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Steep-Slope Monitoring

November 1, 2005 By: Xiufeng He, Wengang Sang, Yongqi Chen, Xiaoli Ding

GPS Multiple-Antenna System at Xiaowan Dam

Xiaowan hydropower station on the Lanchang River in Yunnan province, China, consists of a double-curvature arch dam, 292 meters high. Construction began in January 2002 and is expected to conclude by the end of 2010. Steep slopes in the river valley, both natural and engineered, pose critical problems for construction engineers. Heavy rain or further rock excavation could cause slopes near the arch dam to slide. To reduce landslide risk, engineers have employed several conventional techniques, traditional surveying equipment, and specialized geotechnical instrument to monitor the stability of the high-risk slopes. They also used GPS as a monitoring tool for high-risk slopes.

Usually, several observation points must be monitored to fully understand the stability and any ongoing deformation that could cause slope failure. For example, it required 16 observation points to adequately monitor a high-risk slope measuring 300 by 500 meters, or 0.15 square kilometers.

Architectural model of the completed Xiaowon dam and hydropower station
Architectural model of the completed Xiaowon dam and hydropower station

GPS offers greater accuracy and is highly automated and less labor intensive than the conventional techniques used during the stability monitoring of the high-risk slopes. However, GPS does have disadvantages, the major drawback being the high cost associated with placing a permanent GPS receiver at each monitoring point. Xiaowan power station has many steep slopes; therefore, conventional GPS monitoring methods have significant limitations here.

We have implemented a new approach linking a single GPS receiver with multiple antennas mounted at several monitoring points. We developed a dedicated electronic switching device — the GPS multiple-antenna switch (GMAS) — to connect the receiver with the antennas, significantly reducing the required hardware investment.

Other technologies include a new electronic switching device for GMAS, General Packet Radio Service (GPRS) wireless data communication, and a microamplifier.

System Description

Figure 1 outlines the GPS multiple-antenna system for slope-deformation monitoring. The system includes three main parts: the GPS GMAS with antenna array and low-noise amplifier, control center, and the GPRS wireless data communication system. The GMAS is the core of the slope-deformation monitoring system.

Figure 1 The outline of a GPS multiple-antenna deformation monitoring system
Figure 1 The outline of a GPS multiple-antenna deformation monitoring system

Switching Device. Adjacent photos show the GMAS, an electronic switching device designed for a multiple-antenna GPS deformation-monitoring system. Patented in 2002 in China, the GMAS connects eight antennas to a single GPS receiver. By switching antenna array in turn, the receiver monitors eight separate points. GMAS with different interfaces and connectors support two modes of commercial GPS receivers. One type can connect directly with any standard survey-type GPS receivers and antennas. In the other mode, GPS cards are embedded into the GMAS.

The GMAS sequentially allocates time to each antenna. The main parameter entered into the GMAS is the time allocated to an antenna in each round of measurements. The receiver makes standard pseudorange and carrier-phase observations for each antenna.

Two options are available when allocating time to each antenna. The receiver can be connected to an antenna for one epoch of GPS measurements only (say, for 10 seconds) and then connected to the next antenna in rotation. A time series from each antenna is then built up, with additional data added each time the antenna is revisited. This option causes cycle slips in the data series as the GMAS switches from one antenna to the next. The well-known methods of cycle-slip detection and reconstruction do not work in this case. However, when the rate of deformation of the monitored feature is low, the integer ambiguities for each epoch may be determined based on the prior known coordinates of the monitored points.

Front of the multiple-antenna switching device
Front of the multiple-antenna switching device

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