Hipparcos catalogue tie With conventional celestial reference frames
The Hipparcos catalogue contains a large number of stars whose positions are known at an unprecedented accuracy level. It is therefore important to relate it both to the past (FK5) and to the future (ICRF). Useful information can be gained in both of these procedures. By looking back and relating Hipparcos to FK5, advantage can be taken of the long time spans of the FK5 proper motion measurements. By looking forward and placing the Hipparcos catalogue in the ICRS (see Sect. 5.1), its optical star positions are transplanted to a coordinate frame that is the closest contemporary approximation to an inertial system. This chapter discusses some of the details of such catalogue links.