VLBI校准源在绝对和相对天体测量方面都具有重要作用，前者如天球参考架的改进及其在射电和光学波段的连接，后者如提高VLBI视差和自行测量与航天器测定轨的精度。目前，VLBI校准源的数据库基本来自绝对天体测量，比如测地VLBI观测以及快照模式的类测地VLBI观测。然而，目前尚无来自相对天体测量（比如VLBI相位参考观测）的校准源数据库。BeSSeL为相位参考观测模式的VLBA重点科学项目，其主要目的是测量大质量恒星形成区脉泽的视差和自行，研究银河系旋臂结构和运动学。由于脉泽一般远比河外校准源强，可以作为相位参考源，来增加弱的校准源的观测积分时间和UV覆盖，从而获得绝对天体测量难以达到的高质量校准源图像。本项目将基于BeSSeL的存档数据，提取约200颗校准源的多历元和多波段（C，Ku和K）的位置和近4000张高质量图像信息，建立开放的数据库，为天体测量和天体物理研究提供重要的观测信息。

VLBI calibrators are very import for both absolute and relative VLBI astrometry, e.g., the improvement of the international celestial reference frame and the link between radio and optical frame，the improvement of the accuracy of parallax and proper motion measurement and spacecraft positioning and orbit determination. Currently, the database of VLBI calibrators is mainly from the absolute VLBI astrometry, such as geodetic VLBI and snap-shot geodetic-like VLBI observations. However, there is no database of VLBI calibrators from the relative VLBI astrometry, i.e., VLBI phase-referencing. The BeSSeL Survey is a VLBA Key Science project. The primary goal of the survey is to study the spiral structure and kinematics of the Milky Way, by measuring distances and proper motions to masers in regions of massive star formation across large portions of the Milky Way. To measure the distances of masers via trigonometric parallaxes, the relative positions between the masers and extragalactic VLBI calibrators are determined with phase-referencing at different epochs spanning at least 1 year. The Masers are usually served as phase reference since they are much stronger than the calibrators, allowing integration times about 1 hours for the weak calibrators. In addition, the calibrators are often observed though the whole observing program, providing excellent uv-coverage for imaging. From the BeSSeL survey, there will be about 4,000 high quality images of about 200 VLBI calibrators distributed near the Galactic plane. In this project, we will extract of the positions and images of the VLBI calibrators and build and database for open use, providing observational information for both astrometry and astrophysics.