Integrating GPS and Laser Distance Meters for Landslide Monitoring

__________________________________________________________________________________ European Geophysical Society, XXVII General Assembly, Nice, France,...
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__________________________________________________________________________________ European Geophysical Society, XXVII General Assembly, Nice, France, 21 - 26 April 2002

Integrating GPS and Laser Distance Meters for Landslide Monitoring Luca Manetti, Alfredo Knecht, Matteo Frapolli, Haiye Lou 1 1

GEODEV SA Earth Technologies, Manno (Switzerland)

ABSTRACT In recent years, systems for landslide monitoring have shifted towards ever more automatic and autonomous operation. Moreover, technologies and instruments are available to reliably interconnect distributed components. Specifically, the measurement, logging, data processing and interpretation activities may be carried out by separate units at different locations in near real-time. Building on the results of a previous development project which focused on land movement monitoring with GPS, the system has been generalized to accommodate a range of other sensors. In particular a laser distance meter has been integrated. First results confirm an expected increase in robustness of the combined measurement network, which is particularly important in unfavorable stand-alone GPS reception conditions. Due to the modular architecture of the system, other sensor types may be supported with minimal effort. Examples range from simple inclinometers to motorized theodolites. Measurements are transmitted via cellular or point-to-point radio links to a control centre, which provides for post-processing and network management. The control centre may be remotely accessed via an Internet connection. The resulting system in characterized by autonomy, reliability and a high degree of automation.

1. INTRODUCTION Owing to the advantages of high accuracy, all-weather conditions, no requirements of inter-visibility between measuring points, GPS is playing more and more important role in high precision positioning missions in structure/construction health and land formation movement monitoring. For achieving a particular purpose, a proper configured GPS measurement system can meet most of the possible static and dynamic measurement needs in such applications for absolute positioning and relative displacement. In other words the required precision and accuracy can be approached with an architecture of the GPS differential system based on the choice from different types of the GPS receives in single/dual (L1 or L1/L2) frequency carrier phase, capability of GLONASS signal, data sampling rate, communication between GPS receivers

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MANETTI, KNECHT, FRAPOLLI, LOU

and Control Unit and the method of data processing. Most of constructional and geotechnical engineering measuring and test tasks can be perfectly performed with aid of the available GPS technologies. In fact, it has been recognized that one of the factors of hindrance to the widespread use of permanent monitoring with GPS is the cost of the remote sensor. MMS (Movement Monitoring System) is a GPS sensor unit composed system developed to meet abovementioned monitoring purposes with considerations of low cost, robustness, environment tolerance, autonomous power supply, maintenance free, Internet accessibility and flexibility in system reconfiguration. In order to increase the capability of MMS in various fields and situations with higher feasibility in remote monitoring, ancillary sensors are equipped and tested with the remote GPS sensor units. In particular a laser distance meter has been integrated and the primary results confirm an expected increase in robustness of the combined measurement network, which is particularly important in unfavorable stand-alone GPS signal reception conditions. This additional instrument provides a way to compensate the loss of accuracy and precision of measurement due to the factors of lower satellite visibility, dense foliage and multiple path effect.

2. MONITORING SYSTEM DESCRIPTIONS

Figure 1: System architecture

The system consists of a number of small mobile measuring stations with a GPS receiver unit installed on the object to be monitored, plus one or more reference GPS stations installed at a measuring-object-independent location. The reference stations units are physically identical to the mobile station units. Depending on the application, remote units are individually linked, either by cable, radio or cellular modem to a Control Unit, which is responsible for data collection, GPS data post-processing and monitoring for correct operation of the network. It is remotely accessible through a dedicated communication channel, including an Internet dial-up connection.

__________________________________________________________________________________ European Geophysical Society, XXVII General Assembly, Nice, France, 21 - 26 April 2002 MANETTI, KNECHT, FRAPOLLI, LOU

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The slow dynamics associated with geotechnical movements (mm/year to cm/day) do not impose high acquisition frequencies. For such applications, 15 to 30 minutes of intervals between each measurement sessions are more than adequate if the goal is to observe possible slow land movements or trends. On the other hand, single frequency GPS static measurements requires about 10-20 minutes for receiving/sampling GPS observation and navigation data as one measurement session, which will be post-processed and make one positioning resolution. For this reason, using low-cost single frequency GPS receivers perfectly meets the application requirements with higher accuracy at acceptable expense. Every measuring station will thus collect GPS observation data for a sufficient amount of time according to the actual onsite measuring conditions, and the data will be transmitted to the Control Unit for post-processing. The Control Unit has the task of collecting the data from all receivers in the network while overseeing the single stations for correct operation. The GPS data will be processed together, and the result will consist of the relative position of the various mobile measuring stations with respect to the reference stations. The Control Unit also manages the operation of additional sensors connected to the remote measuring stations, be it for the data acquisition or for the data transmission phases. Acquisition and transmission of additional sensor data may be carried out asynchronously to GPS receiver cycles.

Figure 2: Remote GPS sensor unit

3. REMOTE SENSOR UNIT

Cellular GPS

Accumulator

/

Radio CPU

Internal Sensors

Solar Panel

Power Management

Data Acquisition

Figure 3: Sensor unit block diagram

External Sensors

A remote sensor unit is a complex device consisting of a GPS receiver, a communication transceiver, an ancillary data acquisition unit with multiple channels, and a power supply management unit. A central processing unit oversees overall operation and schedules measurement and communication tasks.

Figure 3 shows a block diagram of the sensor unit. All electronic subsystems are mounted in an environmentally tolerant sealed enclosure.

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MANETTI, KNECHT, FRAPOLLI, LOU

The GPS receiver is a single-frequency receiver module characterized by low power consumption and good sensitivity. Since the stability of the local receiver reference clock is a factor influencing the precision of the carrier phase measurement, a high-quality clock source has been selected. Ancillary sensors may be added to each station where desired. To this purpose, the unit is equipped with three analog input ports and two digital input ports. As part of the initial station configuration, these additional measurement channels may be defined in terms of measurement interval, scale and resolution. When the equipment is in use, measurements from the ancillary sensors can be asynchronously logged and transmitted to the Control Unit on demand. Three spare RS-232 serial ports are also provided for connection to "intelligent" external sensors. Presently, a laser distance meter and a motorized theodolite have been successfully connected. A CPU runs a real-time operating system and a multi-task program written in standard "C". The program oversees the operation of the all sensors, manages the data acquisition and implements the communication protocols. Activation and deactivation of each single subsystem are sequenced according to the tasks configuration. Care has been exerted in developing the program to reduce power consumption of the sensor unit to a minimum. The CPU spends most of the time in a "doze" state with very low power consumption, and periodically wakes up to attend to measurement or communication tasks. An important requirement is the ability for the measuring units to operate for extended intervals in total autonomy and without requiring on-site human intervention. Fitting each station with an accumulator and a solar panel enables autonomous operation over an extended period of time, while the unit itself is essentially maintenance-free. The Control Init can be programmed to initiate special actions when a criteria is met such as a relative position differences or the velocity of movement are found to be over a certain pre-set limit.

4.

EXTERNAL SENSORS FOR ENHANCING ROBUSTNESS OF GPS MEASUREMENTS In the field of geotechnical movement monitoring, additional sensors are useful mainly to increase the precision and robustness of positional measurements from the main sensor or providing extra physical parameters related to the movement. A landslide is a kind of local geologic structure change phenomena from steady to unsteady. The behavior of the land movement highly depends on the local geographical, geological and climate condition. Hence, in addition to the aid of GPS means for absolute or relative position, measuring it is also important and very helpful to have more data concerning different aspects that affect the slope

Figure 4: Integrated laser distance meter

__________________________________________________________________________________ European Geophysical Society, XXVII General Assembly, Nice, France, 21 - 26 April 2002 MANETTI, KNECHT, FRAPOLLI, LOU

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movement and its development from the susceptible area. The remote sensor units provide a flexible interface for connecting different sensors to meet such a sophisticated monitoring task. In the MMS system, an additional distance/orientation sensor generally serves to compensate one or more shortcomings inherent in the GPS measurement. A typical example is when a measuring station has to be installed in a location with limited visibility of the sky, although a good sky visibility is important for the precision and quality of GPS measurements. In such a case, the GPS sensor can be supplemented by, for example, a laser distance meter measuring the distance between the point in question and another one. This arrangement allows the question point to be integrated in the measurement network, compensating the loss of measurement precision and quality due to the non-ideal conditions for GPS. Figure 4 shows the integrated laser distance meter developed by the Soil Mechanics Laboratory of EPFL, Lausanne, which can be connected as an external sensor to the remote GPS sensor unit. The following table compares the performances of GPS sensor with that of the laser distance meter. Table 1: GPS and laser distance meter performance comparison

Max. baseline length Max. acquisition rate Accuracy on single measurement (horizontal) Accuracy on single measurement (Vertical) Dependency on Pressure and Temperature Dependency on day / night

L1 Phase GPS with post processing (static) 10 – 15 Km 2-4 meas. / hour ± 5..10 mm ± 7.. 15 mm weak weak

Laser distance meter 500-600 m (with reflector) Up to 30-40 meas. / min ± 1.5 mm + 3ppm ± 1.5 mm + 3ppm strong strong

The laser distance meter reveals a better performance then the GPS in short baseline determination. On the other hand the dependency of this instrument on atmospherical and physical factors is stronger than that of GPS. The following plots highlight the abovementioned dependency of the laser distance measurements on temperature and sun light conditions. In application, a proper filtering applied can improve the precision.

__________________________________________________________________________________ European Geophysical Society, XXVII General Assembly, Nice, France, 21 - 26 April 2002 6

MANETTI, KNECHT, FRAPOLLI, LOU

Day / night laser distance measurements (uncorrected) 258.115

day

258.110

day

distance [m]

258.105 258.100 258.095 258.090 258.085 258.080

night

night

night

18:18 20:51 23:23 02:02 06:06 08:38 11:11 14:01 16:35 19:05 21:37 00:07 02:40 05:10 07:40 10:21 12:52 15:22 17:52 20:22 22:52 01:22 05:50

258.075

time

Figure 5: Day / night laser distance meter measurements

Standard deviation day / night - Laser distance measurements (uncorrected) standard deviation [mm]

7.00 day

6.00

day

5.00 4.00

night

night

night

3.00 2.00 1.00 18:30 21:00 23:30 02:00 04:30 07:00 09:30 12:00 14:30 17:00 19:30 22:00 00:30 03:00 05:30 08:00 10:30 13:00 15:30 18:00 20:30 23:00 01:30

0.00

time

Figure 6: Day / night laser distance meter standard deviations

In principal, on one side, any high accuracy distance/displacement/orientation sensor can provide complementary contributions to the GPS measuring system, increasing its robustness and accuracy in spatial positioning aspect. In another side, it can also be used to put extra concerns to the move of some specific objects. With the introduction of the laser distance sensor, inclinometers or motorized theodolites, the enhancement can be in following two aspects:

__________________________________________________________________________________ European Geophysical Society, XXVII General Assembly, Nice, France, 21 - 26 April 2002 MANETTI, KNECHT, FRAPOLLI, LOU

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1. For relative displacement measuring:

• Such as crack width, rock or constructions distance/position with a specific reference point within the involved area 2. For integrating the movement to absolute coordinate with GPS measuring network:

• Compensate the shortcoming of GPS measurement due to the satellite visibility limitation from heavy foliage covering or block hindering or the possible multi-path effect from the surrounding objects; • Height measuring enhancement: the high acquisition rate of a laser distance meter can help to provide higher accuracy measurement in height range, where the precision of the GPS can be defined as 1.5 times worse as the one of the horizontal coordinates; • Extend GPS capability to where GPS receiver is not suitable to be used such as deep valleys, caves and heavy foliage covered areas, even indoor as well. This means that a GPS measuring station is used as a relay. Legend: optional sensors with digital out put

LS R:laser distance meter THEO:theodolite [A]:3 single-ended and 2 differential analog input ports [B]:2 digital input ports [C]:RS-232 serial port

GSM antenna

THEO solar panel

battery LS R GSM / radio modem

optional sensors with analogue out put

GPS strain gauge

M ain board :master board

[B]

Pressure transdusor [A]

clinometor piezometer

[C]

temptrure gauge GPS antenna

Figure 7: Remote sensor unit complexes

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MANETTI, KNECHT, FRAPOLLI, LOU

For a purpose to support a consistent post monitoring analysis, a set of complementary monitoring values is expected. Connecting other sensors to a remote measuring station can provide necessary complementary data for determining and understanding the behavior of the monitored object. Such sensors can include sensors for strain gauge, stress, acoustic, etc. in structural monitoring applications, groundwater, rain, etc. in the filed of geotechnical monitoring.

5. CONCLUSIONS Building on the results of a previous development of land movement monitoring with GPS, the system has been generalized to accommodate a range of other sensors. As an example, the integrated laser distance meter has been discussed. The results confirm an expected increase in robustness of the combined measurement network, which is particularly important in unfavorable stand-alone GPS signal reception conditions. Due to the modular architecture of the system, other sensor types can be supported and easily integrated to form a comprehensive monitoring capability with all concerned aspects.

ACKNOWLEDGEMENTS The authors would like to thank Bernardo Ferroni, Emilio Di Francesco and Gianpaolo Nodiroli of the Computer Integrated Manufacturing Institute, University of Applied Sciences of Southern Switzerland (SUPSI) as well as Gilbert Steinmann and Christophe Bonnard of Soil Mechanics Laboratory, EPFL Lausanne for their valuable contributions to this project. The original research and development project has been financed in part by the Swiss Commission for Technology and Innovation, CTI.

REFERENCES

Hofmann-Wellenhof, B., Lichtenegger, H., & Collins, J. GPS Theory and Practice. Springer Verlag, Wien New York, fourth, revised edition 1997. Seeber, G., Satellite Geodesy: Foundations, Methods, and Applications. Walter Gruyter, Berlin New York 1993. Leick, A., GPS Satellite Surveying. John Wiley and Sons, Inc., New York, 2nd edition 1997. Lowry, A. & McLeod, R., PMos: A real time precise DGPS continuous deformation monitoring system, SAGEM Australasia Pty Ltd, 1997 Behr, J., SCIGN-USGS, Hudnut, K., King., N., USGS, Monitoring Structural Deformation at Pacoima Dam, California Using Continuous GPS, 1997 Hyzak, M., Tucker, C., Duff, K., GPS Networks for Structural Monitoring: From Highway Bridges to Mach 10 Rails, Applied Research Laboratories, University of Texas at Austin 205 1997 Manetti, L., Knecht, A. Permanente und autonome Erdrutschüberwachung mit GPS, Mensuration, Photogrammetrie, Génie Rural, N° 7, 2000, Switzerland

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