  We report here on multi-frequency VLBA observations of three extragalactic
sources within 1 degree of the Galactic Center. These sources have been used as
astrometric reference sources for VLA and VLBA determinations of the proper
motion of Sagittarius A*, the compact nonthermal radio source in the Galactic
Center. Each source has a main component with a brightness temperature in
excess of 10^{7.5} K, confirming that the sources are active galactic nuclei.
The sources have simple structure that can be characterized by one or two
Gaussian components. The frequency dependence of the structure indicates that
the positions of Sgr A* determined by the VLA astrometry of Backer & Sramek
(1999) at 4.8 and 8.4 GHz should have an offset of \sim 2 mas. This offset is
in the same direction as the 5 mas shift measured by Backer & Sramek (1999).
The structure is unlikely to bias the 43 GHz VLBA results of Reid et al (1999).
Motions of components in the calibrator sources could lead to errors in the
proper motion of Sgr A* on the order of a few km s^{-1}.
  All three sources show frequency dependent structure consistent with
scattering which is significantly stronger than that of the Galactic scattering
model of Taylor & Cordes (1993) but significantly weaker than that of the
hyperstrong Galactic Center scattering. Combined with other observations, this
suggests the existence of a new component of Galactic scattering located
several kpc from the Galactic Center.
