AGORA: Alignment of Gaia optical and radio reference frames

自2014年以来，盖亚卫星以高精度的光学波长扫描可观测的宇宙。欧洲航天局(ESA)的这次天体测量任务创造了几项记录：随着第二次数据发布(DR2)，以前所未有的相对精度包含了超过17亿个天体。对于地球上和卫星和航天器上的应用，只有将GAIA数据参考到相应的高精度参考系，才能适当地利用它们。有关大地测量、天文学和空间飞行的参照系、代表地球表面的国际天文学框架和太阳系星历可通过国际天文参照系通过空间大地测量技术VLBI(甚长基线干涉测量)获得。AGORA项目通过准确地确定GAIA相对于ICRF的时变方向，从而允许访问GAIA数据。基于传统的甚长基线测量网络和国际大地测量和天体测量服务机构的新的甚长基线全球观测系统，将确定一个一致的天文参考系，其精确度足以满足盖亚飞行任务整个期间的定向要求。活动星系核和它们的光学对应物，即宿主射电星系，是GAIA星表和ICRF中可用的天体。因此，这些都非常适合建立相互导向。然而，所有射电星系在光学波长上都非常微弱，因此不能提供足够的精度来定位明亮部分的Gaia天体。在AGORA项目中，计划对射电恒星进行VLBI观测，并将对其进行分析，以提供明亮的Gaia天体的适当方向。由于射电恒星在射电波长上非常微弱，它们的观测必须使用特殊的相对测量技术。项目完成后，GAIA数据的转换参数将可用于上述参考系统中的应用。因此，地面光学观测和其他应用，如卫星的姿态控制，可以根据GAIA数据以显著提高的精度加以利用。

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.