Monitoring rock deformations is important to assessing the stability of rock structures, i.e., slopes, tunnels, dams, foundations, etc, in order to confirm the validity of the design during/after construction and to assist in answering specific questions on a project. Various instruments and systems have been developed to achieve successful monitoring. A new displacement monitoring system using GPS has been developed by the author and his colleagues. The system can measure three-dimensional displacements automatically with mm accuracy, and it is now being widely used in various rock and geotechnical engineering projects in Japan. In this paper, further improvements to GPS displacement monitoring and its applications for monitoring slope stability are described.
The Global Positioning System (GPS)  began to be used for displacement monitoring in the fields of Rock Engineering and Mining Engineering in the mid-1980s [2, 3]. Although it had the potential to automatically monitor three-dimensional displacements over extensive areas, it was not applied much in practice. The reasons were uncertain accuracy, troublesome handling, and high costs at that time. A new type of GPS displacement monitoring system has been developed by the authors [4–6]. It can simultaneously measure the three-dimensional displacements of many points over an extensive area and display the results on a computer monitor in real time. The accuracy of the measurements has been improved to 1–2 mm in three-dimensional directions by using a data processing method , although the accuracy of the conventional GPS is 5–10 mm. Since the system is being widely used, higher accuracy is required. There are three main error factors in GPS monitoring, namely,
signal disturbances due to overhead obstacles, and
multipath effects . In particular, the errors caused by tropospheric delays are the most difficult to correct, and it is said that they are the ultimate error factors in GPS monitoring.
In this paper, the modified Hopfield model is adopted to overcome the above-mentioned errors, and it is applied to verify improvements in the accuracy of the measurements through a practical application.
The GPS continuous displacement monitoring system  is illustrated in Fig. 1. Receivers (sensors) composed of an antenna and a terminal box are set on a measurement point and the reference point, and they are connected to a control box into which a computer, a data memory, and a network device have been installed. The data emitted from the satellites are transferred to the control box through cables. The server computer, which is located at an office away from the measurement area, controls the entire system to acquire the data from the control box and to analyze the data in order to obtain the displacements at all the measurement points. Figure 2 illustrates a web system combining the above monitoring system with the trend model (see section 2.2) and a provider service system . The monitoring results are provided to users on the web through the Internet.