  We focus on how astrometric surveys might be exploited for the study of
Galactic halo substructure originating from the disruption of satellite
systems. The Sagittarius (Sgr) dwarf spheroidal tidal stream provides a useful
template to explore present and future capabilities of proper motion surveys
for finding and studying substructure as well as for using individual tidal
streams to probe the global properties of the Milky Way. The configuration of
the Sgr stream is particularly well suited for measuring the flattening of the
halo and for providing an independent means for measuring the speed of the
Local Standard of Rest. For the various proposed experiments, the required
proper motion accuracies are well within the capabilities of the Space
Interferometry Mission (SIM PlanetQuest).
