誌 (ISSN:00380830)
- 巻号頁・発行日
- vol.50, no.2, pp.53-65, 2004-06-25 (Released:2010-09-07)
- 参考文献数
- 48
- 被引用文献数
- 2
GEONET (GPS Earth Observation Network System), which is a nationwide GPS array and data analysis system of the Geographical Survey Institute, started in 1994. Since then, it has been expanded and improved to monitor crustal deformation of Japanese islands quickly and accurately. Today, it is the densest GPS observation network in the world with 1, 200 GPSbased control stations and some other stations. GEONET has contributed its continuous data to various fields of earth science. In seismo tectonics study, it provided the features of coseismic crustal deformation as well as the steady state crustal deformations by plate motion. Five interplate slow slip events were also found by GEONET and has been playing important role to the recent plate coupling studies. In volcanological study, it provided the sequence of magma activity utilized for the estimation of eruption. GEONET also provided a new measure to detect ionospheric and meteorological signals and have been contributing to atmospheric science. The observed 1 Hz data of almost all stations are now provided to commercial users for positioning services in real time. GEONET is becoming a kind of infrastructure of the locationbased information society. The major present research subjects are improvement of the detectability, development of real-time analysis of crustal deformation, etc. It is also important for the future use of GEONET to maintain observation circumstances properly and to be adapted to the GPS modernization and new GNSSs.
A rigorous program of adjustment for any geodetic network must have a useful file of geoidal heights in its own to adjust strictly observations by GPS for space vectors. The program, by which geoidal heights can be automatically computed without missing its fine undualtion at every point where vertical deflection is observed, was newly added to the Universal Program of geodetic net-adjustment. The conventional method to find geoidal heights successively along routes linking neighboring vertical deflection points seems not elegant mathematically, because selections of routes are too arbitrary. It ishighly mathematical to find out geoidal heights at all vertical deflection points badjusting simultaneously all observation equations of vertical deflections which are ex pressed as the function of their geoidal heights. Although it seems geophysically reason able that geoidal heights are expressed by a curved surface with the mathematical formula written by coordinates on the surface of the earth, we are afraid that it might miss local fine undulation of geoid. In order to adjust rigorously geodetic network including GPS-observations, it is desirable that every vertical deflection point has its more reliable geoidal height. Vertical deflection at a point is decided by a local shape of the geoid around it. Then we think a new method as follows : Q We make a local geoid by using geoidal heights (rough height+small unknown correction) at several vertical deflection points inside a circle drawn around a vertical deflection point. Q2 We make both observation equations for I(north-south component) and (east-west component) of vertical deflection at thecentral point by considering the difference between geodetic longitude (latitude) and computed astronomical longitude (latitude) to be the direction of normal to the local geoid through the point. (3) We can find better geoidal heights at all vertical deflection points by solving simultaneously all observation equations by means of the method of least squares. How to derive a local geoid mathematically is the most important problem in abovementioned method. A curved surface with higher order terms requests many surrounding points. Reliability of local geoid made by using remote points deteriorates. Vertical deflection points are distributed locally in remarkable high and low densities. We adopted the following sophisticated way with much freedom after considering those peculiarities above.