An Appropriate GPS Technology for Landslide Monitoring at East-West Highway, Perak, Malaysia
2. The use of GPS Technology in Landslide Mapping
A landslide phenomenon is one of the terms to be used in describing the down-slope of soil, rock and organic material under the influence of gravity. Landslide studies can be organized into three phases, detection and classification of landslides; monitoring activity of existing landslides; analysis and prediction of slope failures in space (spatial distribution) and time (temporal distribution). The second stage in a landslide study is typically monitoring the movement of a landslide. This involves the comparison of landslide conditions over time, including the aerial extent of the landslide, the speed of movement and the change in the surface topography. Using either geotechnical methods or GPS technology the magnitude, direction of the slide, and the boundary of the landslide block can be determined. The GPS is a radio navigation, timing and positioning system with a wide set of applications. By tracking the electromagnetic waves that are sent continuously to the earth, the system can obtain the 3D coordinates (F , l , h or x, y, z). The GPS system has become a valuable complement or extension to the conventional surveying methods (theodolite, tapes, EDM, total stations, etc). Field surveys are usually carried out within a frequently given period. The results are discontinuous over time, and related to the cumulative movements of the surface points. The accuracy required for the measurement of landslide displacement should be, in many cases, at least in the order of centimeters (Josep A. Gili et. 2000). Therefore, the basic question that arises is whether the satellites orbiting 20,200 km above the earth can be used to measure coordinates or displacement of landmark points located at the ground surface with centimeter accuracy. Measurement of landslide displacements can be undertaken by means of either static or kinematic method. The choice depends on the practical considerations : (i) the accessibility, (ii) number of points, (iii) precision and (iv) distance from point to point. Nowadays, the most productive methods i.e. modern positioning technique available for determining single points with precision of milimetres or centimeters is Rapid Static (RS) and Real Time Kinematic (RTK). The RS method is a development of the classical static method, with improved algorithms that accelerates the ambiguity resolution procedures. For instance, measurement of one baseline with six and more satellite available is required for only a few minutes of data logging. This time increases to 15 and 20 min with five and four satellites, respectively. In this method, post-processing must be carried out. The data files from different receivers are merged in order to obtain the solution of the baselines between station points. In the RTK method, the information of code and carrier phase observable received at both extremes of the baseline (base station and rover station) is merged to compute the precise position on the spot. The base receiver transmits a message containing its position, the pseudo-ranges measured to the rover through code correlation and phase measurements of the carrier received from the available satellites. Prior to obtaining the first results, it is necessary to spend a few minutes to initialize the system. The RTK calculates new positions from the old ones, through continuous tracking of the satellites in real time. In this procedure, therefore, the post-processing of the field data is not required. Any problem encountered with the equipment can be localized and solved immediately. It works quickly and gives results with precision at centimeter level for every second, even during movement. In RTK, corrections are transmitted to the rover via a local UHF data link; this transmission is quite directional. Therefore, unless repeaters are used, the RTK method needs an almost direct line of sight between base and rover. In general, this method also needs four or more satellite to work. Due to the continuous tracking of satellite, it is very sensitive to the loss of reception and to the quality of the signal. Even short interruptions will produce a loss of the initialization. To recover it, at least five satellites are needed. Study of mass movement has been monitored using satellite as instrument to monitor monuments that have been installed in a stable or deform area. It gives the reference point on monument in each acquired observable. The concept of monitoring the landslide with GPS technology (Georg Gassner et., 2002, Hitoshi Kondo et., 1996) is shown in Figure:1.  Figure: 1 - Monitoring scheme; reference station (R,K) in stable area; rover station in the deformation area. |