Independent determination of the systematic and random errors of the   proper motions in the PPMXL catalog using quasars
  Using a sample of 117053 quasars identified in the PPMXL catalog, the systematic errors of the proper motions in the PPMXL catalog are estimated independently as -2.0 and -2.1 mas yr$^{-1}$ in $\mu_{\alpha}\cos\delta$ and $\mu_{\delta}$, respectively. For objects with SDSS $r$ magnitude between 16.0 and 21.0 mag and SDSS $g-r$ color between -0.4 and 1.2, there are no obvious magnitude and color dependence in the systematic errors of the proper motions in the PPMXL catalog. The random errors of the proper motions in the PPMXL catalog change from $\sim4.0$ to $\sim8.0$ mas yr$^{-1}$ along with the increase of the magnitude of the objects. No color dependence in the random errors of the proper motions in the PPMXL catalog is found. There are obvious right ascension $\alpha$ dependence of the systematic errors in both components of the proper motions in the PPMXL catalog. For quasars with declination $\delta>0$, the systematic errors of $\mu_{\delta}$ in the PPMXL catalog increase linearly with $\delta$ with a slope of 0.07 mas yr$^{-1}$ deg$^{-1}$.   For comparison, using a subsample of quasars identified both in the PPMXL and SDSS DR7 catalogs, the systematic errors of the USNO-SDSS proper motions in the SDSS DR7 catalog are derived as 0.9 and 0.5 mas yr$^{-1}$ in $\mu_{\alpha}\cos\delta$ and $\mu_{\delta}$, respectively. There are no obvious magnitude, color, and $\alpha$ dependence in the systematic errors of the USNO-SDSS proper motions. For quasars with declination $\delta>0$, the systematic errors of $\mu_{\delta}$ in the SDSS DR7 catalog increase linearly with $\delta$ with a slope of 0.03. The random errors of the USNO-SDSS proper motions change from $\sim2.5$ to $\sim6.0$ mas yr$^{-1}$ along with the change of the magnitude and color of the objects. 
Zhen-Yu Wu, Jun Ma, Xu Zhou (National Astronomical Observatories,   Chinese Academy of Sciences)
astro-ph.GA