The angular velocity solution from individual stars (Eq. 2 (click here)) was made available to the Link Subgroup of the Hipparcos Science Team as our best estimate of the extragalactic calibration of the Hipparcos proper motion system.
Kovalevsky et al. (1996), according to the precepts of Lindegren & Kovalevsky (1995), have combined the corresponding solutions from the different link groups into one unique solution, which has been applied in the production of the final Hipparcos catalogue. The standard deviations of the components of the time-dependent rotation are 0.25 mas/a. They result from a compromise between formal errors and the root mean square of adjusted, weighted errors of the various link groups.
The difference of the adopted calibration for Hipparcos and our
preferred solution is
where the errors result from the combination of our internal errors and
the 0.25mas/a-error for the mean solution.
The 
-difference in 
may result from the uneven
distribution of our link fields on the sphere. One should note that
the probability for the true value to lie inside a 1
-interval
in a one-dimensional case belongs to a 
-ellipsoid in
case of the three-dimensional 
-vector. The difference vector of
Eq. (6 (click here)) has the length 0.97 mas/a. If we approximate all its
three errors by one and the same value 0.40 mas/a, the 
-fold is
0.68 mas/a, and the ratio 0.97/0.68 = 1.43 tells us that the occurrence of
the difference vector corresponds to a 
error in a one-dimensional
case (probability of surpassing is still 15%). Regarding this, our solution
compares favourably with the mean solution.
Acknowledgements
We are very grateful to Wolfgang R. Dick, Christine Ducourant, Roman Galas, Aysel Karafistan and Dimitris Sinachopoulos, who contributed to this project in the past. Michael Geffert has provided invaluable data for the Hipparcos link. Arnold R. Klemola's measurements on Lick astrograph plates are an important contribution to this project. Various plate archives allowed us to borrow and measure some of their photographic plates. We thank the Astronomisches Institut Münster for generous allocation of PDS scan time and the C.A.I. Paris for the possibility to measure with the MAMA machine. Financial support from the Bundesministerium für Forschung und Technologie (FKZ 50OO9101) is gratefully acknowledged.