A Review on the Problems in the Establishment of Celestial Reference Frames

The differences in the requirement of application and the method of construction lead to the multiform existing celestial reference frames. These frames can be classified into three categories: the Dynamical frame (ephemerides), the Optical Frame (star catalogues)and the Extragalactic Radio Frame (extragalactic radio source catalogues). Modern ephemerides are mainly based on ranging observation. Due to the insufficiency of the accurate data, it is very difficult to improve the precision of modern ephemerides. Since the dynamical models of the Sun and the rotation of the Galaxy are not very accurate at present, it is difficult to determine precisely the proper motions of stars, hence the precision of catalogues becomes lower and lower as the time flying.The idea of using extragalactic objects to define the celestial reference frame is quite old, even before the confirmation of such object existances. Any object that is sufficiently distant may have no detectabal proper motion, thus it would be more desirable to define the celestial reference frame rather than stars. Since use of the modern radio astrometric techique in 1960/s, the observation precision has been improved continuously, which laid an indispensable foundation of establishing the high precision extragalactic radio celestial reference frame.There are great progresses in the establishment of the extragalactic radio celestial reference frames in recent years and the International Earth Rotation Service (IRES)has made a lot of contributions. In the same time,there are deficiencies in the IERS method of this establishment and in the IERS extragalactic radio source catalogues. Though the extragalactic radio reference frame is a very good approximation to the ideal quasi inertial reference frame, it should be extended to other frames in practice. This extension can be accomplished by various methods, but most are still in the stage of theoretical discussion or in tests. It is still far for us from the goal to establish a quasi inertial celestial reference frame which is accessible to astrometry in visual as well as in radio wavelengths.