The real-time attitude of the HIPPARCOS satellite (see Turon, this symposium) is monitored by a star mapper viewing a 40 arcmin. Wide band of the sky, ahead of the main field of view, through a system of four vertical and four inclined slits (fig. 1). The resulting on-board precision will be 1 arcsec. rms and on-ground attitude reconstitution will later improve this value to 0.1 arcsec. rms.

The TYCHO project (approved by ESA in 1981) consisted in introducing dichroic beam splitters and a pair of redundant photomultipliers (with a bandpass close to Johnson B and V) into the science payload. This configuration will give a complete survey of the sky down to 11th magnitude in B, during the two and a half years of the mission. The data reduction is prepared by a scientific consortium (TDAC) which will be described in more details in section 4. The work related to the TYCHO Input Catalogue is described in section 2. An overview of the data reduction process is given as section 3, and aspects of the TYCHO outputs are discussed in section 5.