After a CCD image of the Galilean satellites of Jupiter is obtained by 
a long focal-length telescope, we can compare the theoretical positions 
of these satellites with their pixel positions so as to obtain the 
calibration parameters of the CCD field of view. In theory, the measurement 
of relative positions for two satellites would have good accuracy when 
their separation is small enough. 120 CCD images taken by 1-m telescope 
at Yunnan Observatory in February 2003 are used to perform the experimental 
test. After we improve the centroid algorithm for the satellites and adopt 
the former halo-removal techniques (Peng et al. 2003, Peng 2005), the 
results show that the positional measurement of two small-separation 
satellites has precision as good as 0.01-0.02 arcsec. This precision 
has comparability as that from rarely happening mutual events of Galilean 
satellites. Moreover, because of its much more opportunities, it is 
believed that this kind of observations together with mutual event 
observations can be better used for the research in their celestial 
mechanics of Galilean satellites.