Mass structure deformation monitoring using low cost Differential Global Positioning System device
Azar, Ramin S. and Mohd Shafri, Helmi Zulhaidi (2009) Mass structure deformation monitoring using low cost Differential Global Positioning System device. American Journal of Applied Sciences, 6 (1). pp. 152-156. ISSN 1546-9239
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Monitoring of engineering structures has become of importance particularly after the possibility of destructive natural catastrophes has increased. To this end, big engineering structures like suspension bridges, viaducts, tunnels and high buildings etc have been subjected to continuous monitoring surveys (1). The technological developments in high precision point positioning systems together with no-human data transmission techniques without any atmospheric obligation have led to easy adaption of such monitoring systems for the objects in question (1). With the development of Global Positioning System (GPS), the sampling frequency of GPS receiver can reach about 20 times per second, while the location precision can approach 5~10 mm. So, GPS can be used in the displacement monitoring of large structures (5). The carrier phase double difference mathematic model can be adopted to survey the structures with GPS. This model can remove the error between the clocks in the satellite and the receiver. Because the errors of orbit and atmosphere are connected with the distance between the datum point and monitoring point, a GPS receiver antenna should be placed near to the target bridge as a reference station. This point should be stable, and there are no buildings above 5[degrees] to envelop or reflect signals. Another GPS receiver antenna should be placed on the monitoring point, which is often at the mid-span, quarter span or the top of towers and there had better be no structures to envelop the signal, either. At least 5 satellites signal should be received at the same time, and the data will be stored in the computer. When the data is processed, the displacement of each time point can be obtained (5). The results are provided in WGS 84 geodetic coordination. (The horizontal and vertical displacement of bridges can be computed by coordinate projection and translation). In DGPS positioning, we usually assume the position of the reference station is exactly known in WGS 84. In practice, however, the position of the reference station in WGS 84 may not be exactly known due to some practical difficulties. One situation is that there are no known points available in the positioning area and we have to use a point positioning method to determine the position of the reference station. Another case is that we have known points available in the working area but, the coordinates may not be in WGS 84 and we have to transfer the coordinates to WGS 84. Therefore, it is of significance to investigate the effect of the reference station position errors on the remote station position. GPS monitoring of structures is just being recently adopted in Malaysian infrastructure industry and this study was conducted to evaluate the possibilities and limitations to run such tasks on different type structures in Malaysia. More work is being planned to expand GPS monitoring on high rise buildings and towers in Malaysia. As a mid traffic bridge, Seri Wawasan bridge in Putrajaya was selected as study site to be monitored from March to May 2008. Limitations for using low cost GPS devices were assessed and suggestions were made. Although several methods such as RTK were available but here post processing was selected since the equipment were not able to receive real time signals. In this research we have chosen post-processing method with use of two Leica GS20 GPS system with the cost of about $USD 13,000. This GPS system has an accuracy of about 1m on the field and 10cm after single frequency post-processing but when they are post processed in double frequency phase their accuracy will reach to 1cm, which is still not enough if we are doing very precise measurements such as static load. Since we are intended to achieve an experiment in using low cost GPS systems to monitor movements of bridge due to wind or traffic movement it would roughly show us if the bridge is stable or the variance of its movements is too high. If a static load analysis or high precision method were to be carried, it would be better always to use real-time method with real-time connection to correction station. The cost of these devices would then go up to three or four times more (3), (4). Another way would be to establish two reference stations, which also increases the number of devices needed.
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