  We aim to evaluate the possibility of improving the ICRS realization starting
from the ICRF2 catalogue by investigating the coordinate time series of radio
sources observed by VLBI between 1979 and 2016. Sources with long observational
history are selected as the candidates and the least squares fits with special
handling of the weights are performed to derive the linear drifts of the source
coordinates. Then the sources are sorted based on the normalized linear drift
(i) over the whole sky and (ii) in four homolographic areas divided by
declinations. The axial stability of the reference system and sky distribution
defined by the selected sources are evaluated, which are acted as the criterion
for the final source lists. With our improved source selection scheme, two
groups of sources are proposed and considered suitable for defining a more
stable and homogeneous celestial reference system compared to the current
ICRF2. The number of sources in the final lists are 323 and 294, respectively,
and the global rotation of the axes derived from apparent motion of the sources
are about two times better than the ICRF2.
