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USNO-B1.0 and 2MASS are the most widely used full-sky surveys. However, 2MASShas no proper motions at all, and USNO-B1.0 published only relative, notabsolute (i.e. on ICRS) proper motions. We performed a new determination ofmean positions and proper motions on the ICRS system by combining USNO-B1.0 and2MASS astrometry. This catalog is called PPMXL {VO-access to the catalog ispossible via http://vo.uni-hd.de/ppmxl}, and it aims to be complete from thebrightest stars down to about $V \approx 20$ full-sky. PPMXL contains about 900million objects, some 410 million with 2MASS photometry, and is the largestcollection of ICRS proper motions at present. As representative for the ICRS wechose PPMX. The recently released UCAC3 could not be used because we foundplate-dependent distortions in its proper motion system north of -20$^\circ$declination. UCAC3 served as an intermediate system for $\delta \leq-20^\circ$. The resulting typical individual mean errors of the proper motionsrange from 4 mas/y to more than 10 mas/y depending on observational history.The mean errors of positions at epoch 2000.0 are 80 to 120 mas, if 2MASSastrometry could be used, 150 to 300 mas else. We also give correction tablesto convert USNO-B1.0 observations of e.g. minor planets to the ICRS system.

The PPMXL catalog of positions and proper motions on the ICRS. Combining USNO-B1.0 and 2MASS