SURVEYING & MAPPING >» DEFORMATION MONITORING

i'ioring

Although an effective tool for deformation monitoring ot large structures and high-risk slopes, GPS

can also entail a high-cost disadvantage in iarge projects. A remote-controlled monitoring system

using an electronic switching device for a multiple antennas watches the steep siopes under con-

struction at the Xiaowan hydropower station in China.

X iaowan hydropower stationon the Lanchang River inYunnan province, China, con-sists of a double-curvature

arch dam, 292 meters high. Construedonbegan in January 2002 and is expected toconclude by the end of 2010. Steep slopesin the river valley, both natural and engi-neered, pose critical problems for construc-tion engineers. Hean' rain or farther rockexcavation could cause slopes near the archdam to slide. To reduce landslide risk, engi-neers have employed several conventionaltechniques, traditional .survejing equipment.and specialized geotechnical instrument tomonitor the stability- of the high-risk slopes.

XIUFENG HE is a professor at the College of CiuilEngineering, Hohai University in Nanjing, China.

WENGANG SANG is a Ph.D. student at the sameinstitution.

VONGQI CHEN is chair and head of the Department ofLand Surveying and Geo-lnformatics. Hong KongPolytechnic Universitv in Hong Kong.

XIAOLI DING is a professor in the same department.

20 G P S W o r l d NOVEMBER 2005

They also used GPS as a monitoring tool forhigh-risk slopes.

Usually, several observation points mustbe monitored to Rilly understand the stabil-ity and any ongoing deformation that couldcause slope failure. For example, it required16 observation points to adequately moni-tor a high-risk slope measuring 300 by 500meters, or 0.15 square kilometers.

GPS offers greater accuracy' and is highlyautomated and less labor intensive than theconventional techniques used during thestability monitoring of the high-risk slopes.However, CiPS does have disadvantages, themajor drawback being the high cost associ-ated witb placing a permanent CiPS receiverat each monitoring point. Xiaowan powerstation bas many steep slopes; tberefore, con-ventional GPS monitoring methods havesignificant limitation.̂ here.

We bave implemented a new approachlinking a single GPS receiver witb multipleantennas mounted at several monitoringjx)ints. We developed a dedicated electronicswitcbing device — the GPS multiple-antenna switch (GMAS) — to connect the

receiver with tbe antennas, significantlyreducing tbe required hardware investment.

Otber tecbnologies include a new elec-tronic switching device for CTM-XS, (GeneralPacket Radio Service (GPRS) wireless datacommunication, and a niicroamplifier.

System DescriptionFigure 1 outlines the GPS multiple-antenna

An experimental version of this method-

ology appeared in the March 2000 issue

of GPS World, in an "Innovation" column

by Xiaoli Ding andYongqi Chen, oo-

authors of this article, vi/ith four others.

The photo on this page shows a differ-

ent, completed dam in the same region

of China, with the same double-curva-

ture arch structure. The photo at top of

the next page depicts an architectural

modei of the Xiaowan dam, with Number

Two steep slope, referenced later, on the

righthand side.

The magazine's cover shows a base

station on Number Two steep slope.

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Antenna Array A• • • / \

^ 1 Low Noise Amplifier

GPRSTerminal

• ARCHITECTURAL MODEL of the completed Xiaowondam and hydropower station

system for slope-deformation monitoring. The system includesthree main parts: the GPS GMAS with antenna array and low-noise amplifier, control center, and the GPRS wireless datacommunication system. The GMAS is the core ofthe slope-defonnation monitoring system.

Switching Device. Adjacent photos show the GMAS, anelectronic switching device designed for a multiple-antennaGPS deformation-monitoring system. Patented in 2002 inChina, the GMAS connects eight antennas to a single GPS re-ceiver. By switching antenna array in turn, the receiver moni-tors eight separate points. GMAS with different interfaces andconnectors support two modes of commercial GPS receivers.One type can connect directly with any standard survey-typeGPS receivers and antennas. In the other mode, GPS cardsare embedded into the CJM\S.

The GMAS sequentially alkxates time to each antenna. Themain parameter entered into the GMAS is the time allocatedto an antenna in each round of measurements. The receivermakes standard pseudorange and carrier-phase ohservations foreach antenna.

Tivo options are available when allocating time to each an-tenna. The receiver can he connected to an antenna for oneepoch of GPS measurements only (say, for 10 seconds) and thenconnected to the next antenna in rotation. A time series fromeach antenna is tlien built up, with additional data added eachtime the antenna is re\'isited. This option causes cycle slips inthe dat;i series as the GMAS switches from one antenna to thenext. The well-known methods of cycle-slip detection and re-construction do not work in tbis case. However, when the rateof defomiation ofthe monitored feature is low, the integer am-biguities for each epoch may be detennined based on the priorknown c<3ordinates ofthe monitored points.

Alternatively, the receiver could remain connected to eachantenna for a set period of time to allow it to acquire a numberof epochs of data from an antenna before it switches to the next.For this second option, the data from each antenna can beprocessed as a short-time series using standard double-differ-encing techniques with initial-ambiguity resolution.

In defomiation monitoring, depending on the type and na-ture of tbe monitored features, sometimes the timing — thatis, tbe rate of acquiring and processing the data — is impor-

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DataCommunication

AlarmDevice

Data ProcessingControl Center

A FiGURE 1 The outline of a GPS multiple-antenna deformation monitoringsystem ^

• F R O N T O F the multiple-antenna switching device

• B A C K O F the multiple-anlenna switching device

NOVEMBER 2005 G P S World 21

»>SURVEYING & MAPPING D E F O R M A T I O N M O N I T O R I N G

• LOW-NOISE n

A G P R S • iMsmJtter device

tant to obtain a warning of antenna mo-tion as soon as possible. In other cases, theaccuracy of measurements is more impor-tant. Therefore, users must choose an ap-propriate option for a given situation.

For the Xiaowan hydropower station,therefore, we chose the second option forhigh-aeeuracy measurements.

Microamplif ier. 'ITie antennas were con-nected to the CiMAS by coaxial cables, hutthere is a practical limit to how long thesecan be. Even with low-loss cables, signalattenuation reduces the signal-to-noise levelbelow a usahle threshold for cables muchlonger tban approximately 30 meters. Low-noise preamplifiers help overcome thesignal-to-noise losses for longer cable runs;alternately, a system of fiber-optic cables canbe used.

As a general requirement, the length of acoaxial cable between a GPS antenna andtbe GMAS must be less than 30 meters. Thiswould severely limit the slope-monitoringarea at Xiaowan. From an engineering view-point, we developed low-noise niicroampli-fiers for GMAS.

The maximum distance between antennaand GMAS is now as great as 1 kilometer.These microamplifiers have been used suc-cessfully for steep-slope monitoring oftheXiaowan hydropower station.

Data Link. Currently a fixed telephone

22 GPS World NOVEMBER 2005

line, a dedicated data line, a wireless globalsystem for mobile communications (GSM)data line, and radio transmitters can be usedas data communication links between thecontrol center and the slope zone. The steepslope in this study is in dangerous area.Because ofthe dangerous terrain and thecost of data communication, we chose GPRScommunication technology, as being lessexpensive and more convenient than othercommunication methods.

GPRS is a standard for wireless commu-nications which runs at speeds as fast as 115kilobits per second, and it is particularly wellsuited for sending and receiving large vol-umes of GPS raw data. Using GPRS devicesdeveloped by the authors and the user-defined communication protocol, we havesent large volumes of GPS raw data auto-matically from steep slope at the Xiaowanhydropower station to the control center.

ImplementationThe physical conditions ofthe Xiaowanhydropower station are complicated bothseismically and geologically in its outer sur-roundings; however, the dam site itself isin a relatively safe zone witb considerablestability'. The arch dam is designed to reston a V-shaped deep valley foniied by mas-sive mountains on both banks, more than1000 meters above the river surface. Theconstruction involves slope-cutdng. Tbe sta-bility of steep slopes in the vicinity ofthedam is interrelated to the whole construc-tion project. Fhe photograph above showswhere the shoulder of the arch dam islocated at the Niunber 'Iwo steep slope. 'TTieslope is 700 meters high, and the mean slopegradient is approximately 40^5 degrees.Thus, we needed to place a strong empba-sis on monitoring steep slope Number Two.

We set up the GPS network to perfonnthe monitoring work. It included 16 moni-toring stations and two base stations. Figure2, a depiction of Number Two steep slopeproduced by Autocad software, shows tbedistribution ofthe GPS network, with analternative version ofthe same networkshown in Figure 3.'Iwo base stations, Bl andB2, stand some distance from the moni-tonng zone, and 16 monitoring stations aredistributed across Number Two steep slope.Photographs at tbe right and on the cover

• NUMBERTWO SLOPE seen

A FIGURE 2steep slope

GPS network of NuniberTwo

A FIGURE 3 GPS monitoring network of theNumberTwc steep slope

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DEFORMATION MONITORING

of the magazine show the GPS antennas atthe base station and monitoring station,respectively.

The screen shot just below the base stationphoto shows the working status of 16 moni-toring stations, displayed on the computerscreens in the control center. When the indi-cator lights flash, it indicates that the GMASchannels works well and that GPS raw dataarc being transmitted to the control centerfrom the monitoring stations and base stations.

Results and AnalysisWe have conducted studies using the GMASdeformation-monitoring system for theNumber Two steep slope at the Xiaowanhydropower station since June 2004. The po-sition solutions of aO monitoring points wereobtained based upon raw GPS data from themonitoring zone. Standard coordinates ofmonitoring points from the traditional sur-veying techniques enabled us to compare thedeformation results.

6PS• GPS ANTENNA at base station

We set the observation time for eachantenna at 10 minutes, with the receiver-sam-pling interval at 15 seconds. A dedicated soft-ware package processed the data.

Table 1 shows positioning accuracies of allmonitoring points in WGS-84 frame. TableI shows that the positioning errors are lessthan 3 millimeters in the horizontal direction

and are less than 7 millimeters in the verti-cal direction. Note that 18 surve^'-quality GPSreceivers were required in the conventionalGPS network; however, we used four survey-quality GPS receivers for our study of theNumber Tivo steep slope at the Xiaowan hy-dropower station using the CJMAS defomia-tion-inonitoring s^'stem.

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24 G P S World NOVEMBER 2005 CIRCLE CIRCLE 17 www.gpsworld.com

TABLE 1 Positioning

Monitoring points

TP01

TP04

TP09

TP14

TP15

TP18

TP29A

TP33

TP22

TP25

TP30

TP36

TP29

TP35

errors of monitoring points

X

1.8

1.6

2.0

2.3

1.7

2.5

2.6

2.0

1.1

1.3

1.1

1.3

1.2

1.2

WGS-84 frame (mm)

Y

5.4

4.5

5.8

6.5

4.9

5.8

5.5

5.3

3.2

3.83.1

3.7

3.3

3.5

Z

2.0

1.8

2.2

2.7

1.9

2.9

2.6

2.5

1.3

1.5

1.3

1.4

1.4

1.4

ConclusionWe have implemented a remote-controlledGPS steep-slope monitoring sj'stem basedon GM4S at the Xiaowan hydropower sta-tion and conducted a study using the sys-tem on Number Two steep slope for morethan one year. The results illustrate thatthe GiMAS system is stable and can pro-vide precise positioning accuracy. Thenumber of survey-quality receivers has

been reduced significantJy compared withthe number required by a conventionalGPS network.

Thus, the GMAS system represents aneconomical new technique for deformationmonitoring using a large-scale network.This system is a potentially cost-effectivesolution for deformation monitoring ofconstructed and natural structures such asdams, slopes, and landslides.

A WORKING STATUS of GMAS monitoring

system ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

AcknowledgmentsThis research work was supported by GrantNumber 50279005 from the NationalNature Science Foundation of China. ®

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