  Direct evidence for the existence of dark matter and measurements of its
interaction cross-section have been provided by the physical offset between
dark matter and intra- cluster gas in merging systems like the Bullet Cluster.
Although a smaller signal, this effect is more abundant in minor mergers where
infalling substructure dark matter and gas are segregated. In such low-mass
systems the gravitational lensing signal comes primarily from weak lensing. A
fundamental step in determining such an offset in sub- structure is the ability
to accurately measure the positions of dark matter sub-peaks. Using simulated
Hubble Space Telescope observations, we make a first assessment of the
precision and accuracy with which we can measure infalling groups using weak
gravitational lensing. We demonstrate that using an existing and well-used mass
re- construction algorithm can measure the positions of 1.5x1013 M
substructures that have parent halos ten times more massive with a bias of less
than 0.3 . In this regime, our analysis suggests the precision is sufficient to
detect (at 3 {\sigma} statistical significance) the expected mean offset
between dark matter and baryonic gas in infalling groups from a sample of 50
massive clusters.
