## 3 Study Method  
In this study, a static displacement monitoring program was developed to acquire accurate 3D measurements by photographing the VLBI antenna's main reflector with CCD cameras and analyzing the results with close-range photogrammetry. Close-range photogrammetry is a method for accurately measuring the positions and shapes of facilities using stereo images [16; 17; 18]. For this purpose, the interior orientation parameters, including the focal length of the camera, coordinates of the principal point, and lens distortion, and the exterior orientation parameters, including the camera orientation and position of the perspective center, were calculated [18; 19]. Among the four fixed pillars around the VLBI antenna, a total station was installed in the fixed pillar closest to the control point, and a ground control point survey was conducted for the feature points on the VLBI antenna surface. An S8 Total Station from Trimble was used to conduct the ground control point survey, shown in Figure 4.  
Attaching a reflective target on the VLBI antenna surface for the ground control point survey would likely reduce the positioning accuracy of the VLBI antenna due to radio interference. Thus, the feature points on the antenna surface were measured using a non-target method. The non-target measurement using the total station showed an accuracy and error range of \(\pm\)(2 mm + 2 ppm \(\times\) D) in the standard mode, and \(\pm\)(4 mm + 2 ppm \(\times\) D) in the tracking mode. In this study, the 3D ground coordinates of the feature points on the VLBI antenna surface were measured. After the local coordinates were set, a ground control point survey was conducted for the feature points on the surface of the VLBI antenna. Ground control point measurements are required to calculate the interior and exterior orientation parameters of the camera, and to verify the results. Figure 3: The components of the process for measuring static displacement of the VLBI antenna surface using close-range photogrammetry at the SGOC. The results of the ground control point survey were used as the reference coordinates for calculating the camera orientation parameters and for evaluating the accuracy based on the differences when the target coordinates were recalculated after determining the orientation parameters. To match the image coordinates of the feature points on the VLBI antenna surface with the results of the ground control point coordinates, the feature points of the VLBI antenna surface were labeled, as shown in Figure 5. Then, labeled feature points easily distinguishable with the naked eye were selected. Because the selected feature points were evenly distributed on the VLBI antenna surface, the main reflector shape of the antenna could be accurately perceived. The labeling of the feature points in Figure 5 was classified into two categories. For those indicated with a blue box, the outermost edges of the upper top side in the rectangle were used as the feature points. For those indicated with a red box, one dot in a certain direction in a group of two or four dots was selected and used as the feature point. To calculate accurate interior and exterior orientation parameters of the camera, image coordinates were collected from the 188 labeled feature points in Figure 5, and a ground control point survey was conducted. The 188 image coordinates used to calculate the accurate internal and external orientation parameters of the camera were used as the conjugate point in the stereo images. The conjugate point  
Figure 4: Ground control point survey for the feature points on the VLBI antenna surface. Figure 5: Target labeling for control point surveying and acquisition of image coordinates. was the same point in the stereo images and the object of the 3D coordinate calculation after obtaining the orientation results.