## 1 Introduction  
Very Long Baseline Interferometry (VLBI) is a technology that precisely determines the location of a measurement point by analyzing the differences in signal arrival time between observation stations. The signals in question are radio waves from quasars, which are at cosmological distances from multiple radio telescopes on the surface of the earth. VLBI is one of the leading space geodetic techniques, along with Global Navigation Satellite System (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbit Determination and Radio positioning Integrated on Satellite (DORIS). VLBI is being used to develop the International Terrestrial Reference Frame (ITRF) and to determine the Earth Orientation Parameter (EOP) in the field of geodetic surveying. Its applications are increasing, as it supplements other space geodetic techniques. The VLBI system is operated to obtain a baseline vector measurement with 1 cm accuracy. To maintain the performance of a VLBI antenna and its absolute position measurement accuracy, it must be monitored constantly and its deformation rate inspected continually. This diligence is needed because even a slight displacement of a few millimeters in the main reflector of the antenna directly affects VLBI measurement performance, which is utilized as the datum for national geodetic control points. Among the various displacements that the main reflector of the VLBI antenna experiences, static displacement is the major shift requiring monitoring because it causes permanent deformation. As these displacements cause absolute or relative deformations of the VLBI antenna shape and position, the 3D position coordinates of the antenna's main reflector must be measured against a solid point on the ground. In fact, reduced antenna gain of electromagnetic wave reception from a quasar is expected due to the structural deformation of the main reflector in a VLBI antenna. Therefore, the importance of structural deformation monitoring of the main reflector is of critical importance. Furthermore, the main reflector has a high potential for deformation in the VLBI structure. These factors were the motivation for this work, in which the investigation of the monitoring system for the main reflector is addressed using an efficient algorithm and close-range photogrammetry. Many studies have been conducted to monitor the structural deformation of VLBI antennas. To monitor the gravitational deformation of VLBI primary reflectors, studies applying a laser scanner and terrestrial surveying have been conducted. One study measured the patterns according to the gravitational deformation of VLBI primary reflectors, and accurately quantified the changes in focal length and incoming signal path [1]. In another study, the time series thermal deformation of a VLBI antenna was modeled under the premise that thermal changes in the VLBI influence the deformation of the antenna and displace the VLBI reference point [2]. Optical techniques based on close-range photogrammetry have been shown to be very effective and accurate (millimeter level or better) for static load test measurements [3; 4; 5; 6]. In this study, total station observations and close-range photogrammetry using Charge Coupled Device (CCD) cameras were used to measure the 3D position coordinates of multiple points on the surface of a VLBI antenna, relative to a solid point where no displacement occurs. Finally, a static displacement monitoring program for the VLBI antenna was developed, which can be utilized as a continuous and automated monitoring system for structural deformation in the near future. Other studies that performed structural health monitoring using cameras are as follows. [PERSON] et al. [7] monitored the shape of beam structures by calculating the strain and curvature of images captured with a camera. Shape modeling was studied using photogrammetry, and curvature calculation resulted from performing the health monitoring of a specific structure. [PERSON] et al.