Impacts of the image alignment over frequency for VLBI Global Observing
  System
  The VLBI Global Observing System, which is the next generation of geodetic
VLBI and is called VGOS, observes simultaneously at four frequency bands in the
range 3.0--10.7 GHz (expected to be extended to 14 GHz). Because source
structure changes with frequency, we aim to study the source position estimates
from the observations of this new VLBI system. Based on an ideal point source
model, simulations are made to determine the relation between the source
positions as determined by VGOS observations and the locations of the radio
emission at the four bands. We obtained the source positions as determined by
VGOS observations as a function of the source positions at the four frequency
bands for both group and phase delays. The results reveal that if the location
of the radio emission at one band is offset with respect to that at the other
bands, the position estimate can be shifted to the opposite direction and even
by more than three times that offset. The VGOS source positions will be very
variable with time and very imprecise in the sense of relating to the locations
of the radio emission at the four bands, if the effects of source structure are
not modeled. The image alignment over frequency is essential in order to model
the effects of source structure in VGOS observations, which is the only way to
mitigate these strong frequency-dependent impacts on VGOS source positions.
