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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 0.15square kilometers.　

　　　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.

　　　The screen shot just below the base station photo shows the working

　status of 16 monitoring stations, displayed on the computer screens in the

　control center. When the indicator lights flash, it indicates that the

　GMAS channels works well and that GPS raw data are being transmitted to

　the control center from the monitoring stations and base stations.

	Low-noise microamplifier. The antennas were connected to the GMAS by coaxial
	cables,but there is a practical limit to how long these can be. Even with low-loss
	cables, signal attenuation reduces the signal-to-noise level below a usable threshold
	for cables much longer than approximately 30 meters. Low-noise preamplifiers help
	overcome the signal-to-noise losses for longer cable runs;alternately, a system of
	fiber-optic cables can be used.

As a general requirement, the length of a coaxial cable between a GPS antenna and the GMAS must be
less than 30 meters. This would severely limit the slope-monitoring area at Xiaowan. From an engineering
viewpoint, we developed low-noise microamplifiers for GMAS.

The maximum distance between antenna and GMAS is now as great as 1 kilometer. These microamplifiers
have been used successfully for steep-slope monitoring of the Xiaowan hydropower station.

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