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GPS Baseline Solutions with Tropospheric Correction by Using the JMA
Numerical Weather Model for Monitoring Volcanoes
ῌῌAkimichi TAKAGIῌ,ῌῌ, Keiichi FUKUIῌ and Yoshinori SHOJIῌ
Japan Meteorological Agency (JMA) has installed and is operating GPS networks around major active
volcanoes since ,*** in order to monitor volcanic ground deformation. At present, ,, volcanoes are undercontinuous observation using about +** GPS stations. At most observation points single-frequency receivers areadopted in consideration of power saving and mobility in rugged environments.
GPS baseline solutions include errors due to tropospheric delays which are inhomogeneous in the actualatmosphere. Generally, computation of baseline solutions is done by using a simple atmospheric model assuming
horizontal homogeneity. But if the adopted model is not consistent with the actual atmosphere, troposphericdelays cannot be accurately estimated, resulting in poor position estimates. Especially with regard to the volcano
observation, the errors in the vertical component of baselines become large according to the large troposphereerrors. In the case of baseline solutions between receivers with large vertical di#erence, the time variation of thevertical length is superposed by seasonal noise caused by spatial and temporal variations of refractive index of theatmosphere. For accurate monitoring of volcanic activities, more precise positioning in the vertical component
is desirable, which should be realized by incorporating more accurate atmospheric model into the analysisprocedure.
For this purpose, an improved analysis process was developed, based on the JMA’s operational meso-scale
numerical weather analysis (MANAL). The MANAL is applied to daily meso-scale numerical weather prediction as initial ﬁeld. Generally in the di#erential analysis process of positioning, zenith tropospheric delay (ZTD)
is estimated by least-squares method together with the positioning. In this case, initial value of ZTD is given froma simple atmosphere model. In our approach, ZTD between both receiver sites is calculated from MANAL, andthen the conventional analysis process is done ﬁxing ZTD between sites. In calculation, analysis software package
Bernese Ver. /.* was used, while a part of the program was personally modiﬁed.
This correction strategy using MANAL was applied to the baseline solutions at Asamayama volcano, whereground deformation has been observed associated with the eruption activity from ,**2 to ,**3. Consequently wecould approximately eliminate the vertical seasonal noise at a baseline whose vertical di#erence reaches to +./ km.
This approach is quite convenient and e#ective for GPS observation at local and steep areas such as volcanoes.
Key words : GPS baseline solution, tropospheric correction, JMA numerical weather model, Asamayama volcano +ῌ+-*/ῌ**/,
Seismology and Volcanology Research Department,
Meteorological Research Institute, +ῌ+ Nagamine,
Tsukuba, Ibaraki, -*/ῌ**/, Japan.
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Present address : Earthquake and Disaster-Reduction
Research Division, Research and Development Bureau,
Ministry of Education, Culture, Sports, Science andTechnology - Japan.
 +ῌ+-*/ῌ**/,Forecast Research Department, Meteorological Research Institute, +ῌ+ Nagamine, Tsukuba, Ibaraki,-*/ῌ**/, Japan.
Corresponding author : Akimichi Takagi
e-mail : atakagi@mext.go.jp