The "PPM Star Catalogue" provides a high-precision astrometric reference star catalogue on the new IAU co-ordinate system of FK5 (J2000). It replaces the "SAO Catalogue" and "AGK3" as an astrometric reference. It provides more than 180,000 reference stars on the northern and southern celestial hemispheres and has a much higher degree of accuracy than the two older catalogues. Both the mean errors of the proper motions (0.43"/century) and of the present-epoch positions (0".27) provide an improvement of more than a factor 3 compared to SAOC. The star density is increased by 35 percent. On average 6.2 measured positions were used per star to compute mean positions were used per star to compute mean positions and proper motions. This redundancy ensures high reliability of the PPM data. In addition to the astrometric data (that is, positions, proper motions and their mean errors) the following data are given for each star: photographic magnitude; spectral type; HD number; SAO number; AGK3 number: and Durchmusterung number. The physical units used for the data and the arrangement of the stars (in declination bands of 10 degrees) are the same as in the SAOC.
The authors are scientists at the Astronomisches Rechen-Institut, Heidelberg, Germany. For ten years they have been active in the field of positional astronomy. They worked on the determination of Comet Halley's orbit, on the evaluation of measurements by ESA's astrometry satellite HIPPARCOS, and on the production oof high-precision star catalogues such as FK5 and PPM.