Tomography of Galactic star-forming regions and spiral arms with the
  Square Kilometer Array
  Very Long Baseline Interferometry (VLBI) at radio wavelengths can provide
astrometry accurate to 10 micro-arcseconds or better (i.e. better than the
target GAIA accuracy) without being limited by dust obscuration. This means
that unlike GAIA, VLBI can be applied to star-forming regions independently of
their internal and line-of-sight extinction. Low-mass young stellar objects
(particularly T Tauri stars) are often non-thermal compact radio emitters,
ideal for astrometric VLBI radio continuum experiments. Existing observations
for nearby regions (e.g. Taurus, Ophiuchus, or Orion) demonstrate that VLBI
astrometry of such active T Tauri stars enables the reconstruction of both the
regions' 3D structure (through parallax measurements) and their internal
kinematics (through proper motions, combined with radial velocities). The
extraordinary sensitivity of the SKA telescope will enable similar "tomographic
mappings" to be extended to regions located several kpc from Earth, in
particular to nearby spiral arm segments. This will have important implications
for Galactic science, galactic dynamics and spiral structure theories.
