  We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is
the first of its kind in Solar Cycle 24. This is the first GLE event to be
fully observed close to the surface by the Solar Terrestrial Relations
Observatory (STEREO) mission. We determine the coronal mass ejection (CME)
height at the start of the associated metric type II radio burst (i.e., shock
formation height) as 1.38 Rs (from the Sun center). The CME height at the time
of GLE particle release was directly measured from a STEREO image as 2.32 Rs,
which agrees well with the estimation from CME kinematics. These heights are
consistent with those obtained for cycle-23 GLEs using back-extrapolation. By
contrasting the 2012 May 17 GLE with six other non-GLE eruptions from
well-connected regions with similar or larger flare size and CME speed, we find
that the latitudinal distance from the ecliptic is rather large for the non-GLE
events due to a combination of non-radial CME motion and unfavorable solar B0
angle, making the connectivity to Earth poorer. We also find that the coronal
environment may play a role in deciding the shock strength.
