Hou Tao, Fan Xiangtao, Guohuadong, Nie Yueping, Zhu boqin
State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing Applications
Chinese Academy of Sciences, P.O.X 9718, Beijing, China
Email: wuhanht@sohu.com
Abstract—on the 24th General Assembly of International Scientific Radio Union in 1993, astronomers from 10 countries, including China, proposed a tentative large radio telescope project, also called SKA (Square Kilometer Array). Firstly, using space-borne remote sensing images, topographic maps and other relevant data, through image processing and spatial characters of remote sensing images interpretation, the surrounding sites of SKA in a 1000 km radius circle region centered in Guizhou Province has been searched and a lot of Karst depressions were found compatible for the construction of SKA. According to historical records, remote sensing images analysis and local engineering geological and hydrological survey, many aspects were studied and evaluated in order to select the optimal site, such as natural geography, controlling factors of physiographic feathers growth, Karst depressions shapes, hydrology, engineering stability, weather and radio environment in the Karst region of south Guizhou.
Secondly, using the high-resolution QuickBird remote sensing images, 1:10000 scale topographic maps and telescope parameters as the data source, this paper specifically studied the surface fitting between the digital terrain model (DTM) of Dawodang Karst depression and the spherical cap of FAST. In 3D simulation scene the quantities of earth excavation and filling are computed as the indication of construction cost, and through acquiring the excavating depth and filling depth between the telescope’s sub-surface and land and viewing the topography the construction difficulties can be evaluated. The 3D simulation scene of the surrounding environment of Dawodang Karst depression and the FAST model are constructed. During the process of adjusting telescope model positions, the digital elevation model and telescope reflector sphere surface model were combined to compute the quantities of excavation and earth filling in real-time and acquire the height and depth data between the telescope’s sub-surface and land. The 3D simulation experiment results indicate that Dawodang 3D scene based on the DEM and high-resolution remote sensing images can better reflect the 3D topography shape and directly give relevant parameters of FAST site selection in Dawodang Karst depression.
Key Words: Karst depression, large radio telescope model, sphere caps fitting, 3D simulation
I. Introduction
On the 24th General Assembly of International Scientific Radio Union in 1993, astronomers from 10 countries, including China, proposed a tentative large radio telescope project, also called SKA (Square Kilometer Array). Since 1994, the Chinese research team leaded by National Astronomical Observatories has been working on the research for the construction of Five-hundred–meter Aperture Spherical Telescope, simply called FAST, as a pre-project for SKA. One of the essential differences comparing with other countries’ plans is that China proposed to use Karst depression as the main antenna reflector surface support and the pending construction site addresses for FAST, and to construct approximately 40 units to build up SKA in the Karst region of Guizhou Province[1, 2].
II. Study of Site Selection
The main factors that influence the site selection of telescope can be generalized as follow:
1) Geometry conditions of depression
The geometry conditions reflect the fitting between the telescope sphere cave and Karst depression and determine the total quantity of filling and excavating of earth. Moreover, close depression can reduce the quantity and wind overload. Having more surrounding hills can be better for the non-close depression [3, 4].
2) Geography condition
Good solid base and rock construction are the most important indications for constructing the telescope. Under ground river and other karst water system directly control the output of water flow in depression.
3) Weather condition
The temperature and water falling are complicated at the area surrounding depression and influence the design and use of telescope.
4) Electromagnetic wave environment
Because the telescope usually receives the weak electromagnetic signal from the remote space, there is no permitting other electromagnetic disturbing,otherwise, telescope will not work.
5) Social and economical condition
Constructing and maintaining the telescope need some necessary social and economical conditions such as traffic, power and human resource.
So, it is a complicated systemic analysis process to select the optimal candidate site from many karst depressions in Guizhou province. In essence, it is to find out the optimal combination from the conditions of depression geometry, geology, weather and electromagnetic wave. The method is described in detail as follow:
1) Using the remote sensing images and geological maps, according to the distribution of carbonate rock, the region where Karst depression can not develop is excluded. Then using the large scale topographic map and remote sensing images, according to the geometry conditions of constructing telescope, several depressions are initially selected.
2) These selected depressions are locally reviewed. The factors such as stratum, lithology and rock structure are investigated in detail. Combining the prior geometry conditions, these depressions are qualitatively analysis and evaluated.
3) According to the selected depressions, the distribution map of candidate sites is plotted. The key region in which depressions centralize is confirmed.
Through the specific analysis of the tectonics, engineering geological and hydrological data in these two regions, study showed that there is no fault seen in the Mesozoic and Cenozoic red basin layer in the Wanjin period. Other faults and joints have been filled by calcite veins and conglomerations with firm cementation. The geological structure is simple and the original structure is well preserved. The compact structure and well-proportioned texture also have good engineering geological attributes.
More than 300 Karst depressions were found suitable for the SKA construction and two optimal sites were picked out, named Dawodang and Shangjiachong, respectively.
III. Methodology and technologies of 3D simulation
As a synthesize technology, has been springing up in the end of 20 century, Virtual Reality technology integrates several latest information technology such as computer graph, artificial intelligence, Multi-media, Sensor surveying and parallel processing technology.
Because the VR system gives a direct display for building, some faults of design can be found out in time. VR system makes progress in the representation of site plan from the traditional method to the new digital technique and solved the difficult problem of no good visualization in plan evaluation.
The technical route of 3D simulation reference to the figure1 and the detailed steps of reconstruction are listed as below:
1) Data processing: Using high-resolution Quickbird images and 1:10000 scale topographic map as basic data source. Because the original contour precision is five meters, using the re-sampling method, the higher precision DEM is obtained.
2) Rectifying the remote sensing images as the scene texture. Based on the DEM, terrain 3D model is created. In the 3DStudioMAX environment, according to the size of telescope, the virtual telescope model is created.
3) Based on the parameters of telescope, the reflection sphere space model is computed and its values are stored in 2D array.
4)After all models are added into the 3d scene driven engine, during the moving the telescope model, the information such as quantities of filling and excavating earth and orientation can be queried in real time by computing the DEM and reflection sphere space model.
The method of constructing the telescope reflector sphere surface model is firstly to project the telescope model onto a square grid which is plotted into many unit grids. Then comparing the telescope’s aperture circle radius and the distance of every unit grid to the whole grid’s center, those unit grids being occupied by the projected telescope’s sub-surface are marked. At last, the height of the marked unit grids is calculated according to the sphere radius, aperture circle radius and flare angle of telescope. The method of computing the quantities of excavation and earth filling in real-time is firstly to determine the coordinate of the left-up corner of the reflector sphere surface model in the terrain grid. Then the depth of earth filling is calculated by the height of terrain minus heights of telescope’s sub-surface. The height of earth excavating is contrarily calculated. The total quantities of excavation and earth filling are calculated by summing the results of multiplying the area of every unit grid by the depth of earth filling or the height of earth excavating. The whole process of computing the quantity of filling and excavating earth references to figure 2
The query of fitting information process is described in detail.Given the moving condition, the corner coordinate of telescope model relative to the terrain grid is （x0+xcount, y0+ ycount）, to self is (0, 0). In reverse, if the position of mouse clicking point on the terrain is (tx,ty), to the telescope model at top the coordinate is （tx-x0-xcount, ty-y0-ycount）. After obtaining the (tx,ty), equal to obtaining the index of DEM array, the altitude value can be queried. After obtaining the （tx-x0-xcount,ty- y0-ycount）, equal to obtaining the index of a unit grid of telescope model, the altitude of telescope model can be queried. Through the comparison of these two values, the filling depth and excavating height information can be computed.
Result AnalysisWhen telescope model not horizontally moving, temporarily vertically moving up, the quantity of filling earth is increasing and the quantity of excavating earth is decreasing to 0. At this time, the terrain traversing the surface of telescope model is gradually decreasing and disappears at last in the 3D scene display. If telescope model vertically moving down, the quantity of excavating earth will gradually increase and the quantity of filling earth will be zero. During this process the telescope model sink into the terrain in the 3D display. It indicates the display effect is consistent to the computing result.
Given the condition of filling and excavating earth respectively, theoretic result is
Pai*r*r*deltaheight=3.14159 X250 X 250 X（1008-904）
=3.14159 X2.5 X2.5 X1.04 X106=2.0420335 X107
Real result is
（0.615708+1.43609）X107=2.051798 X107
The error is
（2.051798 X107-2.0420335 X107）/2.051798 X107
= 0.0097645/2.051798=0.004759=0.48%
I. Conclusion and Discussion
The 3D simulation experiment results indicate that Dawodang 3D scene based on the DEM and high-resolution remote sensing images can better reflect the 3D topography shape and directly give relevant parameters of FAST site selection in Dawodang Karst depression. The expected goal and effect can be realized by viewing the whole project environment in different angles and observing the site’s surrounding topography in a large scale, whereas the traditional method shows its deficiency. In the Dawodang 3D scene, the telescope model, about
Acknowledgment
This work was partially supported by the National Key Technologies R&D Program Olympic Games Dedicated Projects "Research of dynamic monitor for Beijing Olympic Green Environment"(Grant No. 2002BA904B07). Special thanks to Songyang for his support in words check.
References
[1] Nan R D, Peng B. A chinese concept for the 1 km2 radio telescope. Acta Astronautica, 2000, 46(10~12): 667~675
[2] Nan R D,Five-hundred meter Aperture Spherical Telescope.SCIENCE IN CHINA Ser. G Physics, Mechanics & Astronomy,2005, 35(5): 449~466.
[3] Liu Hong , Song Jianbo, Wang Wenjun ,Peng Bo , Nan Rendong ,Research on Three - Dimension Simulation and Parameters Optimization System for the Site of Large Radio Telescope. Mining Research& Design,2005,25(4) : 42~46.
[4] Song Jianbo，Liu Hong，Wang Wenjun，Peng Bo，Nan Rendong , Methods of Selecting Site for Square Kilomater Array in KARST Rerion of Guizhou，China.Earth and Environment,2005,33(3): 63~68.
