Abstract

This study investigated the efficacy of tropospheric delay error correction in GPS displacement monitoring using the results of field measurements taken at the Tokuyama Dam in Japan, a rockfill dam with a height of 161 m. The dam body exterior displacement, measured with the GPS displacement monitoring system, showed the remarkable influence of the tropospheric delay error in the height direction component due to the difference in height between the GPS sensors. In this study, therefore, the tropospheric delay error was corrected by two methods. One is a correction method applied to estimate the amount of tropospheric delay with the modified Hopfield model using local meteorological data, and the other is a method that corrects the error by directly measuring the amount of tropospheric delay from GPS measurements taken at two fixed points at different elevations. The results obtained with these two correction methods confirmed that the tropospheric delay error in the height direction of the GPS displacement was remarkably reduced by both methods.

1. Introduction

The GPS displacement monitoring system has come to be used for the maintenance management of long mine slopes, road slopes, dam body observation, tunnel construction management, and the safety monitoring of various structures. By using the static method, which is one of the relative positioning methods, the most reliable and accurate results are obtained with GPS measurements. However, in order to obtain high-quality GPS results, attention must be paid to the error factor for which appropriate correction is required.

This study reports the results of the long-term continuous displacement measurement of a large rockfill dam, conducted from 2006 up to the present time, by the GPS displacement monitoring system for the safety management of the embankment. By analyzing the long-term GPS data in detail, the study examines the measurement accuracy and the error factors in order to obtain high-quality GPS displacement data. A typical error is the tropospheric delay error caused by the difference in height between GPS sensors. In dams with high bank heights, the height difference between sensors may be large, and the influence of the tropospheric delay error is of concern. The results of long-term GPS displacement measurements taken at the Tokuyama Dam are introduced here along with tropospheric delay correction. There is a conventional method used as the tropospheric delay error correction method that employs meteorological data (air temperature, barometric pressure, and relative humidity) obtained at the measurement site. However, it is not always easy to acquire such meteorological data in real time at the actual measurement site and to incorporate the data into the GPS measurements. In this study, therefore, following the method proposed by Soda et al., reference points (fixed points) were set at two different elevations and the tropospheric delay error was estimated from the GPS measurement results between the reference points, comparing it with that of the conventional method.

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