The proper motions in the Hipparcos Input Catalogue are derived from a
wide variety of sources. The correspondence between the letters used
in field 24 and the source of the proper motions, along with the number
of stars whose proper motion was taken from this source, is given in
Table B1 
(page ).
[]For most stars the source of the proper motions and positions is identical. In addition, data were taken from high proper motion surveys, in particular from the Luyten (LHS, NLTT, L, and LP) and Giclas surveys, either via the SIMBAD data base or through data newly included in the INCA data base. These proper motions have relatively large errors, of the order of 0.02 arcsec per year. However, if they are used for stars whose positions were newly obtained either by plate measurements (mean epoch around 1976) or by meridian circle observations (mean epoch 1986), the final accuracy obtained at epoch 1990 for the input to the Hipparcos observations (Jahreiß 1989) was still within the margin tolerated by the satellite operation requirements.
Consistency checks were performed when independent
machine-readable positions were available for epochs sufficiently well
separated in time, and some new proper motions were individually
computed using first epoch positions from the Astrographic Catalogue
(AC, see van Biesbroeck 1963). About 4000 entries of
the Hipparcos Input Catalogue remain without known proper motions.