Determination of distances is a fundamental problem in studies of objects within our Galaxy. Spectroscopic or kinematic distances have a high degree of uncertainty, and that uncertainty generally propagates to even higher levels of uncertainty in source physics. The large increase in the number of stellar detections with the SKA combined with milli-arcsecond resolution at centimeter wavelengths will enable astrometry on a large number of radio stars at sub-milli-arcsecond accuracy. This level of astrometric accuracy would allow accurate parallax measurements out to distances of a large fraction of a kpc. It would also allow the direct measurement of binary motions for nearby (within a few 100 pc) non-thermal radio binaries such as RS CVn and Algol systems and weak thermal emitters such as Be star binaries.
One of the chief problems with using stellar positions to tie the optical and radio coordinate frames to very high accuracy is the binary nature and high degree of variability of the few bright non-thermal radio stars for which accurate radio astrometry is currently available. Thermal emission from stellar photospheres and the winds of single stars is quasi-steady state and centered on the position of the optical image of the star. These objects are ideally suited for matching optical and radio reference frames, a problem that will become more critical when coordinated optical and radio studies on sub-arcsecond scales become commonplace with the new generation of optical and radio facilities. Combined with the observations from missions like Hipparcos, radio astrometric positions for a large number of thermal emitting, single stars will provide an extensive grid of accurate measurements to tie the optical and radio reference frames over the entire sky with milli-arcsecond precision.
Dimensions of order 1 AU have crossing times of order one year for velocities of 10's of km s-1. Hence milli-arcsecond resolution images will allow direct measurements of the angular expansion of slowly expanding objects such as planetary nebulae and wind interaction shells with the ISM. Combined with spectroscopic data, such measurements can provide accurate distances for a significant population of objects out to kpc distance scales.