AGORA: Alignment of Gaia optical and radio reference frames

自2014年以来，盖亚卫星以光学波长高精度扫描可观测的宇宙。欧空局（欧空局）的这一天体测量使命创造了几项记录：第二次数据发布（DR 2）已包括超过17亿个天体，其相对精度前所未有。对于地球上以及卫星和航天器上的应用，只有在能够以高精度将盖亚数据引用到相应的参考系统时，才能适当地利用盖亚数据。通过空间大地测量技术VLBI（甚长基线干涉测量），可通过国际天体参考框架（ICRF）获取大地测量、天文学和空间飞行的相关参考框架、代表地球表面的ITRF和太阳系星历。AGORA项目通过精确确定盖亚相对于ICRF的时变方向，赠款了获取盖亚数据的途径。基于原有的VLBI网络和新的VLBI全球观测系统（VGOS）的IVS（国际VLBI测量和天体测量服务）将确定一个一致的天体参考框架，这是足够准确的，以满足在整个盖亚使命的持续时间的方向的要求。活动星系核和它们的光学对应物，即宿主射电星系，是盖亚目录和ICRF中的对象。因此，它们非常适合于建立相互定向。然而，所有的射电星系在光波长上都非常微弱，因此无法提供足够的精确度来定位盖亚天体的明亮部分。在AGORA项目中，计划对射电星进行VLBI观测，并将进行分析，以确定明亮的盖亚天体的正确方向。由于射电星的射电波长非常微弱，因此必须使用特殊的相对测量技术进行观测。项目完成后，盖亚数据的转换参数将可用于上述参考系统。因此，可以根据盖亚数据利用地面光学观测和其他应用，如卫星的姿态控制，并大大提高精度。

Since 2014, the Gaia satellite scans the observable universe in optical wavelengths with high precision. This astrometric mission of ESA (European Space Agency) establishes several records: with the second data release (DR2) there have been more than 1.7 billion objects included with unprecedented relative precision. For applications on Earth and on board of satellites and spacecraft, the Gaia data can only be utilized properly, if they can be referred to the corresponding reference systems with high precision. Relevant reference frames for geodesy, astronomy and space flight, the ITRF, that represents the Earth surface, and the solar system ephemerides are accessible via the International Celestial Reference Frame (ICRF) through the space geodetic technique VLBI (very long baseline interferometry). Project AGORA grants the access to Gaia data by establishing precisely the time-variable orientation of Gaia with respect to ICRF. Based on the legacy VLBI networks and the new VLBI Global Observing System (VGOS) of the IVS (International VLBI Service for Geodesy and Astrometry) a consistent celestial reference frame will be determined, which is accurate enough to meet the requirements for the orientation during the entire duration of the Gaia mission. Active galactic nuclei and their optical counterparts, the hosting radio galaxies, are objects that are available in the Gaia catalogue as well as in the ICRF. These are therefore very suitable for establishment of the mutual orientation. All radio galaxies, however, are extremely faint in optical wavelength and thus do not provide enough precision for orientation of the bright fraction of Gaia objects. In project AGORA, VLBI observations of radio stars are planned and will be analysed for providing the proper orientation of bright Gaia objects. Since radio stars are extremely faint in radio wavelengths, their observation has to be done with special relative measurement techniques. After completion of the project, transformation parameters of Gaia data will be available for applications in the reference systems mentioned above. Thereby, ground-based optical observations and other applications, such as the attitude control of satellites, can be utilized based on Gaia data with substantially improved accuracy.